J. Phys. Chem. 1995, 99, 10438-10445
10438
Reactions of Rui, Rh+, Pd+, and Agf with H2, HD, and D2 Yu-Min Chen,' J. L. Elkind,' and P. B. Armentrout* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 Received: February 15, 1995: In Final F o m : April 24, 1995@
Guided ion beam mass spectrometry has been used to examine the kinetic energy dependence of reactions of the second-row transition-metal cations, Ru', Rh+, Pd+, and Ag', with molecular hydrogen and its isotopologues. By using a meter long flow tube ion source, we are able to create Ru', Rh+, Pd+, and Ag+ ions that are believed to be in their electronic ground state terms and primarily in the lowest spin-orbit levels. Corresponding state-specific reaction cross sections are obtained. Analysis of the cross section data yields 0 K bond dissociation energies of Do(Ru+-H) = 1.62 f 0.05 eV, Do(Rh+-H) = 1.67 f 0.04 eV, Do(Pd+-H) = 2.07 +c 0.04 eV, and Do(Ag+-H) = 0.41 rt 0.06 eV. This thermochemistry is compared with theoretical calculations and previous experimental measurements. Periodic trends in these bond dissociation energies and the bonding character of these metal hydride ions are discussed. Results for the HD reactions indicate that Rh+,Ru+, Pdf, and Ag+ ions react via direct mechanisms. The reaction mechanisms and reactivity differences among these ions are explained using simple molecular orbital concepts.
Introduction The study of gas-phase reactions of atomic transition-metal ions (M+) with molecular hydrogen, reaction 1,
M-
+ H, - MH+ + H
(1)
can be viewed as a useful starting point in understanding the activation of chemical bonds at a transition-metal center. Although simple, this reaction allows a detailed study of the activation of a covalent single bond, directly analogous to activation of the C-H and C-C bonds of hydrocarbons. Also, by measuring the energy threshold of reaction 1, the bond dissociation energies (BDEs) of metal hydrides can be obtained. This thermochemistry is of obvious fundamental interest and also has implications for understanding a variety of catalytic reactions involving transition-metal systems.' Previously, we have systematically investigated reaction 1 for many elemental ions,' including all first row transition metal ion^.^,^ For these latter systems, MH+ BDEs were derived from these results, and an analysis of the periodic trends in these BDEs indicates that the bonding of M+-H involves extensive 4s orbital character on the In addition, it has been shown that the reactivity and mechanism of reaction 1 can be explained on the basis of the electron configuration and spin state of the metal ions."* Reaction 1 is most facile when M+ has an empty valence 4s orbital and a pair of valence 3dn electrons. When an H? molecule approaches M+ in a perpendicular geometry (C?, symmetry), M+ donates 3dx electrons into the o*(H?) antibonding orbital and accepts electrons from the a(H2) bonding orbital into the empty 4s orbital. These interactions effectively weaken and lengthen the HZbond while simultaneously building electron density between the metal and H atoms. In contrast, occupation of the valence 4s or 3do orbitals leads to a more repulsive interaction between M+ and Hl. This kind of repulsive interaction is lessened when the Hz approaches M+ in a collinear geometry (C-, symmetry), such Present address: Department of Chemistry, MIT, Cambridge, MA 02139.
= Present address: Texas Instruments, Inc., 12201 Southwest Freeway, MS 621, Stafford, TX 77477. @Abstractpublished in Advance ACS Ahsrructs, June 1, 1995.
0022-365419512099-10438$09.0010
that the reaction occurs by a direct mechanism. These ideas can be summarized by three "rules" and reaction behavior that is characterized by distinct differences in the results for reactions with HD.9 (1) If M+ has an electron configuration with empty 4s and 3da orbitals, such as for a 3d" configuration where n < 5, the reaction is efficient and may proceed by an insertion mechanism. These processes are characterized by product branching ratios in the HD system, a(MH++D)/a(MD++H), that are near unity, consistent with the statistical behavior of a long-lived intermediate. (2) If either the 4s or 3da orbital is occupied and the M+ state is low-spin, such as for 3d" ( n > 5) or low-spin coupled 4 ~ ~ 3 d ~configurations, I-l the reaction occurs efficiently via a direct mechanism. These processes are characterized by a product branching ratio in the HD system of 3 to 4, consistent with arguments concerning conservation of angular momentum. (3) If either the 4s or 3da orbital is occupied and the M+ state is high-spin (the highest spin it can possibly have), such as for high-spin coupled 4s'3dn-' configurations, the reaction is inefficient and tends to react impulsively. These processes are characterized by a product branching ratio in the HD system that favors MD+ H by a large factor and that exhibits shifts in the thresholds for the H, and Dz systems vs the HD system. Research on reaction 1 and the analogous processes with D2 and HD for second-row transition-metal ions is less extensive than for first row transition metals. Mandich, Halle, and Beauchamp (MHB)'O have used ion beam methods to study reactions of Ru+, Rh+,and Pd+ with D2. Elkind and Armentrout (EA)'.'' have used guided ion beam techniques to study reaction 1 for most of the second-row metals, although they report only metal hydride ion BDEs and do not provide any detailed results. The only detailed experimental results for reactions of secondrow transition-metal ions with Hz, D2, and HD that have been published are those for Y+.I2 BDEs reported in these work^^.'^.'' for the late transition metals are listed in Table 1. Good agreement between the values for PdH+ are found, while the results for RhH+ from MHB and EA are in poor agreement. These values can also be compared with results from two theoretical studies".'' (Table 1). The results of Schilling, Goddard, and Beauchamp (SGB)I3are smaller than values from the more comprehensive calculations of Pettersson et al. (PBLP),I4 and the disagreement becomes larger as one moves
+
0 1995 American Chemical Society
Reactions of Ru', Rh',
J. Phys. Chem., Vol. 99, No. 26, 1995 10439
Pd+, and Ag' with Hydrogen
TABLE 1: Second-Row Transition-Metal Hydride Bond Dissociation Energies (eV) at 0 K' M+-H Ru+-H Rh+-H Pd+-H Ag+-H
MHB~ 1.74(0.13) 1.78(0.13) 1.91(0.13)
EA' 1.52(0.13) 2.00(0.13) 0.65(0.13)
this work 1.62(0.05) 1.67(0.04) 2.07(0.04) 0.41(0.06)
PBLP 1.64 1.80 2.12 0.46
TABLE 2: Electronic States of Second-Row Transition-Metal Cations+
SGBe 1.37 1.51 1.76 0.09
" Uncertainties in parentheses. Mandich, Halle, and Beauchamp, ion beam mass spectrometry, ref 10. Original 298 K values are adjusted to 0 K by subtracting 0.039 eV = 3k~T/2. Elkind and Armentrout, guided ion beam mass spectrometry, refs 7 and 11. dPettersson, Bauschlicher, and Langhoff, theoretical calculation, ref 14. e Schilling, Goddard, and Beauchamp, theoretical calculation, ref 13. from Y+ to Ag+. The experimental value of EA for PdH+ is in reasonable agreement with the results of PBLP, but differences in the values for RhI-I+ and AgH+ are outside experimental error. MHB's results are in good agreement with PBLP for RuH+ and RhH+ and worse for PdH+. It is possible that the discrepancies between the theoretical and experimental values could be the result of electronic excitation in the M+ beams used to measure these BDEs. In all three early ion beam studies, metal ions were generated in a surface ionization (SI) ion source that creates mainly ground state but also some excited state ions. The accuracy of the BDE measurements depends critically on how these excited states are handled in the interpretation of the data and whether they exhibit different reactivity than the ground state ions. In this paper, we initiate systematic studies of the reactions of second-row transition-metal ions with H2, D2, and HD. This is largely prompted by our recent development of a dc discharge flow tube (FT) ion sourcetsthat can generate intense beams of the Ru+, Rh+, Pd+, and Ag' ions under conditions that we believe produce the ions in their ground electronic state terms and primarily in the lowest spin-orbit levels. Thus, the interpretation of these results to determine MH+ BDEs have fewer complexities associated with the presence of excited state ions than the previous studies. Given the revised MH' thermochemistry derived here, we reexamine the bonding character of the second-row transition-metal hydride ions vs that of the first-row metal hydride ions. The theoretical calculations indicate that d orbital character in the M+-H bonding increases significantly from the first row to the second row, especially for R u H ~ RhH+, , and PdH+.'3.'4,'6 In addition, we examine the reactivity of these metal ions with HD for the first time. This enables us to examine whether the reactivity "rules" outlined above for the first-row metal ions can be extended to explain the reactivity and reaction mechanisms for the secondrow transition-metal ions. Among the ions studied in this paper, the ground states of Ru+, Rh+, Pd+, and Ag+ have 4d" electronic configurations with empty 5s and occupied 4do orbitals (Table 2).17 Thus, these states fall under "rule 2". If applicable to these second-row transition metals, this rule predicts that the reactions of these metal ions with dihydrogen should occur efficiently by direct mechanisms.
Experimental Section General Procedures. The guided ion beam instrument on which these experiments were performed has been described in detail p r e v i o u ~ l y . ' ~Ions ~ ' ~ are created in a flow tube source, described below. The ions are extracted from the source, accelerated, and focused into a magnetic sector momentum analyzer for mass analysis. Mass-selected ions are slowed to a desired kinetic energy and focused into an octopole ion guide that radially traps the ions.Ig The octopole passes through a
ion
state
electron energy config (eV)
Ru+ a4F
4d7
a4P a6D Rh+ a3F
4d7 5s4d6 4d8
a'D a3P Pd+ a2D
4d8 4d8 4d9
a4F
Ag+ alS a3D a'D
5s4d8 4d1° 5s4d9 5s4d9
P .
300K
4.5 3.5 2.5 1.5 (avg) (avg) 4.0 3.0 2.0 2.0 (avg) 2.5 1.5 (avg) 0.0 (avg)
0.000 0.189 0.309 0.385 1.056 1.266 0.000 0.298 0.444 1.012 1.385 0.000 0.439 3.369 0,000 5.034
99.95 0.05
(avg)
5.709
population (%)< 700K 11OOK 2200K 96.23 3.36 0.34 0.07
87.80 9.58 5. According to "rule 2" in the Introduction, such ions should react with dihydrogen efficiently in a direct process. This is consistent with the experimental observations as shown most clearly by the branching ratio in the HD system, which favors formation of MH+ D by a factor of about 2 at threshold and more as the energy is increased. Very similar behavior is observed in the reactions of HD with the first-row transition-metal ions having 3d" ( n = 7 - 10) electron configuration^.^^^^^^^ analogous to the 4d" ( n = 7- 10) configurations for these second-row transitionmetal ions. The variation in the reactivities of Ru+, Rh+, and Pdf (as measured by the cross section magnitudes) can be rationalized by considering the occupation of the 4da orbital. As noted in the Introduction, the d a orbital will interact with the o(H2) bonding orbital, such that the most favorable interactions are expected when the 4do orbital is singly occupied. The ground states of Ru+, Rh', and Pd+ have 4d7, 4d8, and 4d9 electronic configurations, respectively, such that the probability of having a singly occupied 4do orbital decreases from 60% to 40% to 20%. This is roughly proportional to the decrease in reactivity observed experimentally. Because the ground state of Ag+ has a 4d"' electron configuration, the 4do orbital is always doubly occupied, helping to explain why Ag+ shows the lowest reactivity of the metals studied here. Note Added in Proof. We inadvertently omitted several relevant references to the theoretical studies of Balasubramanian and co-workers on the potential energy surfaces for RuH?', RhHr+, and PdH2+.40 These surfaces are for the evolution of M+ + H) to the MH:+ molecule as a function of the H-M-H angle but do not examine the MH+ H dissociation asymptote. The key observation in these studies is that there are no stable MH2+ species relative to the ground state M+ H? reactants except for the M'(H2) adduct complexes. Species corresponding to inserted H-M'-H intermediates are found in shallow wells or as inflection points at H-M-H angles between 90" and 120". The energies of these intermediates lie between the energies of the reactants and the MH+ H asymptotes in all
+
+
+
+
three systems. Thus, the direct nature of the reaction mechanisms observed here is consistent with these calculated potential energy surfaces. It is also worth noting that the calculations indicate distinct differences in the relative repulsiveness of surfaces corresponding to those with and without 4do orbitals occupied, although not all surfaces correlating with the ground state M+ H2 asymptotes are calculated. These differences are in accord with the relative reactivities noted above for the different metal ions.
+
Conclusion In this study, we are able to create Ru+, Rh+, Pd+, and Ag+ ions in their ground state terms by using a flow tube (FT) ion source. Thus, we are able to obtain corresponding state-specific reaction cross sections. Analyses of these data yield bond energies for the four metal hydrides and deuterides as listed in Tables 1 and 3. These values refine previous experimental determinations and agree well with theoretical results. Simple promotion energy concepts can be used to rationalize the periodic variations in these second-row transition-metal hydride ion BDEs and are consistent with theoretical calculations, which indicate that the extent of d character in these second-row MH' bonds is large. The branching ratios observed in the M+ HD reactions indicate that the ground states of Ru-, Rh-, Pd+, and Ag- react by a direct mechanism. The reactivities and mechanisms in these systems can be explained by using the molecular orbital concepts generalized from previous studies on the reactions of the first-row transition-metal ions with dihydrogen.
+
Acknowledgment. This work was supported by the National Science Foundation under Grant CHE-922 1241. References and Notes ( I ) Crabtree, R. H. Ckern. Rei,. 1985, 85. 245. (2) Armentrout, P. B. h f . Ret.. Pky.s. Chrm. 1990. 5. 115. (3) Elkind. J. L.: Armentrout. P. B. J. Phys. Ckern. 1987, 51, 2037. (4) Armentrout. P. B. lnf. Rev. Phys. Ckern. 1990, 9. 115. In Selecfive Hydrocarbon Activation: Principle.,and Progress: Davies. J. A,, Watson. P. L.. Greenberg. A,. Liebman. J. F.. Eds.: VCH: New York. 1990: p 467. In Gas Phase Inor,q" Chemistn: Russell. D. H., Ed.: Plenum: Neh York. 1989: p 1. 15) Armentrout. P. B.; Kickel, B. L. In Organornefullic lor1 Ckemisfr?: Freiser. B. S., Ed., in press. Armentrout, P. B.; Clemmer, D. E. In Energetic.\ of Orgunometallic Species: Simoes. J . A. M.. Beauchamp. J. L.. Eds.; Kluwer: Dordrecht. 1992: p 321. Armentrout. P. B. ACS Symp Ser. 1990. No. 428. 18. Armentrout, P. B.; Sunderlin, L. S. In Tran.sitior7 Metal Hydrides: Dedieu. A , , Ed.: VCH: New York. 1992: p 1. (6) Armentrout. P. B.; Georgiadis, R. Polyhedron 1988. 7. 1573. ( 7 ) Elkind. J. L.; Armentrout. P. B. Inorg. C h e m 1986, 25. 1078. (8) Armentrout. P. B. Aniiic. Rei. Phys. Chem. 1990. 41, 313: Scieitce 1991. 251. 175. (9) Armentrout. P. B. ACS Symnp. Ser. 1992. No. 502. 194. ( I O ) Mandich. M. L.; Halle, L. F.: Beauchamp. J. L. J. Ain. Che171. Soc. 1984, 106, 4403. (1I )
Bond energies for PdH' and RhH+ taken from unpublished work
of Elkind and Armentrout are cited in ref 6. (12) Elkind. J. L.: Sunderlin, L. S.: Armentrout. P. B. J. Phyx Chern. 1989. 53. 3151. ( 1 3 ) Schilling. J. B.: Goddard 111. W. A , ;Beauchamp. J. L. J. Am. C k e m Soc. 1987, 109. 5565. (14) Pettersson. L. G. M.: Bauschlicher. Jr.. C. W.: Langhoff. S . R.: Partridge. H. J. Chern. Phxc. 1987, 87. 481. ( 1 5 ) Schultz, R. H.: Armentrout, P. B. Int. J . Muss Spectrorri. I017 Processes 1991, 107. 29. (16) Ohanessian. G.: Goddard 111. W. A. Acc. Chem. Res. 1990. 23.
386. (17) Moore. C. E. Aromic Energy Levels. Ntifl. Sfand. Ryf: Datu Ser.. "v'fl. Bur. Srcirid. (NSRDS-NBS) 1971. 35. Vol. 11. (18) Ervin, K. M.: Armentrout. P. B. J. Chem. Phys. 1985. 83. 166. (19) Teloy. E.: Gerlich. D. Chern. Pkys. 1974. 4 . 117. Gerlich. D. Diplomarbeit. Unibersity of Freiburg. Federal Republic of Germany. 1971.
Reactions of Ru+, , ' h R
J. Phys. Chem., Vol. 99, No. 26, 1995 10445
Pd+, and Ag+ with Hydrogen
Gerlich, D. In State-Selected and State-to-State Ion-Molecule Reaction Dynamics. Part 1. Experiment; Ng, C.-Y., Baer, M., Eds.; Adv. Chem. Phys. 1992, 82, 1. (20) Chantry, P. J. J. Chem. Phys. 1971, 55, 2746. (21) Chen, Y.-M.; Armentrout, P. B. J. Chem. Phys., in press. (22) Kickel, B. L.; Armentrout, P. B. J. Am. Chem. SOC. 1995, 117, 4057. (23) Clemmer, D. E.; Chen, Y.-M.; Khan, F. A,; Armentrout, P. B. J. Phys. Chem. 1994, 98, 6522. (24) Haynes, C. L.; Armentrout, P. B. Organometallics 1994,13,3480. (25) Kickel, B. L.; Armentrout, P. B. J. Am. Chem. SOC. 1995, 117, 764. (26) Severs, M. R.; Chen, Y.-M.; Elkind, J. L.; Armentrout, P. B. Work in progress. (27) Sunderlin, L. S.; Armentrout, P. B. J. Phys. Chem. 1988.92, 1209. (28) van Koppen, P. A. M.; Kemper, P. R.; Bowers, M. T. J. Am. Chem. SOC.1992, 114, 10941. (29) Armentrout, P. B. In Advances in Gas Phase Ion Chemistry; Adams, N. G., Babcock, L. M., Eds.; JAI: Greenwich, 1992; Vol. 1, p 83. (30) Weber, M. E.; Elkind, J. L.; Armentrout, P. B. J. Phys. Chem. 1986, 84, 1521. (3 1) Calculated from heats of formation given by: Gurvich, L. V.; Veyts, I. V.; Alcock, C. B. Thermodynamic Properties of Individual Substances, 4th ed.; Hemisphere: New York, 1989; Vol. 1, Part 2.
(32) Elkind, J. L.; Armentrout, P. B. J. Phys. Chem. 1985, 89, 5626. (33) Elkind, J. L.; Armentrout, P. B. J. Phys. Chem. 1986, 90, 6576. (34) Elkind, J. L.; Armentrout, P. B. J. Chem. Phys. 1986, 84, 4862. (35) Elkind, 3. L.; Armentrout, P. B. J . Phys. Chem. 1986, 90, 5736. (36) Armentrout, P. B. In Structure/Reactivity and Thermochemistty of Ions; Ausloos, P., Lias, S. G., Eds.; Reidel: Dordrecht, 1987; p 97. (37) The zero-point energy differences between MD+ and MH' are 0.032,0.036,0.037, and 0.013 eV for M = Ru, Rh, Pd, and Ag, respectively. These are calculated from the differences in the MD+ and MH+ vibrational frequencies, which are taken from ref 14 for MH+ and estimated for MD' by using a Morse potential to scale the MHf frequencies. (38) E,(5s'4dn-') is easily calculated as the mean energy of the lowest electronic states that have high-spin and low-spin 5s14dn-'configurations. Calculation of Ep(4dn)from spectroscopic data is more complicated and described in detail in ref 10. Values obtained in this fashion are provided in ref 7. In many cases, it is more convenient and nearly as accurate to use theoretical values listed in ref 39. (39) Carter, E. A.; Goddard 111, W. A. J. Phys. Chem. 1988, 92, 5679. (40) Das, K. K.; Balasubramanian, K. J . Chem. Phys. 1990, 92, 6697. Das, K. K.; Balasubramanian, K. J . Phys. Chem. 1991, 95, 6880. Zhang, H.; Balasubramanian, K. J . Phys. Chem. 1992. 96, 6981. JP950436E
