J. Phys. Chem. 1993,97, 11673-1 1676
11673
An Investigation of the Electronic Structure of Some Tris( q5-cyclopentadienyl)thorium(1V) and -Uranium(IV) Complexes by Relativistic Effective Core Potential ab Initio Calculations and Gas-Phase UV Photoelectron Spectroscopy+ Santo Di Bella, Antonino Gulino, Giuseppe Lanza, and Ignazio L. Fragalii' Dipartimento di Scienze Chimiche, Univmith di Catania, V a l e A . Doria 8, 951 25 Catania, Italy
Tobin J. Marks' Department of Chemistry, Northwestern University, Evanston, Illinois 60208-31 13 Received: June 2, 1993'
Comparative relativistic effective core potential ab initio calculations for both Th(1V) and U(1V) Cp3AnL (Cp = q5-C5H5; L = CH3, BH4) complexes are reported. The Cp-An bonding appears to be dominated by metal 6d orbitals interacting with ligand ~2 orbitals. Metal 5f orbitals provide a smaller contribution but are crucial for stabilization of the Cp3An cluster. The stability of the An-CH3 bonding depends upon interactions involving metal 6d,~-basedorbitals directed along the An-CH3 vector. The L = BH4 ligand interactions are mediated by d,, and d,, atomic orbitals, which are even better suited for favorable overlap and, hence, for greater metal-ligand ?r covalency. Ground 3A2states have been found to be the most stable for the U(1V) complexes. The experimental H e I/He I1 photoelectron data are consistent with the quantum chemical calculations and indicate a close similarity between ground-state properties of the present Th(1V) and U(1V) complexes.
Introduction Cyclopentadienyl (Cp = q5-C5H5) complexes currently represent a central area in the organometallic chemistry of 5f elements, since this important ancillary ligand has proven to be as useful and as flexible as in organotransition metal chemistry.' Within this important class of actinide organometallics, the complexes related to the Cp3An fragment have been and are still actively investigated, since they can interact with a large variety of L ligands, thus affording extensive classes of Cp3AnL species. For these reasons, a number of aspects of the electronicstructure of these complexes have been investigated by a variety of theoretical methods2 and experimental technique~.'.~ Relativistic effective core potential (RECP) ab initio calculationss have opened a new approach to the study of the electronic structure of actinide molecules, since relativistic effects can be taken into account and, in contrast to the case of nonempirical local density functional6 or semiempirical' methods, the nonlocal exchangeintergrals are explicitly evaluated,thus allowing a more accurate description of the bonding. The use of RECP ab initio methods has been, however, of limited applicability to the large organometallic molecules containing 5f elements, since they are computationally demanding. To date, only a very detailed investigation of U($-CsHs)2 has appeared in the literatureanNo comparable studies have been reported either for a thorium organometallic complex or for cyclopentadienyl thorium and uranium derivatives. In both cases, however, the issue of 5f vs 6d participation in the bonding, hence the relevance of relativistic effects, is an important, unresolved issue. Moreover, there is evidence2J*.b that the energy of the metal 6dg orbital relative to the 5f manifold is crucial for the stability of the An-L bonds. It therefore becomes of interest both to study electronic structural differences upon changing from U to Th complexes, where a different pattern of 5f vs 6d energies is expected, and to investigate the differences encountered on passing from a-only to *-only bonding L ligands. In this contribution, we report a comparative study of the electronic structure of four Cp3AnL (An = Th, U; L = CH3, f Part 1 1 of the series Photoelectron Spectroscopy o f f Element Organometallic Complexes. For part 10 scc ref 2 1. Abstract published in Alance ACS Abmacrs, October 1, 1993.
0022-3654/93/2097-11673$04.00/0
BH4) complexes by relativistic effective core potential ab initio calculationsand by gas-phase photoelectron spectroscopy(PES).9
Experimental Section Cp3ThL complexeswere synthesized according to the published procedures.1° They were purified by sublimation in vacuo. The purity was checked by NMR and IR spectra. PE spectra were recorded as described elsewhere.P*.b Resolution measured on the He Is-' line was always around 25 meV. The intensities of He I1 spectra were corrected only for the He 1119 "satellite" contributions(10%on referenceN2 spectrum). The spectra were run in the 130-160 OC range without any evidence of thermal decomposition.
Computational Details RECP ab initio calculations were performed by using the restricted Hartree-Fock (RHF) method for the closed-shell states and the restricted open Hartree-Fock (ROHF) method for the open-shell states. For the degenerate states of Cp3UL (e.g., 2E arising from the (e)' configuration),the generalized valence bond (ROHF-GVB) formalism was used, in order to obtain a symmetric charge density. The ionization energies (IEs) of selected lower lying molecular orbitals (MOs) were evaluated using the ASCF procedure. The RECPs, for the central atoms, explicitly treat the 6p electrons for thoriumll and 6s, 6p electronsfor uranium.'2 The [3s,2p,2d,2fl contraction basis set was used in both cases.11J2 Double-{ Dunning's basis sets13 were adopted for C, B, and H atoms. The HOND08 d e l 4 was run on an IBM ES/9OOO computer. Closed-shell ('Al) ground-state configurations were studied for Th(1V) complexes. Severallow-lying states were investigated in the case of homologous U(1V) complexes, due to the F configuration. In all calculations, the geometrical parameters of the UCp3 fragment were fixed with a typical 117' Cp(centroid)U-Cp(centroid) angle and 2.54 A U-Cp(centroid) distance.ls The U-CH3 and U-BH4 bond distances and bond angles were taken from known structures of Cp3U(n-C4H9)16 and Cp3U(BH4),17assuming C3" symmetry. X-ray crystallographic data are not available for the present Th complexes. Their geometries 0 1993 American Chemical Society
Di Bella et al.
11674 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
TABLE I
(") MO t l6ap -2.81 7ald 8.09 15al 8.45 2Oe 8.84 14al 9.74 19e 9.88 1st 13.62
TABLE I1 A. Ab Initio Eigenvalues and Mulliken Population Analysis for CplThCHj IE," Th ASCF eV 7s pb 6d 5f 3Cp CHI character 0 0 39 14 55 1 r3+d9 7.59) 7,80 (a) 0 0 0 6 94 0 *z+fy(3~fi 6.85 0 5 4 1 50 40 C H s + r z + d$ + PI 1 rz+dyr+d, 8.57 (b) 0 1 8 3 87 0 0 17 3 42 38 r i + C H , + d $ 9.06 (c) 0 1 15 0 83 1 rz+d(&fi +dxz 0 6 5 0 8 5 4 r l
}
B. Ab Initio Eigenvalues and Mulliken Population Analysis for Cp3UCH3 ("1 MO
t
IE,# ASCF eV Sr pb 6d 5f 3Cp CH3
17ale -2.77 2 l d 12.85 6.67
::::i:iz)7*74
8.77 15aI 9.64 19e 9.80 Me 13.69
2Oe
0 0 6.56 0 0 0 0 0 )8*47 0 9.01 0 0
9 36 53 0 99 1
0 0 9 91 4 3 1 57 0 7 3 89 0 19 4 36 1 11 0 88 5 5 0 72
A. Ab Initio Eigenvalues and Mulliken Population Analysis for CpaThBH, (CV) Th t ASCF IE,#eV 7s pb 6d 5f 3Cp B&
MO
2119 -2.65
0 0 32 12 56
7af 2Dc
0 0 0 0
8.23 7.73 7.85 (a) 8.56 8.47 (b)
15al 9.41 8.94 8.82 (b') 0 4 6 1 88 19e 9.93 9 . 3 3 ( ~ ) 0 2 13 0 82 14a1 12*01 11.01 (d) 0 1 3 0 11 18e 12.92 0 0 14 1 21
}
17e 13.89
2
0 35 1 41 0 18
0 9 3 0 8 3
fdAfi
r~+fyo+fi CH3+r2+d9+pZ r2+dyr+dn rl+CH3+d9 ~ + d ( + f i +dxz TI
a See Figure 1. Value refer to 6p and 7p orbital population. LUMO. HOMO. e Experimental data from ref 3b. /Values refer to 6s and 7s orbital population.
d
MO
Results and Discussion Electronic Structure. Ab initio atomic population data for Cp3ThCH3 provide evidence (Table IA) that the Th-Cp bonding involves MOs (7a2,20e, 14a1,and 19e) which are combinations of the uppermost filled "2 orbitals19 of the cyclopentadienylanion and both 5f and 6d metal subshells. The energy ordering, 7a2 > 20e = 14al > 19e, appears identical to that found in in the other Cp3AnL complexes studied to date.3a.b.20 Moreover, there is evidence that ligand ~2-basedMOs are all significantly engaged in the metal-ring bonding and represent the major source of ligand-to-metal donation. The 5f interactions mostly involve the 7a2 MO and provide a significant stabilization of the Cp3 cluster.3~.b*20The metal 6d subshells are heavily admixed into the remaining ~ r r e l a t e dMOs, and the major contributions are found in the lower lying MOs, namely 14al and 19e. Interesting enough, sizable metal 6d/6p contributions are also found in more internal (xI)ring-based MOs. Interactions with the CH3 ligand appear to be a-only in nature and are contained in the 15al and 14al MOs (Table IA). They involve the metal 6d+based atomic orbital, the contribution of which becomes predominant in the low-lying 14al MO. Variations in electronic structure are observed on passing to theCp3ThBH,homologue (Table IIA). The metal Sfcontribution to the 7a2 MO remains constant. In contrast, smaller metal 6d admixtures are observed in the remaining T2-related MOs, and in particular, the most significant effect is noted on passing from the Cp3ThCHp 14al MO (17% 6d) to the corresponding Cp3ThBH4 15al MO (6% 6d). The metal 6d orbitals are also of relevance in the 13coordination of the BH4 ligand (Table IIA). A 14%d,, d,, contribution is found in the 18e MO. Symmetryallowed interactionsmix some MOs in both the present complexes. They involve the 15a1,14al MOs in Cp3ThCH3and the 20e, 18e MOS in 18e MOs in CppThBH4. The ab initio total charges on the central Th atom, +1.37 eu (L = C H d and +1.02 eu (L = BH4), are indicative of an appreciable metal-ligand covalency in both complexes (Table 111). The observed trend argues for a greater ligand-to-metal
t
(eV) IEJ ASCF eV
z pb 6d 5f 3Cp B&
17ald -2.60 216 13.96 7.22
0 0 10 34 55 6.93 0 0 0 99 1
7a1 8.24 7.71 8.45 2Oc 16al 9.37 19e 9.86 15al 11.92 18e 12.50
8.04 8.45 8.76 9.25
17e
were adapted, after corrections for the differing atomic radii,'* from the structures of the analogous uranium compounds.
+
B. Ab Initio Eigenvalues and Mulliken Population Analysis for Cp,UB*
character
r3+fd+3fi 0 fd+fy.l+fry+
0 6 94 3 5 81
charactep
0 r 3 + d +d,+ dn dA? 0 r~+fi(3+,z, 11 ~ + f ~ . + f + + B-Hb 1 r~+dfi+p, 3 r2+d(Afi+dry 85 B-HI+rl 64 dxz + dyr 5 r 1
}
0 0 0 0 0 10.79 0
14.03
0
0 2 1 1 0
0 10 90 3 6 79 6 1 91 12 0 84 3 1 2 10 1 22
0 7 4 0 8 7
charactep
1 r3+f4*3fi 0 fd+fy.l+f,+ fdAfi
r2+f~~yl) ri+f&+f+ ri+d$ T ~ + & A A +-~ = CHt B-Hb+rg d; d; 2 r 1
0 12 0 3 93 67
\ . . . I
+
+
See Figure 2. Values refer to 6p and 7p orbital population. e Hb = Hmd&, Ht = H-1. LUMO. e HOMO. /Experimental data from ref 3b. g Values refer to 6s and 7s orbital population. a
TABLE III: Ab Initio Electronic Charges for CpJAnL(An = Th, U and L = BH4, C H 3 ) Complexes metal atomic orbital population
charge (eu)
complex
Sf
6d
7s
6,7p
M
CpsThCH3 Cp3ThBH4 Cp3UCH3 CpsUBHd
0.83 0.92 2.70 2.81
1.99 2.22 1.81 1.84
0.00 0.00 2.12' 2.13'
5.81 5.93 6.07 6.12
+1.37 +1.02 +1.30 +1.10
a
3Cp
L
-0.88 -0.49 -0.69 -0.83 -0.66
-0.33 -0.47
4.44
Values refer to 6s, 7s orbital population.
donation in the L = BH4 complexes than in the L = CH3 complexes. The electron distribution in the metal atomic orbitals is similar for the two complexes (Table 111)and indicatesa primary role of the 6d orbitals in the bonding. The present results agree with previous fully relativistic DVM-Xa calculations on theclosely related Cp3ThOCH3 complex,3*as well as with RECPcalculations
-
on Th0211 and C12Th(CH2P(CH3)CH2.2' It has already been noted that the homologous Cp3UL complexes possess various low-lying states. Among them, we find the 'Al (a12)and 3A2(e2) states to be lower-lying, and in particular, the triplet ground states lie -1.5 eV below the singlet states in both the present U(1V) complexes. Ab initio RECP results (Tables IB and IIB) for the3A2ground state of the present CpsUL complexesare similar with those for thorium homologues. Note, however, an increased (relative to the Th complexes) 5f contribution to the r2-related MOs of a2 symmetry (Tables IB and IIB) and, hence, a greater covalency of the An-Cp bonding and a greater overall stability conferred to the Cp3 cluster. As far as the bonding with the L ligand is concerned, the An-CH3 interaction is mediated by comparable admixtures with the 6d AOs in both the U(1V) and Th(1V) complexes. In contrast, a smaller (10% vs 14%) U metal 6d contribution vs Th is found in the Cp3AnBH4 18e MO. The calculated energies of the U 5f orbitals indicate that they are more tightly bound than most of the occupied, mainly ligandbased, orbitals. Nevertheless, this indication can be misleading,
The Journal of Physical Chemistry, Vol. 97, No. 45, 1993 11675
Th(q5-C5H5)3 and U(q5-C5H5)3 Complexes
I
t
a
0
10
1 1 w 7
8
0
lo
1 1 w 7
8 0 lo 11.V Figure 1. Gas-phase He I and He I1 PE spectra of Cp3ThCH3 (7.0-1 1.5 7
eV)
.
kl
I
I
7
8
0
10
11.v
F i p e 2. Gas-phase He I and He I1 PE spectra of Cp3ThBH4 (7.0-1 1.5 eV)
.
TABLE IV Comparison of Relevant Experimental IEs (eV) of CpjThL and CPJUL~ complexes band label
CmThCH3
ConUCH3
7.80 8.57
6.56 7.74 8.47
9.06
9.01
CoqThBHA ~
X
a b
b’ C
d
COIUBHA
~~
7.85 8.47 8.82 9.33 10.86
6.93 8.04 8.45 8.76 9.25 10.79
Data are taken from ref 3b.
since, a priori, Koopman’s theorem would predict relatively high energy ionizations for the 2E(5f-1) (produced from ’A2) state, while previous experimental data3 show that production of the 5f-l ion state results in a low-energy onset band in the corresponding PE spectra. Clearly, relaxation effects upon ionization (Tables I and 11) are large enough to upset the Koopman’s MO sequence,similar to the orbital reorderingobserved upon ionization of metal d subshells in early transition metal complexes.22 Finally, note that large U vs Th differences are observed as far as the nest of lower lying virtual orbitals is concerned. They are metal 5f-based for both of the present U(1V) complexes, while they possess metal 6d character for Th(1V) analogues (Tables I and 11). Therefore, consistent with earlier results,20the 5f orbitals are lowered in energy upon proceeding from Th to U. Photoelectron Spectra. The low IE regions ( e 2 a1> e. We note, however, that ionizations of the 20e and 15al MOs are resolved in the spectrum of Cp3ThBH4 within the b, b’ envelope. The band a in the spectrum of CppThCHp, twice as intense as the corresponding band in Cp3ThBH4, must be taken to also represent the ionizationof the 15al u (Th-C) MO (Table IA). This assignment is entirely consistent with previous PE studies on Cp3UL complexes.3b The aforementioned lower IE shift of the band a in Cp3ThBH4,compared to the case of the uranium analogue, appears indicative of a smaller energetic stabilization of the 7a2 r2Cp-based MO and, hence, of a smaller metal 5f contribution to this particular MO. This experimental observation is in excellent agreement with the present computational results on Cp3UBH4 and Cp3ThBH4 (Table 11). Moderate changes of relative Cp3ThL PE intensities are observed upon switching to the He 11 radiation (Figures 1 and 2). It has been demonstrated23 that 5f cross sections have a ‘delayed” maximum at the He I1 wavelength and, therefore, PE bands due to MOs having metal 5f contributions must be expected to have a less pronounced relative intensity falloff than those having metal 6d admixture. As a consequence,band a moderately increases (relative to bands which follow) in the He I1 spectrum of Cp3ThBH4(Figure 2) because of the metal 5f content. A similar variation is not observed in the spectrum of the L = CH3 complex, probably because of a balancing effect due to the overlapping ionization of the 15al MO, which possesses a 4% metal 6d admixture. As far as band d is concerned, a reduced relative intensity is observed in the He I1 spectrum of Cp’ThBH4 in accordance with the smaller cross section associated with the BZ,-based M O S . ~ ~
Conclusions The present results represent the first comparative RECP ab initio calculations for Th(1V) and U(1V) cyclopentadienyls. As such, they provide an accurate picture of the metal-ligand bonding in representative Cp3AnL complexes. The An-Cp bonding appears to be mediated largely by metal 6d orbitals interacting with ligand T Zorbitals. Metal 5f orbitals provide a smaller total contribution but are crucial for stabilization of the CpSAn cluster through interactions represented by the lower lying MOs of a2 symmetry. Evidence of some metal 6d bonding contribution is also found in lower lying MOs related to the TI Cp-based orbitals. These interactions are nonbonding to first-order, since they involve filled 6p metal orbitals, but acquire some bonding character because of concurrent hybridization with empty 6d AOs. The An-CH3 bonding interactions are contained in the lower lying MOs of a1 symmetry. They mostly involve metal 6d9based orbitals oriented along the An-L vector. These represent an ideal acceptor for the lone pair of the CHI ligand.3b Nevertheless, the stability of the An-L bonding cannot in general be ascribed only to such interactions, since metal-ligand bonding with the L = BH4 ligand is mediated by the d,, and d,, AOs, which are better suited for favorableoverlapand, hence, for greater metal-ligand r covalency. The derived metal atomic charges in the Cp3AnBH4 complexes support this effect. Ground 3A2 states have been found to be the most stable for the present U(1V) complexes. The filled uranium 5f subshells are totally nonbonding (99% Sf) in the present U(1V) complexes. Consistent with experimental PE data, a close similarity between the ground-state properties of both the U(1V) and Th(1V) complexes is found. The differences in the nature of the lower lying empty Cp3AnL orbitals are seen in the different E112 values for the An(1V) An(II1) reductive processlJ5as well as the different relative bond disruption energies for the An-R bonding on passing from U(1V) to Th(1V) complexes.26 In this context, it must be noted that differences in D(An-R) do not depend entirely upon ground-
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11676 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
state electronic structure properties but can be rationalized in terms of the energetics of the metal oxidation state changes accompanying homolytic bond disruption.26 Clearly, further work is desirable in this direction.
Acknowledgment. The authors gratefully thank the Minister0 d e h i v e r s i t a e della Ricerca Scientifica e Tecnologica (MURST, Rome, Italy, I.L.F.) and the National Science Foundation (Grant CHE 9104112, T.J.M.) for financial support. References and Notes (1) (a) The Chemistry of the Acrittfde Elemenrs; Katz, J. J., Seaborg,
G.T.,Morss,L.R.,Eds.;ChapmanandHall: London, 1986. (b) Fundamental and Technologtcal Aspecrs of Organo-f-Elemen?Chemistry; Marks, T. J., FragalB, I. L., Ed,.; D. Reidel Publishing Company: Dordrecht, The Netherlands, 1985. (c) Marks, T. J.; Emst, R. D. In Comprehensive OrganomerallicChemisrry;Wilkinson, G.,Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, 1982;Chapter 21. (d) Marks, T. J. Science 1982, 217, 989. (e) Acfinides in Perspecriue; Edelstein, N. M., Ed.; Pergamon Press: Oxford, 1981. (0Marks, T.J.; Manriquez, J. M.; Fagan, P. J.; Day, V. W.; Day, C. S.;Vollmer, S . H. ACS Symp. Ser. 1980, 131, 1. (g) Orgonomefollicsoff-Elements; Marks, T.J., Fischer, R. D., Eds.; D. Reidel Publishing Company: Dordrecht, The Netherlands, 1979. (h) Marks, T. J. Prog. Inorg. Chem. 1979,25,224. (2) (a) Pepper,M.; Burstern, B. E. Chem.Rev. 1991,91,719.(b) Bursten, B. E.; Strittmatter, R. J. Angew. Chem., In?. Ed. Engl. 1991, 30, 1069. (3) (a) Gulino, A.; Di Bella, S.;FragalB, I.; Casarin, M.; Scyam, A. M.; Marks, T.J. Inofg. Chem. 1993,32,3873.(b) Gulino, A.; Ciliberto, E.; Di Bella, S.;FragalB, I.; Seyam, A. F.; Marks, T. J. Organomerallics 1992,1I, 3248. (c) Vittadini, A.; Casarin, M.; Ajb, D.; Bertoncello, R.; Ciliberto, E.; Gulino, A.; FragalB, I. Inorg. Chim. Acra 1986, 121,L23. (d) Green, J. C.; Kelly, M. R.; Long,J. A.; Kannellakopulos,B.; Yarrow, P. I. W.J. Organomet. Chem. 1981,212,329. (e) FragalB, I.; Ciliberto, E.; Fischer, R. D.; Sienel, G.; Zanella, P. J. Organomer. Chem. 1976, 120,C9. (4) (a) Amberger, H.-D.; Fischer, R. D.; Yunlu, K. Organometallics 1986,5,2109.(b) Bagnall, K. W.; Plews, M. J.; Brown, D.; Fischer, R. D.; Klahne, E.; Landgraf, G. W.; Sienel, G. R. J. Chem. SOC.,Dalron Trans. 1982,1115. (c) Amberger, H.-D. J. Organome?.Chem. 1976,116,219.(d) Andenon, M. L.; Crisler, L. R. J. Organomer. Chem. 1969,17,345. (5) (a) Ortiz, J. V.; Hay, P. J.; Martin, R. L. J . Am. Chem. Soc. 1992, 114,2736.(b) Ermler, W. C.; Ross, R. B.; Christiansen, P. A. In?.J. Quantum Chem. 1991,40,829 and references therein. (6) (a) Painter, G. S.;Ellis, D. E. In?.J. Quantum Chem., Symp. 1970, 3, 801. (b) Ellis, D. E. In?. J. Quanrum Chem., Symp. 1968, 2, 35. (c) Johnson, K. H. J. Chem. Phys. 1966,45,3085.
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