Interactions of nitric oxide and carbon monoxide with palladium and

2321

J. Phys. Chem. 1991, 95, 2327-2339

Interactlons of NO and CO with Pd and Pt Atoms Gregory W. Smith and Emily A. Carter* Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024- 1569 (Received: July 2, 1990)

We report ab initio generalized valence bond and Correlation-consistent configuration interaction studies of CO and NO interacting with Pd and Pt atoms. We find dramatically different bonding mechanisms for the two ligands, which are easily understood in terms of changes in the electronic structure of the metal and the ligand. CO bonds to both Pd and Pt by a u donor/* back-bonding mechanism, yielding linear geometries. Our calculations predict that the ground (‘E+)state of PdCO is bound by 27 kcal/mol, while the ground (l2+)state of PtCO is bound by only 18.5 kcal/mol. By contrast, PdNO and PtNO are both bent, with the dominant bonding involving a covalent u bond between a singly occupied metal do orbital 20 kcal/mol], and the singly occupied NO 27r* orbital. While the ground (2A’) state of PtNO is strongly bound [D,(Pt-NO) NO binds very weakly to Pd [D,(Pd-NO) I4 kcal/mol]. Linear excited states (2Z+and of PtNO and PdNO are predicted to be only weakly bound or unbound. However, corresponding linear cationic states (IZ+and ’n) are strongly bound, but the cationic bent (IA’) states are still the ground states of PtNO+ and PdNO’. These stark contrasts, in which NO binds strongly to Pt but weakly to Pd while CO binds much more strongly to Pd, are due to the preference for closed-shell species to bind strongly to other closed-shell species (e.g., CO to Pd) and for radicals to bind strongly to other radicals (e.g., NO to Pt).

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I. Introduction A great deal of interest exists in the interaction of nitric oxide and carbon monoxide with transition metals, in part because of the crucial role these metals play in automobile exhaust catalysis and because of a fundamental desire to understand the nature of the bonding between various transition metals and different types of ligands such as NO and CO.I-)O NO and CO adsorption has

( I ) Bradshaw, A. M.; Hoffmann, F. M. Surf.Sci. 1978, 72, 513. (2) Ertl, G.; Koch, J. Adsorption-Desorption Phenomena; Ricca, F., Ed.; Academic Press: New York, 1972; p 345. (3) Tracy, J. C.; Palmberg, P. W. J . Chem. Phys. 1969, 51, 4852. (4) Steininger, H.; Lehwald, S.;Ibach, H. Surf.Sci. 1982, 123, 264. ( 5 ) Hoge, D.; Tushaus, M.; Schweizer, E.; Bradshaw, A. M. Chem. Phys. Lett. 1988, 151, 230. (6) Mieher, W. D.; Whitman, L. J.; Ho, W. J . Chem. Phys. 1989, 91, 3228. (7) Winicur, D. H.; Hurst, J.; Becker, C. A.; Wharton, L. Surf.Sci. 1981, 109, 263. (8) Conrad, H.; Ertl, G.;Kuppers, J.; Latta, E. E. Sur/. Sci. 1977,65,235. (9) Jorgenscn, S.W.; Canning, N. D. S.;Madix, R. J. Surf.Sci. 1987,179, 322. (IO) Nyberg, C.; Uvdal, P. Surf. Sci. 1988, 204, 517. (11) Hayden, B. E. Surf.Sci. 1983, 131, 419. (12) Bartram, M. E.; Koel. B. E.; Carter, E. A. Sur/. Sci. 1989,219,467. (13) (a) Comrie. C. M.: Weinberg. W. H.: Lambert. R. M. Surf. Sci. 1976,5< 619. (b) Pirug, G:; Bonze], fi. P.; Hopter, H.;Ibach, H. J . b e m . Phys. 1979, 71, 593. (14) Gland, J. L.; Sexton, B. A. Surf. Sci. 1980, 94, 355. (15) (a) Gorte, R. J.; Schmidt, L. D.; Gland, J. L. Surf.Sci. 1981, 109, 367. (b) Gorte, R. J.: Gland, J. L. Surf Sci. 1981, 102. 348. (16) Kundig, E. P.; McIntosh, D.; Moskovits, M.; Ozin, G.A. J . Am. Chem. Soc. 1973, 95, 7234. (17) Darling. J. H.: 0ade.n. J. S.J . Chem. Soc..Dalton Trans. 1973. 1079. (18) Whyman, R. J. 8rganomet. Chem. 1973,63,467. (19) Misono, A.; Uchida, Y.;Hidai, M.; Kudo, K. J . Organomef. Chem. 1969, 20, P7. (20) Bradford, A. M.; Douglas, G.;Manoj1ovic’-Muir,L.; Muir, K. W.; Puddephatt. R. J. Organometallics 1990, 9, 409. (21) Jack, T. R.; May, C. J.; Powell, J. J. Am. Chem. Soc. 1977,99,4707. (22) Fischer, E. 0.;Shustcr-Woldan, H. Z . Nafurforsch. 1964, 766. (23) Blomberg, M. R. A.; Brandemark, U.; Johansson, J.; Siegbahn. P. E. M.; Wennerberg, J. J . Chem. Phys. 1988, 88, 4324. (24) Pacchioni, G.;Koutecky, J. J . Phys. Chem. 1987, 91, 2658. (25) Blomberg, M. R. A.; Lebrilla, C. B.; Siegbahn, P. E. M. Chem. Phys. Leu. 1988, I50, 522. (26) Gavezzotti, A.; Tantardini, G.F.; Simonetta, M. Chem. Phys. Leu. 1986, 129, 577. (27) Rohlfing, C. M.; Hay, P. J. J . Chem. Phys. 1985,83,4641. (28) Basch, H.; Cohen, D. J . Am. Chem. Soc. 1983, 105, 3856.

been studied extensively on well-defined single-crystal surfaces of Pd and Pt,l-I5 in complexes isolated in argon matrice~,’~.’’ and on clusters.18-22High-resolution electron energy loss spectroscopy (HREELS) and infrared reflection-absorption spectroscopy (IRAS) have been used to determine vibrational frequencies of adsorbed NO and CO on single-crystal surfaces of Pd and Pt,1,495,”2Js while temperature-programmed desorption (TPD) and low-energy molecular beam scattering (LEMS) experiments have extracted the binding energies of NO and CO on Pd and Pt.2J36-9J2-’5 Often these properties are measured as a function of adsorbate coverage. At lower coverages (e < 0.5 ML) (ML = monolayer)), more than one surface metal atom is available to each adsorbed molecule, possibly allowing the formation of bridge-bonded species on the surface. Since our calculations involve only one metal atom, direct comparisons are made only to atop (terminally bonded) surface species. Many experiments have demonstrated that CO binds in both linear atop and bridging geometries on Pd and Pt surfaces.’-’ The interaction of CO and Pd and Pt appears to be insensitive to crystal facial structure but is extremely sensitive to surface coverage. Bridging carbonyls are bound by 34-36 kcal/mol to Pd, while atop carbonyls are bound by only 22-23 kcal/mol, since the atop C O S only appear at high coverages where repulsive lateral interactions reduce their heat of ad~orption.~,~ The C-0 vibrational frequency is strongly coverage and site dependent, with bridging CO’s exhibiting we 1820 cm-’, whereas atop CO’s have o, 2100 cm-I.’ While CO first adsorbs in bridge sites on Pd, CO initially prefers atop sites on Pt (w,(Pt-C) 470 cm-l and o,(C-O) 2100 ~ m - l ) .Bridging ~ C O appears above Bco = 0.17 ML with w,(Pt-C) 380-470 cm-l and w,(C-0) 1855 cm-l.e In contrast to Pd, both bridging and atop CO’s on Pt are bound by 30-35 kcal/mol, for Bco 5 0 . 5 ML.4f47 Several experiments examining NO chemisorption on Pd and Pt surfaces have demonstrated the geometric versatility of NO as an adsorbate, with bridging, linear atop, and bent atop geometries observed.*-I5 At low coverages, NO exists in bridging sites on both Pd8-Io and Pt11-’5surfaces. At higher coverages, linear atop NO is observed on Pt( 11I), while coadsorption of oxygen atoms produces bent atop NO.1ZHREELS data for high coverages of NO on Pd( 100) may be due to the presence of bent atop NO.IO Another study suggests that Pd( 100) precovered with S atoms may cause bent atop bonding of NO to the s ~ r f a c e . ~

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(29) Basch, H. Chem. Phys. Lett. 1985, 116, 58. (30) Bauschlicher, C. W., Jr.; Bagus, P. J . Chem. Phys. 1984, 80, 944.

0022-3654191 12095-2327%02.50/0 0 1991 American Chemical Society

2328 The Journal of Physical Chemistry, Vol. 95, No. 6,1991

Smith and Carter

Experimental binding energies for N O on Pd and Pt vary dramatically with coverage. Complementary vibrational studies indicate that these shifts in the binding energies are due to changes in the adsorption geometry. Typically, bridging NO binds strongly to both Pd and Pt at low ONO: D,(Pd-NO) 31-32 k ~ a l / m o l ~ * ~ 25-36 kcal/m01.'~-'~Atop NO appears at and D,(Pt-NO) higher coverages and is more weakly bound, with binding energies between 14 and 24 kcal/mol for Pd and Pt8*9J2-15 N-O vibrational frequencies for bridging N O on Pd and Pt are lower (we 1500-1600 cm-l)el'3'3b9'4*15b than for atop linear or bent NO on Pd and Pt (we 1680-1 790 ~ m - ~ ) . ~ ~ ~ 9 ~ ~ ~ , ~ ~ 7 ~ ~ ~ Many complexes of CO and NO with Pd and Pt have been Figure 1. Qualitative depiction of bonding between N O and one metal isolated and characterized with IR and X-ray diffraction, including atom: (a) linear *Z+MNO; (b) bent 2A' MNO; (c) 211 MNO. The PdCO and PtCO in low-temperature matrices [w,(C-O) 2050 valence orbitals shown are discussed in detail in the text. c ~ - ' ] . ' ~ JTerminal ~ carbonyls in Pt(C0)2(PPh3)2 and Pd(CO)(PPh3), exhibit C-O stretching frequencies of 1996 and 1954 fashion amounts to allowing NO 5u donation to the metal, concm-l and 1955 cm-I,l9 respectively, while triply bridging COS comitant with metal du back-bonding to the 2u* orbitals of NO, in trinuclear platinum complexes have Pt-C bond lengths of -2.0 leading to a linear 211 state (Figure IC). Since the 2u* level is A and low C-O vibrational frequencies (- 1765-1827 cm-1).20 partially occupied for NO, we expect repulsive effects to inhibit Finally, Pd and Pt complexes with terminal N O ligands exhibit du-pu back-bonding, hence weakening this bond relative to N-O stretching frequencies of 1740-1 790 cm-1.21,22 metal-CO interactions and thus disfavoring the 211state of MNO. Several theoretical studies have been carried out to predict the The rest of this work is outlined as follows. Section I1 presents electronic structure of transition metal-NO and -CO details of the ab initio calculations, section I11 presents results Siegbahn and c o - ~ o r k e r sstudied ~ ~ the 'Z+ ground state of NiCO and discussion for metal-CO and metal-NO interactions, and by using coupled pair functional (CPF), modified CPF (MCPF), section IV summarizes our findings. and multireference contracted configuration interaction (MRCCI) methods. Pd-CO bonding has been studied by Pacchioni and 11. Calculational Details Koutecky2' by employing a nonrelativistic pseudopotential for Pd All electrons on C, N, and 0 were treated explicitly. The and the multireference doubly excited configuration interaction Dunning32valence double-{contractions of the C, N, and 0 (9sSp) (MRD-CI) method. Siegbahn and co-workers have also used the Gaussian primitive bases of Huzinaga" were used, with one set CPF approach to study PdCO and Pd2C0.2s of d-polarization functions added (s(C) = 0.64, s(N) = 0.76, s(0) Gavezzotti et alez6performed Hartree-Fock (HF) calculations = 0.95). Relativistic effective core potentials (RECPs) and the on PtCO using a relativistic pseudopotential for Pt. Rohfling and Pd and Pt basis sets of Hay and Wadt31awere used to represent Hay27performed unrestricted Hartree-Fock (UHF) with Molthe metal valence orbitals, with the Pd (3s3p4d) and Pt (3s3p3d) ler-Plesset second-order perturbation theory (MP2) calculations primitive functions contracted to (3s2p2d). Although all of the on Ni, Pd, and Pt carbonyls, employing the same relativistic calculations reported here utilized these 10-electron RECPs effective core potentials (RECPs) for Pd and Pt that we use in (RECP(10), where the 10 valence electrons are treated explicitly), the current Basch and Cohen2* performed small we did carry out a test of the accuracy of this potential with regard multiconfiguration self-consistent field (MCSCF) calculations, to the prediction of binding energy and bond length. In particular, followed by small valence level CI calculations on PtCO, using we examined the effect of explicitly including the outermost core an RECP for Pt and ECPs for C and 0. In later work, Basch electrons in the calculation. Other authors have shown31cthat carried out H F gradient calculations to predict the vibrational such core polarization effects are quite small for Pt but may be modes of PtCO and PtN0+.29 considerable for Pd. Thus, we examined how the Pd-CO bond The only other theoretical study of N O binding to a metal was length and binding energy varied for two RECPs: the one used performed by Bauschlicher and Bagu~,~O who used CASSCF/ throughout our study, RECP( lo), and the Hay-Wadt 18-electron CISD calculations to find that the ground state of NiNO is the effective core potential,31bRECP( 18), that includes only up linear *E+state, with the bent 2A' state more weakly bound. They through the n = 3 shell of the Pd core (Le., the 4s and 4p electrons suggested that the more diffuse valence s and d orbitals of Pd and are treated explicitly, in addition to the valence electrons). We Pt may reduce the strength of the metal-NO u bond, which might contracted the Hay-Wadt (5sSp4d) primitive basis to (5s3p2d), stabilize the bent states of PdNO and PtNO. Our calculations in order to have similar flexibility in both basis sets. We find that provide strong support for this suggestion (vide infra). the Pd-CO bond distance is lengthened by 0.07 A and the binding Our ab initio theoretical study focuses on both quantitative and energy is correspondingly lowered by 6.4 kcal/mol (H 10%) when qualitative aspects of CO and N O interacting with Pd and Pt the RECP( 18) potential is employed. Thus, while core polarization atoms. We expect to find quite different behavior for Pd versus is somewhat significant for Pd, these effects do not change our Pt and CO versus NO, since Pd and Pt have different ground overall conclusions for Pd or Pt, based on the RECP( 10) potential electronic states (Pd is d'O while Pt is s'd9) and NO is a radical (vide infra). while CO is closed shell. Indeed, qualitatively we find that whereas The geometries of states were optimized by utilizing analytic MCO (M = Pd, Pt) forms only linear, low-spin ground states, gradients of GVB-PP wave functions (generalized valence bond the presence of the 2u electron in NO allows it to either covalently with perfect singlet-pairing restriction^).^^ The first-order wave bond or form a CO-like bond to transition-metal atoms. Two possibilities exist for covalently bound MNO states (Figure 1): (32) Dunning, T. H., Jr. J . Chem. Phys. 1970,53, 2823. (i) a linear 22+state, where the NO 2u* electron is spin paired (33) Huzinaga, S. J . Chem. Phys. 1%5,42, 1293. with a metal d r electron to form a covalent u bond and where (34) (a) The details of the generalized valence bond method may be found the N 2s (NO Sa) pair may form a u-donor bond via donation in: Hunt, W. J.; Dunning, T. H., Jr.; Goddard, W. A., 111 Chem. Phys. Lcrr. into an empty metal u orbital (Figure la) or (ii) a bent 2A' state, 1969, 3, 606. Goddard, W. A., 111; Dunning, T. H., Jr.; Hunt, W. J. Chem. where the NO 2r* electron is spin paired with a metal u electron Phys. Lerr. 1%9,4,231. Hunt, W. J.; Goddard, W. A,, 111; Dunning, T. H.. Jr. Chem. Phys. Lett. 1970.6, 147. Hunt, W. J.; Hay, P. J.; Goddard, W. to form an M-N covalent u bond but where little or no N O 5u A., 111, J. Chem. Phys. 1972, 57,738. Bobrowicz, F. W.; Goddard, W. A., donation occurs (Figure Ib). Restricting NO to bond in a CO-like 111 In Methods ofEIecrronic Srrucrure Theory; Schaefer, H. F., Ed.;Plenum:

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(31) (a) Hay, P. J.; Wadt, W. R. J . Chem. Phys. 1985,82,270. (b) Hay, P. J.; Wadt, W. R. J . Chem. Phys. 1985,82,299. (c) Rohlfing, C. M.; Hay, P. J.; Martin, R. L. J . Chem. Phys. 1986, 85, 1447.

New York, 1977; pp 79-127. (b) Yaffe, L. G.; Goddard, W. A., 111 Phys. Reu. A 1976, 13, 1682. (c) Dupuis, M.; King, H. F. J . Chem. Phys. 1978, 68, 3998. Rappi, A. K.; Upton, T. H. Orgammerallics 1984,3, 1440. Upton, T. H.; Rap*, A. K. J . Am. Chem. Soc. 1985,107, 1206. Schlegel, H. B. J . Compur. Chem. 1982.3, 214.

Interactions of N O and CO with Pd and Pt Atoms TABLE I: Atomic State Splittings at the Hartree-Fock Level for Pd, Pa+,Pt, a d PtCu state TE~ AEc AE(RNHF)" AE(expt)' 3 $0 0.0 0.0 IS Pd (d") -29.10660 'D Pd (s'd9) -29.1 1 1 38 0.0 2.3 21.9 11.5 33.5 ID Pd (s'd9) -29.09308 'F Pd (s2d8) -29.02477 54.3 50.5 77.9 'D Pd+ (d9) -28.877 35 0.0 4.1 4F Pd+ (s'd') -28.785 96 57.3 77.7 'F Pd' (~'d') -28.53280 216.2 'D Pt (s'd9) -26.238 92 0.0 0.0 0.9 IS Pt (dIo) -26.199 95 24.5 20.7 17.5 'F Pt (s2d8) -26.21264 16.5 9.2 21.2 'DPt (s'd9) -26.21538 14.8 38.6 'D Pt+ (d9) -25.95933 0.0 9.6 4F Pt+ (s'd8) -25.946 62 8 .o 27.1 'F Pt+ (S*d') -25.798 64 100.8 91.5 Experimental values are averaged over J states. bHartree-Fock total energy in hartrees. CRelativeenergy at the Hartree-Fock level using a (3sZp2d) basis set and an RECP,'In in kcal/mol. "Relativistic numerical Hartree-Fock in kcal/mol. Reference 37, kcal/mol.

functions for M-NO and MNO+ were at the GVB(4/8)PP level, where a GVB(nI2n)PP wave function involves n GVB electron pairs each described by two natural orbital^.'^.^ The GVB pairs for the 2Af and 2Z+states of MNO and the 'A' and IZ+states of MNO+ were the N-O u and u bonds, the M-N bond, and the N O 5u orbital (derived from the N 2s), while the GVB pairs for the 211state of M N O and the 311 state of MNO+ were the N-O u and u bonds for both states, both metal d u orbitals for the neutral complex, and the N O 5u for the cationic complex. Thus, 311 MNO+ was treated at the GVB(3/6)PP level, in order to maintain the same number of active orbitals for all states of NO bound to M and M+. The first-order wave function for the I Z' state of MCO was a GVB(6/12)PP wave function, where the C-O u and u bonds, both metal d r orbitals, and a metal du orbital were correlated. A GVB(S/lO)PP wave function was employed for the 3Z+ and open-shell IZ+states, both of which involved correlating the C-O u and u bonds and both d r orbitals on the metal atom. Utilizing a GVB(S/IO)PP wave function for the triplet and open-shell singlet states allows the same number (12) of active orbitals as for the GVB(6/12)PP wave function for the I Z' state of MCO. Higher levels of configuration interaction (CI) were then added to the resulting wave f u n c t i o n ~ . ~ ~ J ~ 111. Results and Discussion A. Control Calculations. To check the accuracy of the RECPs, relative energies for low-lying states of Pt and Pd within the (3s2p2d) basis set were compared with experiment and with relativistic numerical Hartree-Fock (RNHF) calculations (Table I). H F theory within the RECP/valence basis set description (HF/RECP) and R N H F theory both describe the Pd 3D-'S splitting poorly. R N H F theory predicts the correct ground state (IS)but with a 3D-lS splitting of only 2.3 kcal/mol, whereas the experimental3' J-averaged 3D-1S splitting is 21.9 kcal/mol. HF/RECP predicts the 3D state to be 3.0 kcal/mol lower than IS. Clearly, inclusion of extensive electron correlation is extremely important for reproducing the splitting in Pd; we have chosen to employ the experimental state splitting in order to obtain reliable energetics (vide infra). The only remaining concern over the use of this RECP is whether an overstabilized s1d9state of (35) Carter, E. A,; Goddard, W. A,, 111 J . Chem. Phys. 1988,88, 3132. ( 3 6 ) The following abbreviations are used: HF = HartreeFock. S,,, = single excitations from all valence orbitals to the virtual space; SD = single and double excitations from the orbital indicated to the virtual space; RCI = all single and double excitationsallowed by symmetry within the indicated GVB pairs such that each GVB pair always contains two electrons;GVBCI = a full CI in the GVB orbital space. (37) Moore, C. E. Atomic Energy k w f sAs Derived From rhe Analyses of OpticalSpccrra;US.Government Printing Office: Washington D.C. 1971; VOI. 111. pp 38-43. 181-185.

The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2329

(3n)

TABLE II: Equilibrium Properties for NO and CO ('E+)at tbe CVB-PPLevel molecule method TE, hartrees Re, A we,cm-' pa NO GVB(2/4)PP -129.32453 1.159 1808.6 NO GVB(3/6)PP -129.32958 1.157 1813.4 -0.017 NO expt 1.151b 1904.2b *0.153e CO GVB(3/6)PP -112.81856 1.134 2273.5 -0,191 CO expt 1.12gb 2169.8b -0.112" Magnitude of dipole moment vector in debye. A positive sign indicates the negative end of the dipole points toward the 0 atom. *Reference 38. cReference 40. dReference 39.

Pd causes problems in the description of PdCO or PdNO. Since we find 3D and 'S Pd to be nearly degenerate, there is no large preference for either state, leading to little bias in the nature of the wave function. In other words, CO and N O can bind readily to dIo and s1d9Pd, respectively, with no inherent qualitative biases. HF/RECP and R N H F both describe the Pt J-averaged 'SJD = 20.7 splitting fairly well: AEWp = 24.5 kcal/mol and URN,, kcal/mol compared with the e ~ p e r i m e n t aJ-averaged l~~ splitting of 16.6 kcal/mol. We have utilized both higher level CI predictions and experimental values for this splitting (vide infra). We have also examined the ID states of Pd and Pt a t the HF level and including electron correlation. The ID state of Pd is correctly predicted to be the second excited state, although the splitting is far too low (1 1.5 versus 33.5 kcal/mol experimentally; Table I). GVBCI( 3/6) calculation^^^ yield an improved value of 19.3 kcal/mol. The ID-IS splitting for Pt is poorly described at the H F level (-9.7 versus 21.1 kcal/mol experimentally). However, GVBCI( 3/6) calculation^^^ a t least obtain the correct ordering of states, with a ID-'S splitting of 5.1 kcal/mol. As discussed in the next section, even the artifically low '&IS promotional energy does not allow strong metal-ligand interactions to take place, and the larger, correct ID-% splitting only makes bonding to the ID state even more unlikely. Although the 3F-1Ssplittings for Pd and Pt are not predicted quantitatively correctly by these methods (e.g., AERECP(Pd)= 51.3 kcal/mol, AEExp(Pd) = 77.9 kcal/mol; AERECp(Pt) = -8.0 kcal/mol, "(Pt) = 3.7 kcal/mol), none of the MNO or MCO states studied dissociate to SFmetal atoms; thus this deficiency should not have a serious effect on the results of our study. Electronic-state splittings for Pd+ and Pt+ at the HF/RECP level are in reasonable agreement with experiment, considering the lack of inclusion of electron correlation. The HF/RECP 4F(s'd8)-2D splitting for Pd+ is 57.3 kcal/mol, compared with the e ~ p e r i m e n t a lJ-averaged ~~ splitting of 73.6 kcal/mol. The HF/RECP level 4F(s'd8)-2D splitting for Pt+ is 8.0 kcal/mol compared with the experimental3' J-averaged splitting of 17.5 kcal/mol. The Pt+ 4F(s2d7)-2Dsplitting at the H F RECP level is 100.8 kcal/mol, compared with the experimentall' J-averaged splitting of 81.9 kcal/mol. Since none of the MNO or MCO states studied dissociate to the 4F states of Pd+ or Pt+, quantitative predictions of these splittings are not necessary, though the qualitative descriptions are reasonable. To check the accuracy of all-electron ab initio calculations on the ligands N O and CO, bond lengths and vibrational frequencies from GVB(3/6)-PP calculations were compared to experiment (Table 11). The predicted bond lengths are 0.006 A Ion er than while those observed experimentally (ReGVB(NO)= 1.157 %EXP(NO)= 1.151 A:* R$vB(CO) = 1.134 A, while &Exp(CO) = 1.128 Aj8). The predicted N-0 and C-0 vibrational frequencies are both within 5% of the experimental values38 (ueGVB(NO)= 1813 cm-l and u,EXP(NO) = 1904 cm-'; weGVB(CO) = 2274 cm-' and u,EXP(CO)= 2170 cm-I), which are quite typic1 errors for such valence DZP basis sets. These values allow us to calibrate our vibrational frequencies and geometries for N O and CO bound to metal atoms (vide infra). Dipole moments were also calculated for N O and C O at their equilibrium geometries at the GVB-PP level. The predicted value

1,

(38) Huber,K.; Herzberg, G. Constants of Diatomic Molecules; Van Nostrand Reinhold Co.: New York, 1979.

Smith and Carter

2330 The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 TABLE III: Calculated Properties of MCO States (M = Pd and Pt) PdCO lz+ OS" Iz+ TEb AE' D,( M-CO)" RJMC-O)' RCW-CO)' e,(M-C-O)g w,(MC-O)* we(M-CO)* we( M - C - O bend)* Pi

-141.968 65 0.0 27.2 1.14 1.96 180.0 2253 428 56 1 1.30

Ptco

lz+

3z+

-1 39.074 24 0.0 15.4 (18.5)" 1.13 1.99 180.0 1976 600 56 1 1.12

-1 41.935 6 1 20.7

-1 41.91 6 65 32.6 e 1.13 4.15 180.0 1889 22 556 -0.79

e

1.13 4.54 180.0 1886 28 555 -0.55

OS" Iz+

3z+

-1 39.062 39 7.4 1.O 1.13 2.99 180.0 1914 348 558 -1.68

-1 39.037 40 23.1 e

1.13 4.05 180.0 1891 32 556 -0.56

#OS = open-shell singlet; correlates with ID Pd and Pt. bTotal energy at the GVB(6/12)-PP level for the 'Z+ state and at the GVB(S/IO)-PP level for the open shell IZ+and 3Z+states (in hartrees). 'GVB-PP relative energy (in kcal/mol). "Adiabatic Pd-CO dissociation energy in kcal/mol at the GVBC1(6/12) level (see text). Value in parentheses is our best estimate for the true De. 'Unbound with respect to ground-state fragments. /Equilibrium bond length in angstroms. g Equilibrium bond angle in degrees. * Vibrational frequency in cm-I. 'Magnitude of dipole moment vector in debye. A positive sign indicates the negative end of the dipole points toward the 0 atom. Pd-C-0

TABLE IV: Electron Distribution in MCO for M = Pd and Pto state IC+ PdCO OSe PdCO 3Z+PdCO Iz+PtCO os, lz+P t c o 3z+P t c o free CO

u r donationb back-bonding'

0.03 0.01 0.01 0.4 1

0.01 0.05

0.17

0.00 0.00 0.20 0.00 0.0 1

M"

Cd

od

9.82 10.02 10.00 10.23 10.02 10.06

6.12 5.93 5.94 5.71 5.93 5.91 5.95

8.06 8.05 8.0'6 8.06 8.05 8.03 8.05

a Electron populations are calculated from Mulliken populations summed over both natural orbitals of the GVB pairs. bTotal electron population donated to M from the CO 5a orbital. 'Total electron population donated to C and 0 from M d r orbitals. "Total electron distribution. Includes only valence orbitals on M. 'OS = open-shell singlet; correlates with 'DM.

of 4 . 1 9 1 D for CO compares well with an experimental value39 of -0.1 12 D (where the minus sign indicates C-0' polarity). It is interesting to note that the polarity of CO is correctly predicted from the theoretical dipole moment, but Mulliken population analysis (Table IV) indicates the opposite polarity. In fact, if the Mulliken population data were taken to indicate a charge of +0.05 e on C and -0.05 e on 0, the dipole moment would be +0.27 D. Thus, theoretical dipole moments are a better measure of charge transfer than Mulliken population analyses. For NO, we find a dipole moment of -0.017 D compared with an experimental4 value of magnitude 0.153 D. Although the sign of the dipole of NO has not been determined experimentally, our result is in qualitative agreement with ab initio calculations of Walch and Goddard41 and Langhoff et al.42that predict values of -0.10 and -0.17 D, respectively. B. Metal-CO Interactions. PdCO: We find the ground state of PdCO to be the I F state, in agreement with previous theoretical s t ~ d i e s . ~ ~ *The * ~ *IZ+ * ~ state of PdCO is formed through a t~ donor/?r back-bonding mechanism. Pd has a 4dI0valence electron configuration in this state, avoiding repulsive Pd 5s-CO 5a interactions and allowing d?r back-donation. The other two PdCO states that we examined are a 3Z+ state that correlates with 3D Pd and an open-shell state that correlates with ID Pd at infinite Pd-C separation. These two states have the Pd 5s orbital partially occupied, resulting in a repulsive interaction with the CO group. The first excited state is found to be the 3Z+ state (T, = 20.7 kcal/mol), and the second excited state is found to be the open shell state (T' = 32.6 kcal/mol, Table 111). Both excited states (39) Rosenblum, B.; Nethercot, A. H.Jr.; Townes, C. H.Phys. Rev. 1958, 109,400. (40) Neumann, R. M. Asfrophys. J. 1970, 161, 779. (41) Walch, S. P.; Goddard, W.A., I11 Chem. Phys. Leu. 1975.33, 18. (42) Langhoff, S. R.; Bauschlicher, C. W., Jr.; Partridge, H.J. Chem. Phys. 1988.89.4909. (43) Entries for the parent molecules and fragments have the following

form: calculation/total energy in hartrees (number of configurations/number of spin eigenfunctions).

I

I

I

1

ONE

ONE

I

I

I

I

I

I

I

.- _ -.

I

Figure 2. GVB(6/12)PP bonding orbitals for PdCO: (a) CO St7 donor bond; (b) one of two identical Pd d r back-bonds; (c) CO covalent u bond; (d) one of two identical CO r bonds; (e) Pd d s lone pair. Contours range from -0.5 to 0.5 au at intervals of 0.04 au.

have purely repulsive Pd-C interactions with respect to their diabatic asymptotes (ID or 3D Pd and CO) and thus are also unbound with respect to ground-state IS Pd (by 21.9 and 33.5 kcal/mol, Table I). Since all electrons in CO are paired, no covalent bonding to the metal is possible. A linear geometry is preferred for PdCO, in order to maximize overlap of the orbitals involved in t~ donation/?r back-bonding. Our calculations predict that the IZ+state of PdCO has a short Pd-CO bond length of 1.96 A (Table 111). By contrast, the open-shell I F and 321+ states are predicted to have very long Pd-CO bond lengths, 4.1 5 and 4.54 A, respectively. All three states have C - O bond lengths of 1.13 A. Our predicted Pd-CO distance of 1.96 A is in good agreement with previous theoretical predictions that range from 1.87 to 2.1 1 A (Table VI). The value of 2.1 1 A is derived from a nonrelativistic treatment of Pd," which points out artifacts that may result from the lack of inclusion of relativistic orbital contractions. Our value

-

The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2331

Interactions of N O and C O with Pd and Pt Atoms TABLE V: l2+MCO Bond Energies for M = Pd and PP

HF GVB(6/12)PP RCI(6/ 12) GVBCI(6/12) -142.04571 (729/3012) -142.05970 (18973/57 168) -141.96865 (64/64) IZtPdCO -141.89592 ( ] / I ) -139.14408 (729/3012) -139.16002 (18973/57 168) -139.00368 (1/1) -139.07424 (64/64) 'E+ P t c o -112.87644 (27/37) -1 12.87765 (45/55) -112.75859 (1/1) -112.81856 (8/8) 12' Cob -29.1 1796 (8/8) -29.138 50 (27/37) -29.13869 (45/55) -29.10660 ( I / ] ) IS Pdb 1s Ptb -26.19995 (1/1) -26.21 1 25 (8/8) -26.219 46 (27/37) -26.226 37 (45/55) 20.2 19.3 27.2 D,(Pd-CO)c 19.3 35.1 28.3 27.9 D,d'lb(Pt-CO)d 30.2 D,ld"b(Pt-CO)' 8.6 8.2 10.5 15.4 (18.5)' 'Reference 36. Aside the total energy in hartrees are the number of spatial configurations/spin eigenfunctions for the given wave function (in parentheses). *Correspondingwave functions are HF, GVB(3/6)PP, RCI(3/6), and GVBCI(3/6). CTheadiabatic bond dissociation energy (0,) is Z ' CO and IS Pd, in kcal/mol. dThe bond dissociation energy DtUb is the energy to dissociate to ! E + CO and IS Pt, in the energy to dissociate to I kcal/mol. #The adiabatic bond energy D,ldiaballows Pt to relax in a spin-forbidden transition to its )D ground state (kcal/mol). The Pt IS-)D splitting (19.7 kcal/mol) was calculated by comparing the total energy of a IS Pt RCI( 1/2) wave function from which single and double excitations were allowed from the correlated d pair (RCI(I/2)*SDd.pi, to the total energy of a 3D Pt HF wave function from which all single and double excitations were allowed from the open shell s and d orbitals (HF*SD,,,,d). /Best estimate for the true De, using the experimental IS3D splitting (16.6 kcal/mol). TABLE VI: ComDarison of Theoretical and Exwrimental Prowrties for l2+PdCO and PtCO we( M C - O

D,(M-CO)" PdCO this work MP2' MRDCIX MRCC112* CPF20h expt PtCO this work MP2' SCF" SCF"

cIP

expt

R,(MC-O)b R,(M-CO)b

27.2 37.4 7.8 28 33 2 2'

1.14 1.185 1.16

15.4 (18.5) 37.4 27.0 14.8 42.7 31 f 10

1.13 1.184

1.96 1.882 2.1 1 1.87 1.91

w,(MC-O)e

w,(MCO)e

bend)c

Pd

428

561

1.30 1.86

1976

600

56 1

1.12 1.79

2157

527

550

2104,' 2073: 1996,' 1954'

468'

2253

2050i 2045/ 2096,k 2092,' 1955"

1.159

1.99 1.977 1.91 1.754 1.707

'Adiabatic M-CO dissociation energy in kcal/mol. Value in parentheses is best estimate. Equilibrium bond length in angstroms. Vibrational frequency in cm-I. dMagnitude of dipole moment vector in debye. A positive sign indicates the negative end of the dipole points toward the 0 atom. ]Matrix isolation.16J7'Atop CO adsorbed 'Reference 27. /UHF value. #Reference 24. hReference25. 'Binding energy for atop CO on Pd(l1 on Pd(100).' 'Atop CO adsorbed on Pd(llI).l "Pd(CO)(PPh3),.I9 "Reference 26. OReference 29. PReference 28. PBinding energy of CO on Atop-bonded CO on Pt(l1 JAtop-bonded CO on Pt( 11 'Pt(C0)2(PPh3)2.'8 Pt( 1 1 of 1.96 A agrees with other relativistic calculations to within -0.08 A, and all methods predict the same WC-O bond length to within -0.05 A. The state exhibits r back-donation from Pd to C O via delocalization of the Pd d r orbitals toward the C atom, with concomitant o donation from C to Pd via delocalization of the C 2s (the CO 50) orbital toward Pd (Figure 2). Donation by the CO Sa and Pd d r bonding orbitals is minimal in the open-shell I P and 'E+ states, due to the large, diffuse Pd 5s orbital that induces o repulsions between Pd and CO. Mulliken population analysis indicates that -0.2 electron is transferred from Pd to CO in the state (Table IV), due primarily to r back-bonding from Pd to C (0.17 electron). The other two states (open-shell and '2') have electron distributions [Pd (-IO), C (-5.9), and 0 (-8,l)I similar to the separated fragments Pd and CO, as expected with such long Pd-C equilibrium separations. The transfer of charge from Pd to C in the IZ+state results in a positive dipole moment (Pd+-CO-) of 1.30 D (Table III), in reasonable agreement with a U H F value of 1.86 D.27 In this case, both measures of charge transfer (dipole moment and Mulliken populations) are consistent with each other. The lack of charge transfer in the open-shell lZ+and 'Z+ states results in small, negative dipole moments (Pd%O+) of -0.79 and -0.55 D, respectively, in the same direction as for free CO. The Pd-CO interaction in the I P state is very strong. Table V depicts the increase in bond strength with increasing inclusion of electron correlation. The best level of calculation, a GVBCI(6/12) wave function, involves a full CI within the G V B valence space of a GVB(6/12)-PP wave function and predicts a Pd-CO bond energy of 27 kcal/mol. This is in excellent agreement with previous relativistic MRCCIl2 calculations of Siegbahn

and c o - w ~ r k e r sthat ~ ~ predict a bond energy of 28 kcal/mol. Siegbahn and c o - w ~ r k e r salso ~ ~ carried out relativistic CPF calculations using a larger basis set (1 ls8p4d3f for Pd and 5s4pld for C and 0) and correlating 20 electrons to obtain a best De(Pd-CO) of 33 kcal/mol (Table VI). UHF/MP2 calculations of Rohlfing and Hay2' give De = 37 kcal/mol, in reasonable agreement with Siegbahn's result. However, as observed by Siegbahn and co-~orkers?~ this agreement is probably fortuitous, since the MP2 De for NiCO is 58 kcal/mol, compared to the best theoretical value of 33 kcal/m~l.~'Pacchioni and Koutecky% used a nonrelativistic pseudopotential for Pd and the MRD-CI method to calculate a nonrelativistic De of 8 kcal/mol, again indicating the importance of including relativistic effects for a proper description of the bonding. The heat of adsorption for atopbonded CO on Pd( 100) is -22 kcal/mol at Oco = 0.5 ML,' which'is lower than our predicted value of 27 kcal/mol. However, the measured surface binding energy is no doubt lower than our value due to coverage-dependent effects (i.e., repulsive lateral interactions between neighboring adsorbed CO molecules). The state of PdCO has a predicted w,(Pd-CO) of 428 cm-' that is much larger than those for the open-shell IZ+and 'Z+ states (22 and 28 cm-I, respectively), as expected from the relative Pd-CO bond strengths of these states (Table 111). We predict a C-O vibrational frequency of 2253 cm-I, downshifted by only 20 cm-' from our vibrational frequency in free CO. A C-O vibrational frequency of 2045-2050 cm-' was observed for PdCO isolated in a matrix,16J7 while the o,(C-0) observed for Pd(CO)(PPh,), is 1955 cm-I.I9 C-O vibrational frequencies of 2096 and 2092 cm-* have been measured2' for C O adsorbed in linear atop sites on Pd( 100) and Pd( 11 l), respectively. Since the ex-

2332 The Journal of Physical Chemistry, Vol. 95, No. 6,1991 perimental vibrational frequency for free CO is 2170 cm-I, we see that the shifts seen experimentally due to CO interacting with Pd range from 74 to 215 cm-I, significantly larger than our calculated shift of 20 cm-I. The discrepancy between the predicted and experimental shifts in w,(C-0) is probably due to basis set truncation. PtCO: We find that the ground state of PtCO is the lZ+state Cjust as in PdCO), with a u donor/* acceptor bond, in agreement with previous theoretical studies.2b29 We again examined two other states of RCO an open-shell IZ+state that correlates with ID Pt and 'Z+ state that correlates with 3D Pt at infinite Pt-C separations. Just as for PdCO, the first excited state was found to be the 'Z+ state (T,= 7.4 kcal/mol), with the open-shell lZ+ state lying higher in energy (T,= 23.1 kcal/mol). The preference for Pt to be 'D leads to a much smaller 'X+-'2+ state splitting for PtCO than for PdCO. In actuality, the 'Z+-IZ+ splitting may be a bit larger than 7.4 kcal/mol, since we overestimate the stability of the 'D state of Pt (Table I). Since we also underestimate the 'D-'D splitting in Pt by -24 kcal/mol, we expect that the true open-shell IZ+state lies much higher than 23.1 kcal/mol above the ground state. Indeed, we find that the repulsions between the partially occupied Pt 6s and CO 5u lead to an extremely weakly bound 3Z+state and a dissociative open-shell IZ+state (Table 111). Thus, Pt must be promoted from its jD ground state to the IS excited state (- 17 kcal/mol higher) in order to avoid u repulsions and form a strong bond to CO. Relativistic contraction of the valence orbitals of Pt results in similar orbital extents for both Pd and Pt, leading to geometries for the IZ+states of PtCO and PdCO that are very similar (Table 111). Our predicted Pt-C bond length (1 -99 A) agrees well with previous theory of Rohlfing and Hay (1.977 A)27and Gavezzotti et al. (1.91 A)26(Table VI). The values obtained by Basch and CohenZ8for the equilibrium Pt-C bond length are somewhat shorter (1.71-1.75 A). This is most likely due to the difference in the ECPs used for Pt. The C-0 equilibrium bond length is comparable (within -0.05 A) for all methods. A much greater discrepancy is found for the 3Z+state, where we find R,(Pt-C) = 2.99 A, while Basch and Cohen2* found R,(Pt-C) = 1.82 A. Qualitatively, however, the same trend is observed in both calculations, with '2? PtCO exhibiting a longer bond length than I Z ' PtCO. The open-shell lZ+state of PtCO is predicted to have a long Pt-CO bond and a low Pt-CO vibrational frequency, indicating that the open-shell IZ+state has a very weak Pt-CO interaction. The bonding orbitals of 'Z+PtCO are qualitatively the same as those for IZ+PdCO (Le., delocalization of metal d r orbitals and the CO 5u orbital to form a u donorln back-bond) and therefore are not depicted here. Long Pt-C bond lengths of 2.99 and 4.05 A for the 3Z+and open-shell 'X+ states, respectively, yield little delocalization of the Pt d* orbitals toward C and little CO 5u donation to the metal, just as in the open-shell IZ+and jZ+ states of PdCO. The propensity for Pt to accept u electrons from CO leads to a net transfer of 0.23 electron from C to Pt in the IX+ state (Table IV), opposite to the direction of charge transfer for PdCO. However, the overall dipole moment for the state of PtCO is positive (Pd+-CO-) and very similar to p(PdCO), which indicates d* back-bonding must dominate. In this case, the Mulliken population and dipole moment data contradict each other, in the same manner as was found for free CO. The predicted value of 1.12 D for p(PtC0) is in reasonable agreement with U H F calculations of Rohlfing and Hay that predict a dipole moment of 1.75 D (Table VI),*' but our value should be more accurate as it is derived from an MCSCF wave function. Little charge is transferred between Pt and CO in the open-shell lZ+and 3Z+ states. Again, the electron populations are what would be expected [valence Pt (-lo), C (-5.9), and 0 (-8.1)] for Pt and CO fragments (Table IV), and the open-shell I F and 3Z+ states exhibit negative dipole moments (Pt--CO+) of -0.56 and -1.68 D, respectively, in the same direction as the value for free CO. The predicted adiabatic Pt-CO bond energy of 15.4 kcal/mol (Table V) is much less than that for Pd-CO (0, = 27 kcal/mol),

Smith and Carter due to the lS-3D promotional cost for Pt and Pd does not incur, Since we obtain a ISAD splitting that is too high by 3.1 kcal/mol, our best estimate of the true Pt-CO bond energy is 18.5 kcal/mol. Notice that the intrinsic (diabatic) Pt-CO bond strength at our best level of calculation (Table V) is 35.1 kcal/mol, considerably higher than the intrinsic Pd-CO bond strength of 27.2 kcal/mol. Thus, the predicted Pt-CO bond energy of 18 kcal/mol is -8 kcal/mol higher than one might have expected if we had assumed the Pt-CO and Pd-CO bond strengths to differ only by the 'S-'D splitting in Pt. Since Pt-Pt bonding involves primarily the 6s electrons," adsorbates on a Pt surface do not feel strong u-repulsive effects due to free 6s electrons. Thus a C O molecule feels primarily an attractive d9 configuration on a Pt surface or cluster, and hence the Pt-CO bond is expected to be stronger on a surface or cluster of Pt atoms. Indeed, the heat of adsorption of CO on Pt( 11 1) is 3 1 f 1 k ~ a l / m o l .Although ~ this value may correspond to the binding energy for bridging CO, the atop CO heat of adsorption is very similar. Temperature-programmed EELS6 has shown that the energy difference between atop and bridge sites is less than 1 kcal/mol. The dominant effect that weakens the metal atomCO bond relative to the surface is the energy required to promote Pt from its 3D ground state to the state required to form the IZ+ state of PtCO (i.e., dIo Pt is needed for maximal ?r back-bonding and u donation). This promotional energy is clearly lessened by the presence of other Pt atoms in Pt metal, such that C O binds strongly to an atop site (in an analogous way with the lZ+state of PtCO). Previous theoretical calculations of the properties of lZ+PtCO have been carried out using SCF, MP2, and CI methods.2b29 UHF/MP2 calculations of Rohlfing and Hayz7yield an adiabatic De of 37 kcal/mol (Table VI); however, this method has been shown to overestimate other metal-CO bond energie~.~'Gavezzotti et a1.26found D,(Pt-CO) = 27 kcal/mol at the SCF level using a minimal basis set (Table VI); basis set superposition errors may be responsible for this large value. An SCF level calculation by B a ~ c hplaces ~ ~ D,(Pt-CO) a t 15 kcal/mol, while a small, energy-selected CI carried out by Basch and Cohen28puts 0,(Pt-CO) = 43 kcal/mol. There seems to be little agreement between the various calculations, showing the sensitivity of the result to the ECP, basis sets, and level of CI. However, our prediction of D,(Pt-CO) = 18.5 kcal/mol arises from the most highly correlated wave function used to date and thus is the most reliable value available. We find a Pt-CO bond energy of only 1 kcal/mol for 3Z+PtCO (Table 111). In fact, the 'Z+ state may be unbound, since basis set superposition effects may be at least 1 kcal/mol. Our result is in severe disagreement with Basch and Cohen,Z8who found a D,(Pt-CO) of 19 kcal/mol for this state. However, the trend in bond energies is the same in both studies: a strongly bound lZ+ state and a weakly bound 'Z' state are predicted. Although the predicted Pd-CO bond energy (27 kcal/mol) is larger than the predicted Pt-CO bond energy (18.5 kcal/mol), the Pd-C and Pt-C vibrational frequencies do not follow this trend w,(Pd-C) = 428 cm-l while w,(Pt-C) = 600 cm-I. The larger relativistic contraction of the Pt 6s orbital results in more donation from the C O 5u pair than for Pd. Significant donation of both u electrons from CO to Pt (0.41) versus Pd (0.03) and K electrons from Pt to CO (0.20, Table IV) yields a larger intrinsic or diabatic (promotionless) bond energy for Pt-CO (35 kcal/mol) than for Pd-CO (27 kcal/mol). The stronger intrinsic bond between CO and Pt is indicative of a stronger interaction near its equilibrium configuration, which will dictate trends in harmonic vibrational frequencies. Thus we expect and observe a larger w,(Pt-C) than w,(Pd-C). Our predicted C-O vibrational frequency for the I F state of PtCO (1976 cm-I) is in good agreement with w,(C-0) for Pt(C0)2(PPh3)2(1996 and 1954 cm-'),I8 with the infrared spectrum16 exhibited by PtCO isolated in an Ar matrix Iw,(C-O) = 2052 cm-I], and the HREELS spectrum4 of C O adsorbed on (44) Wang, H.; Carter, E. A., to be submitted.

The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2333

Interactions of N O and C O with Pd and Pt Atoms

TABLE VII: Calculated Properties of MNO States (M = Pd and Pt) PdNO T E"

AE~ De(M-NO)' Re(MN-O)' Re(M-NO)' B,(M-N-O)' we(MN-O)g we( M-NO)g we(M-N-0 bend)r Ph

2A'

2n

%+

2A'

-158.448 59 0.0 (17.9) 4.1 1.17 1.90 113.3 1686 292 671 -0.98

-158.44221 4.0 (0.0) 4.8 1.16 2.19 180.0 1866 99 524 0.33

-158.435 59 8.2 (26.1) . ,

-155.576 28

d

1.16 4.82 180.0 1816 14 521 -0.16

0.0 20.4 1.17 2.16 112.9 1775 226 433 -0.39

PtNO 2II

22+

-155.559 84

-155.53438 26.3 d 1.16 2.39 180.0 1858 120 523 -0.76

10.3 d 1.16 4.40 180.0 1820 203 521 -0.1s

@Totalenergy at the GVB(4/8)-PP level in hartrees. bRelative energy in kcal/mol at the GVB(4/8)-PP level. Values in parentheses are corrected for the error in the 3D-'S Pd splitting (see text). 'Adiabatic Pd-NO bond dissociation energy in kcal/mol. dunbound with respect to ground-state fragments at all levels of CI attempted. 'Equilibrium bond length in angstroms. /Equilibrium bond angle in degrees. #Vibrational frequency in cm-I. *Magnitude of dipole moment vector in debye. A positive sign indicates the negative end of the dipole points toward the oxygen atom.

Pt(ll1) [w,(C-0) = 2100 cm-I]. S C F calculations by B a ~ c h ~ ~ Another possibility for metal-NO bonding is a combination of a covalent a bond and u-donor bond (Figure la). This 2Z+ on PtCO predicted w,(C-O) = 2157 cm-I, higher than any of the experimental observations (Table VI). An HREELS loss peak at 470 cm-l was assigned to the Pt-C stretch for a high coverage, linear atop state: a bit lower than our predicted value of 600 cm-' and Basch's prediction of 527 cm-l (Table IV).29 This may again be due to repulsive lateral interactions between adsorbed C O molecules. In summary, we have found that C O will bind strongly to Pd and Pt only if the metal valence s orbital is empty. This will allow significant CO 5u donation, which will be accompanied by T back-bonding from occupied metal d* orbitals to the CO 2 ~ * orbitals. These bonding considerations favor Pd, which has a dIo ground state, over ground state, sld9 Pt, since the partially filled Pt 6s orbital will result in u repulsions. The intrinsic ]E+Pt-CO bond is found to be 8 kcal/mol stronger than in IE+ Pd-CO, due to greater C O 5u donation and Pt d r back-bonding. However, the IS-)D promotional cost of 17 kcal/mol required for Pt to form a strong bond to C O results in a smaller adiabatic bond energy for IZ+Pt-CO. On surfaces, adsorbate-adsorbate interactions and metal-metal interactions lead to observed CO-surface binding energies that are different from those for atomic M-CO bonding. Atop-bonded C O on Pd(100) is observed to have a binding energy of -22 kcal/mol at 0,- = 0.5 ML,) while we predict a Pd-CO binding energy of 27 kcal/mol for isolated atop-bonded CO. Thus, lateral interactions between neighboring C O molecules appear to be repulsive by - 5 kcal/mol. Atop C o o n Pt(ll1) is bound by -30 kcal/mol for Bc0 I 0.5 ML,6v7 while we estimate a D,(Pt-CO) of 18.5 kcal/mol. It may be that surface Pt atoms do not require electronic promotion to IS-like states, due to strong metal-metal interactions involving the 6s electrons.u Our calculations on PdCO and PtCO offer an array of new predictions as well as opportunities to compare with previous theory. Analytic gradients of GVB-PP wave functions have been used for the first time to fully optimize the geometries and predict harmonic vibrational frequencies for three states of PdCO and PtCO. Post-HF (GVB-PP) dipole moments for all three states of PdCO and PtCO have also been predicted for the first time. Our prediction of the dissociation energy for 'E+PdCO confirms previous relativistic all-electron calculations of Siegbahn and c o - w o r k e r ~ indicating ,~~ the accuracy of our method, while our R-CO bond energy of 18.5 kcal/mol represents the most accurate value predicted to date. Finally, we also suggest that the open-shell IZ+and states of PdCO and PtCO are probably all dissociative states. C. Metal-NO Interactions. There are several ways for transition metals to form bonds to NO. To form a strong covalent metal-NO u bond, a singly occupied metal u orbital is essential (e.g., we will require 'D Pt or Pd). Since the unpaired electron on N O is in a 27r* orbital, this (2A') covalently bound state will be bent in order to maximize orbital overlap within the u bond (Figure lb). With no strong repulsive interactions present in this 2A' state, we expect it to be strongly bound.

-

state requires a singly occupied metal d a orbital and an empty metal u orbital for donation from the N 2s lone pair. Maximal u- and a-bond overlap is obtained with a linear geometry. Neither low-lying state of Pt or Pd (ISor 'D) satisfies both criteria for strong bonding in the 2Z+state: IS will allow a-donation to be effective with no covalent *-bonding, while )D will allow *-bond formation, but now u donation is inhibited by the lack of an empty u orbital. Thus, we expect the 2X+ state to be weakly bound. It is also possible that N O may bond to a metal in a u donor 17 acceptor fashion similar to MCO bonding (Figure IC). This ll state requires an empty metal u orbital for donation from the N 2s and doubly occupied metal d a orbitals for a back-bonding into the partially occupied N O 2a* (i.e., here we require IS Pt or Pd). A linear geometry is preferred for *IIM N O as for MCO, again to maximize r back-bonding and u donation. Therefore, different electronic states of Pd and Pt will lead to different preferred geometries: while dIo (IS)Pd or Pt will favor a linear configuration of MNO, s'd9 ()D) Pd or Pt should exhibit both linear and bent structures of MNO. PdNO Pd has a IS (dlO) ground state, with its first excited )D (s'd9) state 21.9 kcal/mol higher in energy,)' which suggests that zII PdNO should be preferred over the 2Af and 2Z+states that both require promotion to the 'D excited state. As mentioned in section III.A, the HF method has difficulty predicting the correct ground state for Pd. By increasing the level of electron correlation in our calculations, we were able to predict the C O K ~ C ~ ground state for Pd using the same basis set. Unfortunately, the splitting predicted at this level [)D (HF*SD(open-shell s, d))-'S (RCI(1/2)*SD(d lone pair))]36 is only 1 kcal/mol. Since the discrepancy between experiment and theory is so large, we have used the experimental state splitting wherever this value is needed. All three states examined lie close in energy a t the GVB(4/ 8)-PP level: the bent 2A' state is lowest, the linear 211 state is the first excited state ( T , = 4.0 kcal/mol), and the linear 2Z+ state is the second excited state ( T , = 8.2 kcal/mol, Table VII). However, the 2Afand 2Z+states should have been higher in energy by the 'D-IS promotional cost of 21.9 kcal/mol. Thus, a better estimate for the state splittings would place zII as the ground state, with the ZAfand 2E+states 17.9 and 26.1 kcal/mol higher, respectively. All states of PdNO examined had similar N - O equilibrium bond lengths (2A' R,(N-O) = 1.17 A, 211and R,(N-O) = 1.16 A; Table VII). The Pd-N equilibrium bond lengths reflect the intrinsic or diabatic (promotionless bond strengths of the different states (2A' R,(Pd-N) = 1.90 , 211 R,(Pd-N) = 2.19 A and 2Z+R,(Pd-N) = 4.82 A). As expected, the 211 and 2Z+ states of PdNO are linear, while the ?A' state is bent (0, 1 1 3 O ) . Qualitative features of the bonding in the three low-lying states examined are quite different from one another. The 2A' state exhibits a covalent bond comprised of the N O 2a* orbital (localized mostly on N ) and a Pd du orbital, with some delocalization of the N 2s orbital toward Pd. This bond is strong

I

-

A

-

2334 The Journal of Physical Chemistry, Vol. 95, No. 6, 1991

Smith and Carter

TABLE VIII: Electron Distributions in MNO for M = Pd and Pt within M-N bondu state M N 0 u donationb 2A' PdNO 1 .oo 0.85 0.15 2Z+ PdNO 1.oo 0.79 0.2 1 211 PdNO 0.03 2A' PtNO 0.97 0.87 0.16 2Z+ PtNO 1 .oo 0.79 0.21 211 PtNO 0.06 'A' PdNO+ 1.05 0.69 0.26 'Z+PdNO+ 1.06 0.68 0.26 ,II PdNO+ 0.08 'A' PtNO+ I .06 0.67 0.27 'Z+PtNO+ 1.05 0.69 0.26 ,II PtNO+ 0.12 211 (NO)d 0.79 0.21

0.06 0.09 0.02 0.02

total N 7.01 6.94 6.95 6.95 6.95 6.94 6.27 7.00 6.98 6.89 6.93 6.94 6.94

M

r back-bondinp

9.94 10.01 9.97 10.02 9.97 10.01 9.12 9.09 9.07 9.20 9.16 9.12

0 8.04 8.06 8.07 8.04 8.07 8.06 7.91 7.9 1 7.95 7.90 7.9 1 7.94 8.06

OTable IV,footnote a. bTotal electron population donated to in 4 0 5 u - c A ~ dmolecular orbital. Total electron population donated to 4 and 0 from M dr-derived orbitals. dOccupaiion of 2r* orbital on NO.

enough to polarize the diffuse, singly occupied metal valence s orbital out of the way of the Pd-N bond. Figure 3 depicts the bonding orbitals for 2A' PtNO, which are qualitatively the same as those for 2A' PdNO. Notice that the NO 2 r * character is retained in the metal-N bond pair (Figure 3a) and that the N 2s orbital is slightly distorted toward the metal center for additional bonding (Figure 3b). The NO u and r bonds remain unperturbed in the MNO complex (Figure 3c,d). Figure 3e depicts the metal valence s orbital that shuns the region near the metal-N bond. The 211 state exhibits r back-donation from Pd to NO via delocalization of the Pd d r orbitals toward the N atom, with concomitant u donation from N to Pd via delocalization of the N 2s orbital toward Pd. The bonding orbitals of this state are qualitatively the same as those for '2' PdCO (Figure 2) and thus are not shown. We find that donation by the N 2s orbital and Pd dr-NO 2 r * overlap are minimal in the 22+state, due to the diffuse Pd 5s orbital causing u repulsions between Pd and NO, leading to a purely repulsive (uninteresting) interaction. The valence electron distribution for 2A' PdNO shows transfer of only 0.06 electron from Pd to NO (Table VIII), indicating a truly covalent interaction. Formation of the Pd-N bond results in polarization of the NO 2 r * orbital toward the N atom (0.85 electron on N versus 0.79 electron on N in free NO; Table VIII), in order to increase the overlap (SwN= 0.53) in the Pd-N bond. This polarization of the NO 2r* orbital, along with delocalization of the N 2s orbital toward Pd, leads to a negative dipole moment of -0.98 D for the 2A' state (Table VII). The electron distribution in the linear 211state shows little u donation (0.03 electron) and very little r back-bonding (0.06electron, Table VIII), suggesting only weak interactions for this state, even though it has a reasonable Pd-N equilibrium bond length. The positive dipole moment of 0.33 D for the 211state is consistent with the Mulliken populations, which indicate that d r back-bonding dominates the charge transfer. Lack of charge transfer (or any strong interaction, for that matter) in the 22+state results in a very small, negative dipole moment of -0.16 D, in the same direction as found for free NO. We find that NO binds weakly to Pd atom no matter which bonding mechanism or state is examined. Although a large intrinsic bond energy of 26.0 kcal/mol is predicted for the covalently bound 2A' state at our best level of theory (Table IX), the atomic promotional cost of 21.9 kcal/mol reduces the adiabatic bond energy to only 4.1 kcal/mol. The u donor/r acceptor bond in the 211state of PdNO is predicted to be only 4.8 kcal/mol strong at our highest level of CI (Table X), even though NO is bonding to the ground state of Pd. Indeed, test calculations (vide supra) indicate that core polarization effects will reduce these bond energies by at least 6 kcal/mol, leading to the prediction that *A' and 211PdNO are probably unbound with respect to ground-state Pd and NO. Both linear states (211 and 2Z+) fail to form strong bonds to NO because of repulsions between the N 2s and Pd 5s and 4da electrons. These repulsions are largest for the diffuse Pd 5s,

-.o w l

I

I

I

I

I

t

ONEl

I

I

I

I

1

1

Figure 3. GVB(4/8)PP bonding orbitals for ZA' PtNO: (a) covalent Pt-N bond; (b) N 2s lone pair; (c) NO u bond; (d) NO r bond; (e) singly occupied Pt 6s orbital. Contours range from -0.5 to 0.5 au at intervals of 0.04 au except for the Pt 6s orbital (0.01 au).

leading to a dissociative 22+state. Furthermore, the greater electronegativity of N versus C leads to poor u donation (0.03 electron), and the partially occupied NO 2 r * orbital results in poor r back-bonding (0.09 electron) for 211 PdNO versus '2+ PdCO (Tables IV and VIII), leading to a much weaker donor/ acceptor bond for PdNO. Comparison of our predictions to NO adsorbed on Pd surfaces is complicated by coverage-dependent effects. TPD results of Conrad et a1.* and Jorgensen et al.9 indicate that NO. is bound to Pd( 1 1 1) and Pd( 100) by 17 kcal/mol at high coverages, -24 kcal/mol at intermediate coverages on Pd(100), and -32 kcal/mol at low coverages. The NOsurface bond energy most relevant for our purposes is not 32 kcal/mol, which corresponds to bridging NO, nor 17 kcal/mol, which corresponds to high

-

The Journal of Physical Chemistry. Vol. 95, No. 6, 1991 2335

Interactions of N O and CO with Pd and Pt Atoms

TABLE IX: 'A' MNO Bond Energies for M = Pd and Pto 2A' PdNO HF/-158.37008 (1/1) GVB(4/8)PP/ -1 58.448 59 (16/16) RCI(4/8)/ -158.48098 (811354) GVBC1(4/81/ -158.489 39 (603j2iooj RCISDdI -1 58.496 46 (345oj14356) RCIS'/-I 58.589 72 (5379/32962) RCISDSh/ -158.591 75 (8340/45130) RCISDSGVB'/ -158.596 75 (8745/46425)

TE, hartrecs 2AfPtNO HF/-155.46976 ( 1 / 1 ) GVB(4/8)PP/ -155.576 28 (16/16) RCI(4/8)/ -155.610 10 (811354) GVBCI(4/8)/ -155.61887 (603/2100) RCISDd/ -155.623 41 (3450/ 14356) RCIg/-155.707 05 (5379/32962) RCISDSh/ -155.70887 (8340/45130) RCISDSGVB'I -155.714 13 (8745/46425)

211NO HFp29.267 01 (1 /1) GVB(3/6)PP/ -129.329 58 (8/8)

D,d'lb(Pd-NO)b D,'d(Pd-NO)* -6.7' C 8.9' C

D,(Pt-NO)* -22.7"' 4.9"'

10.2'

C

8.2"'

7.8'

C

9.w

RCISP/ -129.366 57 (63/144)

11.6'

C

1 1.2"'

RCISc/-129.436 96 (327/ 1211)

24.5'

2.6

18.9"

R C W / -129.43696 (327/1211)

25.7'

3.8

20.0"

RCISGVB'I -129.441 61 (365/1287)

26.0'

4.1

20.4"

RCI(3/6)/ -129.35809 (27/76) GVBC1(3/6)/ -129.365 60 (77/188)

'References 36 and 43. bThe diabatic bond dissociation energy DFb(Pd-N0) and D,(Pt-NO) are the bond dissociation energies to 211NO and )D M (kcal/mol). The adiabatic bond energy D,"(Pd-NO) allows Pd to relax in a spin-forbidden transition to its 'S ground state (kcal/mol). The experimental Pd splitting was used.)* tunbound with respect to ground-state fragments. dRCISD = RCI(M-N u, NO u, NOT, 3/6)*SD(M-N a) + RCI(4/8). ) RCI(3/6). 'RCIS = RCI(M-N u, NO u, NO A, 3/6)*SVaI+ RCI(4/8). 8RCIS RCI(N0 u, NO r, #RCISP = RCI(N0 u, NO r, 2/4)*S(N 2 p ~ + 2/4)*SV,1+ RCI(3/6). 'RCISDS RCI(M-N a, NO U, NO A, 3/6)*[SD(M-N U) + SVaJ+ RCI(4/8). 'RCISDSGVB = RCI(M-N U, NO U, NO r, 3/6)*[SD(M-N a) + Sval]+ GVBC1(4/8). 'RCISGVB = RCI(N0 u, NO K , 2/4)*s,,, + GVBCI(3/6). 'Dissociates diabatically to HF )D Pd (total energy = -29.1 11 38 hartrees). 'Dissociates diabatically to HF*SVaI)D Pd (total energy = -29.1 13 37 hartrees). "'Dissociates to HF )D Pt (total energy = -26.23892 hartrees). "Dissociates to HF*SVaI'D Pt (total energy = -26.24004 hartrees).

TABLE X 'I PdNO Bond Energies' ~~

211 PdNO HF/-158.379 10 (1/1) GVB(4/8)PP/ -1 58.44221 (16/16) RC1(4/8)/ -158.47340 (81/354) GVBC1(4/8)/ -158.476 52 (331/1076) RCISC/ -158.564 19 (3048/20398) RCISW/ -158.51446 (9856/54589) RCISDGVB'/ -158.515 16 (10060/55217)

TE, hartrees 211 N O HF/-I 29.267 01 (1 / 1) GVB(2/4)PP/ -129.324 53 (4/4) RCI(2/4)/ -129.34942 (9/17) GVBCI(2/4)/ -129.34990 (11/19) RCISd/-l 29.403 29 (122/323) R C I S P / -129.37924 (261/648) RCISDGVB'/ -129.37964 (263/650)

~

IS Pd HF/-29.106 60 ( 1 / I ) GVB(2/4)PP/ -29.1 14 13 (4/4) RCI(2/4)/ -29.121 16 (9/10) GVBCI(2/4)/ -29.121 23 (11/12) RCISc/ -29.15736 (168/297) RCISDh/ -29.12793 (253/308) RCISDk/ -29.12793 (253/308)

D,(Pd-NO)* 3.4 2.2 1.7 3.4 2.2 4.6 4.8

'References 36 and 43. bThe bond dissociation energy 0,is the dissociation energy to 211N O and IS Pd in kcal/mol. 'RCIS = RCI(Pd d,, Pd d,,, N O r, 3/6)*S,,1 + RCI(4/8). dRCIS = R C I ( N 0 r, 1/2)*S,,, + RCI(2/4). 'RCIS = RCI(d,,, d,,, 2/4)*SvaI. 'RCISD = RCI(d,,, dyz,N O x , 3/6)*[SD(d,,) + SD(d,,) + SD(N 2s)] + RCI(4/8). ERCISD = R C I ( N 0 A, 1/2)*SD(N 2s) RCI(2/4). *RCISD = RCI(d,, d,,, 2/4)*[SD(dJ + SD(d,,)]. 'RCISDGVB = RCI(d,, d,,, N O A, 3/6)*[SD(d,,) SD(d z ) + SD(N 2s)l GVBCI(4/8). 'RCISD = R C I ( N 0 r, 1/2)*SD(N 2s) GVBCI(2/4). 'RCISD = RCI(d,, d,,, 2/4)*[SD(d,) + SD(d,,)f.

+

coverages where lateral interactions perturb the bond strength. Rather, atop-bonded N O at lower coverages is the state closest to our complex. This state of N O on Pd(100) is bound by 24 kcal/mol, which is similar to the intrinsic bond strength of the bent 2A' PdNO (26 kcal/mol). However, HREELS data9J0are inconclusive as to whether atop N O on Pd( 100) is linear, bent, or both, so it may be that -24-26 kcal/mol is the intrinsic bond strength for both bent and linear atop N O on Pd. Indeed, the bond strengths of bent and atop N O are the same (19 f 2 kcal/mol) for N O on Pt(l1 l).12 The dominant effect that weakens the metal atom-NO bond relative to the surface is the energy required to promote Pd from its dIo ground state to the s1d9state required for forming the bent 2Afstate of PdNO. This promotional energy is clearly lessened in Pd metal, such that NO might bind strongly to a bent atop site (in an analogous way to 2A' PdNO). In particular, the electronic configuration of Pd in the bulk has substantial 5s 0ccupation,4~ which provides one explanation of why N O binds more strongly to the bulk metal (no promotional cost is incurred, since some Pd atoms are s'd9 already). Another possibility is that linear atop sites are stabilized on the surface by delocalization of partially occupied valence s orbitals via metal-metal bonding,44 such that u repulsions between the metal and N O are reduced and then N O can bind strongly to a linear atop site (in an analogous way to zZ+PdNO). The Pd-N stretching frequencies (Table VII) reflect the intrinsic bond strengths of the three states (2A' w,(Pd-N) = 292 cm-I, 211 o,(Pd-N) = 99 cm-', and 2Z+ w,(Pd-N) = 14 cm-I). (45) Louie, S. G. Phys. Rev. Len. 1978, 40, 1525.

+

+

-

+

The N - O stretching frequency for ZA' PdNO (w,(N-O) = 1686 cm-l) is 150 cm-I lower than that for 211 PdNO (wJN-0) = 1866 cm-I) and 2Z+ PdNO (w,(N-0) = 1816 cm-I), which are close to the free N O frequency (18 13 cm-' theoretically and 1904 cm-I experimentally; Table 11). The lowering of o,(N-O) in the 2A' state indicates that the N-0 bond has been converted to a double bond by virtue of interaction with the Pd atom. Jorgensen et aL9 and Nyberg and Uvdal'O observed N - 0 stretching frequencies for atop-bonded N O on Pd(100) in the 1678-1750-cm-I range, close to the predicted values for the (zA') bent state and the (2Z+) linear state of PdNO, again suggesting that both bent and linear atop N O may coexist on certain Pd surfaces. HREELS losses corresponding to Pd-N stretches are observed in the 202331-cm-l range,1° in excellent agreement with our predicted Pd-N stretching frequency of 292 cm-I. Since we predict a Pd-N-0 bend at 671 cm-I, a bending mode on Pd should be observed -600 cm-I, similar to that seen on Pt( 11 1) (vide infra). Nyberg and UvdalIo do not see such a mode for N O on Pd( 100); however, we suggest that bent N O may form on Pd( 11 1) precovered with oxygen, as in the analogous experiments on Pt( 111).l2 Indeed, HREELS studies for N O adsorption on sulfur-precovered Pd(100) suggest that bent N O is formed under such condition^.^ Linear and bent N O transition-metal complexes are quite common. N - 0 stretching frequencies21*22for the complexes (C5Ph5)PdN0, (C&-t~lyl)~)PdNO,(C5Ph3Et2)PdN0,and (C5H5)PdN0range from 1755 to 1789 cm-I, also in reasonable agreement (within -5%) of our predicted frequencies for the bent (2A') and linear (2Z+) states. PtNO Pt has a 3Dground state, which suggests that linear 2For bent 2A' PtNO should be preferred over linear 211 PtNO. Indeed, we find the 2A' state of PtNO to be the ground state, where

2336 The Journal of Physical Chemistry, Vol. 95, No. 6. 1991

Smith and Carter

TABLE XI. Calculated Properties of MNO+ States for M = Pd and Pt PdNO+ TE" AE~ De(M-NO)' Re(MN-O)d R,(M-NO)d B,(M-N-O)' w,(MN-OY

we(^-^^)'

w,(M-N-O bend)' Pt

1 A'

3n

-158.247 39 0.0 38.8 1.14 1.99 118.4 1866 722 329 -1.65

-158.227 57 12.4 14.6 1.14 2.28 180.0 21 I9 22 1 523 -1.65

PtNO+

IZ+ -158.22669 13.0 20.7 1.14 2.15 180.0 1943 223 524 -2.12

1A'

3l-I

lZ+

-155.329 13 0.0 31.0 1.14 2.20 117.1 1865 632 317 -1.93

-155.309 17 12.5 11.2 1.15 2.43 180.0 2101 199 523 -1.74

-155.307 82 13.4 15.4 1.14 2.33 180.0 201 3 189 523 -2.18

"Total energy in hartrees at the GVB(4/8)-PP level for the singlet states and at the GVB(3/6)-PP level for the triplet state. bRelative energy in kcal/mol at the same levels as in a. CThebond dissociation energy ( D e ) is the energy to dissociate to 211 N O and 2D Pd+ or Pt+ in kcal/mol. dEquilibrium bond length in angstroms. eEquilibrium bond angle in degrees. YVibrational frequency in cm-I. #Magnitude of dipole moment vector in debye (with the metal ion at the origin). A positive sign indicates the negative end of the dipole points toward the oxygen atom.

a strong Pt-N u bond is formed between a du orbital on Pt and the N O 2 ~ orbital. * In the 2Z+ state, a A bond tries to form between a Pt d r orbital and the N O 2 ~ orbital. * However, this bond is apparently not strong enough to compensate for repulsions caused by the singly occupied 6s orbital on Pt, resulting in a dissociative state. The 211 state again would be formed through a u donor/* back-bonding mechanism, but we find that the ISAD promotional cost cancels any intrinsic bonding for 211 PtNO. In terms of total energies at the GVB(4/8)-PP level, the first excited state is the 2Z+state (T,= 10.3 kcal/mol) and the second excited state is the 211state (T,= 26.3 kcal/mol, Table VII). The ordering of these states tracks their ability to form bonds to the ground state of Pt. Since we make an error of only 5.6 kcal/mol in the S 3 Dsplitting in Pt at the GVB-PP level ( e v e = 22.2 kcal/mol versus AEEXP= 16.6 kcal/mol), we expect the electronic state spectrum for PtNO to be close to these predicted values, with perhaps the T, for 211 PtNO reduced to 20.7 kcal/mol. The 2A' state of PtNO has an equilibrium bond angle of 112.9', essentially the same as that predicted for the 2A' state of PdNO (Table VII). The equilibrium Pt-N bond length is predicted to be slightly longer [R,(Pt-N) = 2.16 A] than that calculated for 2A' PdNO [R,(Pd-N) = 1.90 A]. Pt 6s-N 2s repulsions lead to a very long Pt-N bond length [R,(Pt-N) = 4.40 A] for the 2Z+ state, with essentially no overlap between the Pt d r orbital and the NO 2a* orbital. Although our calculations predict a reasonable equilibrium bond length for the 211 state [R,(Pt-N) = 2.39 A], the predicted Pt-NO vibrational frequency is quite low [w,(Pt-N) = 120 cm-I; Table VII], indicative of a very weak Pt-N bond. The qualitative features of the bonds formed between Pt and N O are essentially identical with the PdNO states. The bonding orbitals of 2A' PtNO are shown in Figure 3, where Figure 3a depicts the covalent bond between the N O 2u* orbital and a Pt du orbital, Figure 3b shows some delocalization of the N 2s lone pair toward Pt, Figure 3c,d shows the rather unperturbed N O u and A bonds, and Figure 3e shows the Pt 6s singly occupied orbital. The bonding orbitals of 211 PtNO are qualitatively the same as those for 211 PdNO and 'Z+PdCO and therefore are not shown. Finally, just as for 2Z+PdNO, u donation by the N 2s orbital and Pt dr-NO 27r* overlap are minimal in the 2Z+state, due to the diffuse Pt 6s orbital causing u repulsions between Pt and NO. The electron distributions for the three states of PtNO examined show little net charge transfer between Pt and NO. The electron populations are what would be expected [valence Pt ( lo), N (-6.91, and 0(-8.1)] for Pt and N O fragments (Table VIII). The lack of significant charge transfer for the 2A', 211, and 22+ states results in small negative dipole moments (Pt--NO+) of -0.39, -0.76, and -0.1 5 D, respectively, in the same direction as for free N O (Table VII). Of three likely candidates for the ground state, only the bent 2A' state is predicted to be bound. Table IX shows that our highest level of CI, RCISDGVB, predicts a strong Pt-N bond of 20.4 kcal/mol, consistent with the high orbital overlap (SPN= 0.47) in the Pt-N bond. Note that the orbital overlaps track the intrinsic

-

covalent metal-N bond strength, with the higher overlap in the Pd-N bond (SwN= 0.53) giving rise to a stronger intrinsic bond strength of 26.1 kcal/mol, even though the adiabatic Pd-N bond strength is so small (vide supra). Although only the bent NO state is stable for Pt atom, we expect that a linear, atop-bonded, 2Z+-like state should compete with the bent state on a Pt surface, since Pt-Pt bonding interactions will polarize the valence s-band orbitals away from Pt-NO bonds." TPD ~ t u d i e s ' ~ reveal J ~ J ~ N O desorption activation energies of 14, 19 f 2, and 25 kcal/mol, for the high-coverage linear atop state,14the bent surface state,'* and the bridging N O ~ t a t e , ~ ~ , ' ~ respectively. The value of 19 f 2 kcal/mol for the bent NO binding energy is in excellent agreement with the predicted Pt-N bond energy in 2A' PtNO. Our predicted vibrational frequency of 1775 cm-' for the N-O stretch of the 2A' state is in close agreement with N-O vibrational frequencies from HREELS observed by Pirug et and Gland and c e w o r k e r ~ ' ~forJ ~N~O adsorbed on several singlecrystal faces of Pt. For high surface coverages, the losses appear at 1790 (Pt(100)),13b1760 (Pt(l10)),'5band 171Ocm-' (Pt(11l)).I4 These workers did not identify any low-frequency bending modes, and thus the above frequencies may be associated with linear atop species. Bartram et a1.12 studied the effect of coadsorbed oxygen atoms on the bonding of N O to Pt( 111). When the Pt( 11 1) surface was precovered with 0.75 ML of O(ad), they observed a new low-frequency mode assigned to adsorbed bent NO. The vibrational frequencies from HREELS for this state are 1775 (N-O stretching mode), 265 cm-' (Pt-N stretching mode), and 510 cm-' (Pt-N-O bending mode), in excellent agreement with our predicted values of 1775,226, and 433 cm-l, respectively, for the bent 2A' state of PtNO. PdNoC and PtNoC: We have seen that occupation of the metal valence s orbital inhibits bonding in the linear states of MNO. To stabilize these linear states, then, we must ionize the metal center. We have therefore examined the interaction of N O with Pd+ and Pt+,since these ions in their ground 2D states have only d valence electrons. We find that the ground states of PdNO+ and PtNO+ are still bent, as in the neutral complexes. These bent 'A' states from even stronger covalent bonds between the metal du orbital and the N O 2n* orbital than the neutrals do. The other two states studied were the linear 'Z+ state that bonds via a covalent A bond and u-donor bond and the linear 311 state that bonds via a u donor/* back-bonding mechanism. Both lZ+and states are strongly bound, in contrast to the corresponding neutral complexes. The 311 and IZ+states for both PdNO+ and PtNO+ are nearly degenerate excited states about 13 kcal/mol above the bent 'A' ground states (Table XI). The equilibrium bond angles for 'A' PdNO+ and PtNO+ are -5' larger than those for 2A' PdNO and PtNO in order to increase N 2s donation to the metal ion (Tables VI1 and XI). The 311 and states have linear geometries in order to maximize T overlap, just as for 211 and 2Z+ MNO. All three states for PdNO+ and PtNO+ have essentially the same predicted N - O equilibrium bond length of -1.14 A (Table XI). The metal-N

The Journal of Physical Chemistry, Vol. 95, No. 6, 1991 2337

Interactions of NO and CO with Pd and Pt Atoms

TABLE MI: 'Af MNO+ Bond Enemies for M = Pd+ and Pt+" TE, hartrees 'Af PdNO+ 'A' PtNO+ HF/-I 58.103 65 (1 / I ) HF/-155.185 84 (1 / I ) GVB(4/8)PP/ -1 58.247 39 ( I 6/ 16) GVB(4/8)PP/ -155.329 13 (l6/16) RC1(4/8)/ -158.277 19 (81/150) RCI(4/8)/ -155.35890 (81/150) GVBCI(4/8)/ -158.285 36 (603/924) GVBCI(4/8)/ -155.367 62 (603/924) RCISD'/ -158.286 61 (3459/6188) RCISDc/ -155.368 26 (3459/6188) RCIS'/ -1 58.377 38 (4776/ 12230) RCIS'/ -155.453 01 (4776/12230) RCISDSg/ -1 58.378 61 (7746/17504) RCISDSg/ -1 55.454 02 (7746/17504)

211 NO HF/-129.267 01 (1 / I ) GVB(3/6)PP/ -129.329 58 (8/8) RCI(3/6)/ -1 29.358 09 (27/76) GVBCI(3/6)/ -129.365 60 (77/188) RCISPd/ -129.366 57 (63/144) -129.43696 (327/1211) -129.43696 (327/1211)

De(Pd+-NO)' -25.5' 25.4' 26.2'' 26.6' 26.8' 38.0' 38.8'

De(Pt+-NO)' -25.41 25.2) 26.01 26.81 26.51 30.3k 3 1.Ok

OReferenceS 36 and 43. 'Table XI, footnote c. 'RCISD = RCI(N0 u, NO 'R, M-N, 3/6)*SD(M-N) + RCI(4/8). dRCISP = RCI(N0 u, NO r , 2/4)*S(N 2pr) + RC1(3/6). 'RCIS = RCI(N0 U, NO r , M-N, 3/6)*S,I + RCI(4/8). fRCIS =Z RCI(N0 u, NO r , 2/4)*S,1+ RCI(3/6). A'RCISDS = RCI(N0 u, NO 'R, M-N, 3/6)*[SVaI+ SD(M-N)] + RCI(4/8). 'Dissociates to HF 2D Pd+ (total energy = -28.87735 hartrees). 'Dissociates to HF*& 2D Pd+ (total energy = -28.879 83 hartrees). 'Dissociates to H F 2D Pt+ (total energy = -25.959 33 hartrees). Dissociates to HF*SVaI2D Pt+ (total energy = -25.967 70 hartrees). Pd*

Pd"N\o ONE1

I

I

I

I

*

.--. '.

ONE

I

I

1

I

I

I

I

I

I

.-- -

-N-0

ONE

ONE

.

ONE

-.

I

I

ONE I

I

I

I

I

I

I

1

I

I

I

Figure 4. GVB(4/8)PP bonding orbitals for 'A' PdNO':

(a) covalent Pd-N bond; (b) N 2s lone pair; (c) N O u bond; (d) N O a bond. Contours range from -0.5 to 0.5 au at intervals at 0.04 au.

equilibrium bond lengths reflect the relative bond strengths (vide infra) of the three states (for Pd, 'A' R,(Pd-N) = 1.99 A, lZ+ R,(Pd-N) = 2.15 A, and 311 R,(Pd-N) = 2.28 A; for Pt, 'A' Re(Pt-N) = 2.20 A, R,(Pt-N) = 2.33 A, and 311R,(Pt-N) = 2.43 A). H F calculations by B a ~ c hon~ l~Z+PtNO' predicted equilibrium Pt-N and N - 0 bond lengths of 1.678 and 1.13 A, respectively. While our N - O bond lengths are in agreement, the large discrepancy in the Pt-N bond length is probably an artifact of Basch's HF wave function, which often predicts artifically short bond lengths. The 'A' states of PdNO+ and PtNO+ form covalent u bonds between the metal du orbital and the 2n* orbital of NO, with significant delocalization of the N 2s orbital toward the metal (as depicted for Pd+ in Figure 4). The states form covalent a bonds between a metal dn orbital and the NO 2n* orbital (Figure 5b), concomitant with u donation from NO to the metal via delocalization of the N 2s orbital into the empty metal s orbital (Figure sa). A small increase in u charge transfer is observed in the cationic complexes (-0.05 electron, Table VIII) relative to the neutral MNO complexes, due to more effective donation by the N 2s orbital and due to increased electrophilicity of M+. The 311states bond through a u donor/n back-bonding mechanism, similar to that for 'Z+ PdCO. However, the positively charged character of the metal and the partially occupied nature of the NO 2n* orbital reduces the extent of delocalization of the metal

Figure 5. GVB(4/8)PP bonding orbitals for 'Z+ PdNO+: (a) N O 5u donor bond; (b) Pd-N covalent u bond; (c) N O u bond; (d) N O u bond. Contours range from -0.5 to 0.5 au at intervals of 0.04 au.

MCO or 211 MNO. By dn orbitals for 311M+NO relative to contrast, the high electronegativity of the metal ion enhances u donation to the metal for 311 M+NO. The valence electron distributions in all of the states of MNO+ show a net charge transfer from NO to M+ (for Pd+, 'A' 0.12 electron, lZ+0.09 electron, 311 0.07 electron; for Pt+, 'A' 0.20 electron, IZ+0.16 electron, and 3110.12 electron; Table VIII), in line with the electronegativity arguments discussed above. The substantial net donation of charge from NO to M+ in all of the states leads to large negative dipole moments (for Pd+, 'A' -1.65 D, '2' -2.12 D, and 311-1.65 D; for Pt+,'A' -1.93 D,IZ+-2.1 8 D, and 311-1.74 D; Table XI). All three of the cationic states of PdNO+ have stronger Pd-N bonds than the corresponding neutral states. The 'A' state has the strongest Pd-N bond (De(Pd-N) = 38.8 kcal/mol), with the predicted bond dissociation energy increasing dramatically with increasing electron correlation (Table XII). The lZ+and 311states are now also strongly bound, with predicted binding energies of 20.7 and 14.6 kcal/mol, respectively. Notice that the largest '2' and 311 Pd+-N bond dissociation energies are obtained for the RCI*SVaIwave function (Tables XI11 and XIV), indicating the importance of orbital shape changes (single excitations) in the description of donor/acceptor bonds. All three states of PtNO+ are also strongly bound, due to the absence of repulsions by the metal valence s electron and the increased electrophilicity of Pt+. The bent 'A' state has the

2338 The Journal of Physical Chemistry, Vol. 95, No. 6,1991 TABLE XIII: 311 MNO+ Bond Enemies for M = Pd+ and Pt+" TE, hartrees 'II PdNO+ 'II PtNO+

HF/-158.16126 (1/1) GVB(3/6)PP/ -158.227 57 (8/8) RC1(3/6)/ -158.25565 (27/126) GVBCI(3/6)/ -158.261 77 (77/300) GVBSDC/ -158.28440 (4767/12106) RCIS'/ -158.351 73 (2922/29956) RCISD'J -158.29291 (6240/42629)

HF/-155.242 69 (1/1) GVB(3/6)PP/ -155.309 17 (8/8) RC1(3/6)/ -155.33746 (27/126) GVBCI(3/6)/ -155.34368 (77/300) GVBSDc/ -155.366 13 (4767/12106) RCIS'/ -155.433 77 (2922/29956) RCISD'/ -155.37465 (6240/42629)

Smith and Carter

'II NO

De(Pd+-NO)b

D.(Pt+-NO)*

HF/-129.267 01 (1/ 1) GVB(3/6)PP/ -129.329 58 (8/8) RCI(3/6)/ -129.35809 (27/76) GVBCI(3/6)/ -129.36560 (77/188) GVB36SDd/ -129.37967 (359/652) RCIS'/ -129.448 71 (843/4430) RCI24SDs/ -129.38774 (732/2691)

10.6h 13.0h 12.7' 1 1.8' 11.3' 14.4 11.5'

10.3' 12.7' 12.6' 11.8' 10.9' 10.9" 11.2'

+

+

"References 36 and 43. bTable XI, footnote c. 'GVBSD = GVBPP(N0 u, NO r,N 2s, 3/6)*[SD(M dxz) SD(M dyz)+ SD(N 2s)l RCI(N0 c, NO , N 2 ~3/6). , dGVB36SD = GVBPP(N0 U, NO T , N ZS, 3/6)*SD(N 2s) + RCI(N0 U, NO T , N 2 ~3/6). , 'RCIS RCI(N0 U, NO T , N 2 ~3/6)*Sv,~. fRCISD = RCI(N0 u, NOT, 2/4)*[SD(M dxz)+ SD(M d,) + SD(N 2s)] + RCI(N0 u, NO r, N 2s, 3/6). rRCI24SD RCI(N0 c, NO x , 2/4)*SD(N 2s) + RCI(N0 u, NO r, N 2s, 3/6). "able XII, footnote h. 'Dissociates to HFSD = HF*[SD(M dxz)+ SD(M dyz)] *D Pd+ (total energy = -28.88679 hartrees). /Table XII, footnote i. "Table XII, footnote j. 'Dissociates to HFSD 'D Pt+ (total energy = -25.969 14 hartrees; see footnote i . "'Table XII, footnote k. T,

MNO+ Bond Energies for M = Pd+ and Pt+O TE, hartrees 'Z+ PdNO+ I Z+ PtNO+ HF/-155.149 70 (1/ 1) HF/-158.066 65 ( I / 1) GVB(4/8)PP/ -155.30782 (16/16) GVB(4/8)PP/ -158.22669 (16/16) RC1(4/8)/ -155.337 37 (81/150) RC1(4/8)/ -158.255 83 (81/150) GVBCI(4/8)/ -158.26260 (331/492) GVBCI(4/8)/ -155.344 17 (331/492) RCIS'/ -158.34985 (2763/7116) RCISC/ -155.429 19 (2763/7116) RCISD'/ -158.291 18 (7815/18101) RCISDC/ -155.37294 (7815/18101) RCISDSg/ -158.36425 (9975/23731) RCISDSt/ -155.443 31 (9975/23731)

TABLE XIV

'II NO HF/-129.267 01 (1/ 1 ) GVB(3/6)PP/ -129.329 58 (8/8) RCI(3/6)/ -129.35809 (27/76) GVBCI(3/6)/ -129.36560 (77/188) RCISd/ -129.43696 (327/1211) RCISD'/ -129.39592 (663/2379) RCISDS'/ -129.45225 (963/3514)

De(Pd+-NO)b De(Pt+-NO)b -48.8' -48.1' 12.4' 11.9' 12.8' 12.5' 12.3' 12.1' 20.7' 15.4' 11.2' 11.1' 20.2) 14.7'

"References 36 and 43. bTable XI, footnote c. 'RCIS = RCI(N0 u, NO r, M-N, 3/6)*S,,, + RC1(4/8). dRCIS = RCI(N0 u, NO r, 2/4)*s,1 + ] RCI(4/8). fRCISD RCI(N0 U, NO T , 2/4)*[SD(N 2s) + S(N 2 p ) ] RC1(3/6). 'RCISD RCI(N0 U, NO A, M-N, 3/6)*[SD(M-N) + SD(N 2 ~ ) + RC1(3/6). 'RCISDS RCI(N0 U , NO r,M-N, 3/6)*[SD(M-N) + SD(N 2s) + S,,J + RC1(4/8). 'RCISDS = RCI(N0 U, NO r, 2/4)*[SD(N 2s) S,,,]+ RCI(3/6). 'Table XII, footnote h. /Table XII, footnote i. 'Table XII, footnote j. 'Table XII, footnote k.

+ +

strongest Pt-N bond, with D,(Pt-N) = 31.0 kcal/mol. Notice that both the 'A' and IZ+states of PtNO+ are unbound at the HV level (Tables XI1 and XIV); this calls into question the utility of the H F calculations by B a ~ c on h ~I~Z+PtNO'. As is usual for covalent bonds, the largest bond dissociation energy is obtained for the most correlated wave function (RCISDS calculation; Table XII). The lZ+and jII states are bound by -15 and -11 kcal/mol, respectively. Little variation (-2.5 kcal/mol) exists in the Pt-N bond dissociation energy of 311 Pt+NO as a function of electron correlation (Table XIII), consistent with the ion-diole nature of the bonding that is reasonably well described even at the SCF level. The largest IZ+PtNO+ bond dissociation energy is obtained for the RCI*SVaIwave function, again showing how crucial single excitations are for a proper description of the bonding in this particular state (Table XIV). The 'A' state M-NO vibrational frequencies (w,(Pd-N) = 722 cm-' and w,(Pt-N) = 632 cm-I) are much larger than for the other two states (IZ+we(Pd-N) = 223 cm-' and jII w,(Pd-N) = 221 cm-I; IE+ w,(Pt-N) = 189 cm-' and 311 w,(Pt-N) = 199 cm-l; Table XI). The M-N vibrational frequencies correlate with the general trend in bond energies shown in Table XI [Le., 'A' De(M-N) > jII, IZ+D,(M-N)]. BaschZ9found Pt-N and N-0 vibrational frequencies for IZ+PtNO+ of 533 and 2167 cm-I, much larger than our values of 189 and 2013 cm-I. Again, this is probably an artifact of the HF method, which tends to predict narrow potential wells. The N - 0 vibrational frequencies are 90-250 cm-I higher than for states of neutral MNO. This, along with shorter equilibrium N-O bond lengths for the cationic states, suggests that the N-O bonds, in addition to the M-N bonds, are stronger in the cationic complexes. This is consistent with the observed decrease in occupation of the N O 2 r * orbital for the cations, leading to stronger N - 0 bonds. IV. Summary We have carried out extensive ab initio GVB/CI calculations to ascertain both qualitative and quantitative features and contrasts in the interaction of CO and NO with Pd and Pt atoms. We have shown that CO binds strongly to Pd (0, 27 kcal/mol) but more weakly to Pt (De 18 kcal/mol), where the differences in bond strengths can be understood as a combination of two effects that

-

-

-

work in opposite directions: electronic promotional costs of 17 kcal/mol weaken the Pt-CO bond, while more effective CO u donation to the Pt 6s orbital acts to strengthen the Pt-CO bond by 8 kcal/mol. Since the large promotional energy associated with exciting 'D Pt to 'S Pt is essential for bond formation to a closed-shell ligand such as CO, while ground-state Pd is already in the bonding IS state, the Pt-ligand bond is necessarily weaker than the Pd-ligand bond (because differential increases in intrinsic bond strengths are small compared to promotional energies). Our calculations of MNO and MNO+ are to our knowledge the first systematic ab initio studies of these systems. We have shown that N O bound to a single Pd or Pt metal atom or ion always prefers a bent structure, confirming the suggestion that linear states might be disfavored for PdNO and PtNO, in contrast to NiNOe30 The linear states are higher in energy because of promotional costs (e.g., for 2Z+ PdNO and 211 PtNO) and repulsive interactions with the valence s and d orbitals on the metal (present for all linear states). N O is predicted to bind either weakly or not at all to Pd, since the (preferred) bent state requires a promotional cost of 22 kcal/mol, which essentially wipes out the intrinsic bond strength of 26 kcal/mol. By contrast, we find that N O binds strongly to Pt, because it can bind to the jD ground state of Pt. N O binds stronger still to the metal cations, for three reasons: (i) it binds to the 2Dground states of both metal ions (Le., no promotional costs are incurred); (ii) strong ion-dipole interactions are present; (iii) no repulsions exist between the N O 5u and metal valence s electrons, since the valence s orbital is empty. Thus, we have shown that Pd and Pt have completely opposite affinities toward binding CO and NO, with Pd preferring CO and Pt preferring NO. This follows a general trend in which closed-shell ligands prefer low-spin metal centers, while open-shell ligands prefer open-shell metals. Finally, it is important to emphasize that different electronic states of metals necessarily lead to different molecular structures. For example, while s1d9Pd and Pt can form either bent or linear N O complexes, dIo Pd and Pt form only linear complexes. Thus, consideration of the local electronic state of the metal and the ligand provides a powerful tool for predicting the relative stabilities of metal-ligand interactions.

J . Phys. Chem. 1991, 95, 2339-2344 Acknowledgment. Support of this work was provided by the Office of Naval Research (Grant No. NO001 4-89-5-1 492) and the donors of the Petroleum Research Fund, administered by the American Chemical Society. E.A.C. also f3ratefUllY aChOWledges a National Science Foundation Presidential Young Investigator Award and a Camille and Henry Dreyfus Distinguished New

2339

Faculty Award. Registry No. Pd, 7440-05-3; F't, 7440-06-4; CO, 630-08-0; NO, 10102-43-9; pd(co), 41 772-86-5; pt(cO), 498 19-49-0; Pd(NO), 132297-45-1; pt(NO), 132297-46-2; Pd(NO)+, 132297-47-3; F't(NO)+, 97223-72-8.

Ab I nltlo Molecular Orbital Study of Boron, Aluminum, Gallium p-Hydrido-Bridged Hexahydrides Charles W. Bock,* Department of Chemistry, Philadelphia College of Textiles and Sciences, Philadelphia, Pennsylvania 191 44, and American Research Institute, Marcus Hook, Pennsylvania 19061

Mendel Trachtman, Cindy Murphy, Bob Muschert, Department of Chemistry, Philadelphia College of Textiles and Sciences, Philadelphia, Pennsylvania I91 44

and Gilbert J. Mains Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 (Received: July 2, 1990)

The structures of H2X(p-H),YHZ,X, Y = B,Al, and Ga, have been optimized at the MPZ(FULL) level by using the 6-31G** basis set for boron and aluminum and, for gallium, by using a Huzinaga's split-valence basis set augmented by polarization functions. The structures containing boron all reflected the higher Lewis acidity of boron and tended toward a tetrahedral arrangement of the hydrogen about this atom. Ga and A1 structures were almost identical, supporting the idea that the 3dI0 orbitals are ineffective in shielding the nuclear charge in Ga. The dissociation energies, i.e., into XH3 and YH3, were remarkably similar when zero point energy and correlation effects were taken into account. Harmonic vibrational frequencies were computed for all of the hexahydrides and, where possible, compared with experiment. Since the spectra of the mixed hydrides are very rich, it should be possible to study the exchange reactions between diborane, digallane, and dialane (when it is synthesized).

Introduction Diborane, H2B(p-H)2BH2ris the prototype structure for phydrido bridging in electron-deficient systems. Consequently, the structural and electronic properties of this novel dimer have been studied extensively, both experimentally and computationally.' In contrast, dialane, H2AI(p-H)2AIH2,and digallane, H2Ga(pH)2GaH2,have proven more difficult to study experimentally. A12H6+was detected vis mass spectrometry as early as 1964, whereas Ga2H,+ could not be detected under similar laboratory conditions.2 Digallane, in fact, eluded experimental preparation until recently when Downs, Goode, and Pulham3 prepared it by using LiGaH, to reduce Ga2CI2H4(dimeric monochlorogallane) and concluded they had produced a hydrogen-bridged diboranelike structure from its vibrational spectrum. Ab initio molecular orbital studies4 have been reported for both dialane and digallane, both finding a double-hydrogen-bridged structure to be the global minimum. Although no experimental frequencies are available for dialane, the agreement between the experimental frequencies and those computed from ab initio calculations for digallane is excellent and lends credence to the predicted dialane structure. Lammertsma, et al. found the dimerization energies to decrease in the order BH3, AIH3, and GaH3, although the optimizations were carried out at different levels, e.g., dialane at' the MP2( 1 ) Liebman, J. F., Greenberg. A., Williams, R. E., Eds. Advances in Boron and Boranes; VCH Publishers: New York, 1988. (2) (a) Breisacher, P.;Siegel, B. J . Am. Chem. Soc. 1964,86, 5053. (b) Breisacher, P.; Siegel, B. J . Am. Chem. Soc. 1965, 87, 4255. (3) Downs, A.J.; Goode. M. J.; Pulham, C. R. J . Am. Chem. Soc. 1989, I l l , 1936. (4) Lammertsma, K.; Leszczynski, J. J . Phys. Chem. 1990, 94, 2806.

0022-3654/91/2095-2339$02.50/0

(FULL)/6-31Go* and digallane at the RHF/3-21G* level. On the other hand, little information is currently available on the structures, frequencies, and stabilization energies of the mixed hexahydrides, GaBH,, GaAlH,, and AlBH6. In fact, gallaborane is the only mixed dimer that has been synthesized and characterized. Pulham et al.5 studied the vibrational spectrum and provided strong circumstantial evidence for a H2Ga(p-H)2BH2 structure with C, symmetry and this bridged structure was also supported by electron-scattering experiments. Barone et a1.,6 using a b initio pseudopotentials, determined that the bidendtate structures of AIBH, and GaBH, are more stable than the very symmetric tridentate structures by about 10 kcal/mol. Furthermore, the two bidentate structures appeared to be best described as (BH4)-XH2+,(X = Al, Ga). No calculations were reported for AIGaH, in either the bidentate or tridentate form. In this paper we shall compare the structures, frequencies, and binding energies for the bidentate forms of all six hexahydrides at similar levels of computation. The computed frequencies should help in the experimental identification of the mixed hexahydrides which have to date escaped synthesis. It will be especially interesting to determine the extent to which gallalane is describable as (AIH4)-GaH2+or (GaH4)-AlHZ+. Computational Methods Ab initio calculations were performed using the GAUSSIAN86 and GAUSSIAN 88 of programs on the CRAY Y(5) Pulham, C. R.; Brain, P. T.; Downs, A. J.; Rankin, D. W. H.; Robertson, H. E. J . Chem. Soc., Chem. Commun. 1990, 177. (6) Barone, V.; Minichino, C.; Lelj, F.; Nino, R. J . Compur. Chem. 1988, 9, 518.

0 1991 American Chemical Society