Organometallics 1995, 14, 5665-5669
5665
[(q-C5&)M]406 (M = Nb,V): Ab Initio Calculations
Predict Equivalent Metal Atoms in a Rectangular Conformation Jose Pedro Sarasa,?Josep-M. Poblet,t Marie-Madeleine Rohmer,§ and Marc Bknard*J Laboratoire de Chimie Quantique, UPR 139 du CNRS, Universitk Louis Pasteur, F-67000 Strasbourg, France, Applications Scientifiques du Calcul Intensif (ASCI), UPR 9029 du CNRS, Orsay, France, and Departament de Quimica, Universitat Rovira i Virgili, Pq Imperial Tarraco 1, 43005 Tarragona, Spain Received July 12, 1995@ Ab initio SCF calculations have been carried out on the organometallic model clusters [(l;l-CgH5)M]406 (M = Nb,V), for the purpose of comparing the energies of the various conformations proposed for the niobium complex [(~pC5Me5)Nb]406.It is shown that a reliable energy comparison requires for this specific problem a fine tuning of the Gaussian basis sets. After the inclusion of diffuse and polarization functions in the basis set describing the metal-oxygen core, it is shown that the most stable conformation for the niobium complex is the quasi-planar form with D2h symmetry, as tentatively proposed by Bottomley on the basis of NMR spectra and magnetic behavior. The observed temperature dependence of the magnetic moment could be explained by the close vicinity of three states with different spin multiplicities. The [(l;l-CgH5)M]406 model also predicts a conformation with Dah symmetry for M = V. This prediction is in contradiction with the reported Td structure of the [(q-C5Me5)Vl406 complex, because the model does not account for the important steric strain induced by the coplanar position of the centroids of the C5Me5 ligands. A change in the structure of the vanadium complex could therefore be expected from the replacement of C5Me5 by less bulky Cp ligands.
Introduction The synthesis and characterization of clusters with general formula [LnM41@2-&)has been extended in the last decade to non-d1° transition metal atoms.1,2 The M4@2&) core of those clusters has an adamantane-like structure similar to the one represented in Figure la. Recently, three clusters have been characterized that provide an alternative to the adamantane-like structure. [(?pC~d%)Ti14Ses,~ (?pC5MedsM08016,~and (q-C5Me5)4Mo50115 display the M4@2-A)3@3-A)3 structure represented in Figure IC. An analysis of the relationship between the observed conformations of those clusters and their electronic structures has been given by Bottomley, based upon extended Huckel calculations.6 The case of [(~,d!5Me5)M]406 (M = Nb, Tal7 for which no X-ray structure could be obtained appears more puzzling. At variance from the other clusters with dl metal ions, the magnetic behavior of those molecules is Universitat Rovira i Virgili. Permanent address: Departamento de Quimica Fisica y Quimica Orgdnica, Universidad de Zaragoza, Ciudad Universitaria d n , 50009 Zaragoza, Spain. Universitat Rovira i Virgili. 5 Laboratoire de Chimie Quantique and ASCI. Abstract published in Advance ACS Abstracts, November 1,1995. (1)Babcock, L. M.; Day, V. W.; Klemperer, W. G. J. Chem. SOC., Chem. Commun. 1987,858. (2)(a)Bottomley, F.;Magill, C. P.; Zhao, B. Organometallics 1990, 9, 1700. (b) Bottomley, F.; Magill, C. P.; Zhao, B. Organometallics 1991,10, 1946. (3)Bottomley, F.;Day, R. W. Organometallics 1991,10, 2560. (4)Harper, J. R.; Rheingold, A. L. J. Am. Chem. Soc. 1990,112, +
@
4037. .
( 5 )Bottomley, F.; Chen, J.;Preston, K. F.; Thompson, R. C. J . Am. Chem. SOC.1994,116, 7989. (6) Bottomley, F. Organometallics 1993,12, 2652. For a general review on cyclopentadienyl metal oxides, see ref 23.
not characteristic of an open-shell ground state. The small temperature-independent magnetic susceptibility observed for [(?pC5Me5)Nb]406 rather suggests a diamagnetic ground state interacting with a low-lying multiplet. This magnetic behavior is therefore incompatible with the ground state configuration laI21e2 obtained from extended Huckel calculations on the adamantane structure as on the &@2-A)3@3'A)3 0118.~'~ In order to solve this contradiction, two other structures with nondegenerate ground states were proposed by B~ttomley.~The structure of Figure Id has been observed previously for the complexes [MzClz(S(CH2)2NMeCH2)zIz (M = Zn,8 Cd9), whereas the structure of Figure lb, a dimer of Cp2Nb2@2-02)units with D2h symmetry connected by two extra bridging oxygens, has apparently no pre~edent.~ In spite of that, Bottomley tentatively assigned this latter structure to [(~,&Mes)Nb1406 because of the lH and 13C MMR spectra suggesting a unique type of q-CsMe5 ligand. The goal of the present work is to carry out ab initio Hartree-Fock calculations on the model system [(qC5H5)Nbl406 in order t o characterize the optimal geometry corresponding t o each of those conformations, t o compare those energies, and t o detect the origin of the low-lying paramagnetic states leading t o the temperature-independent magnetic susceptibility. Similar calculations will be reported concerning the isoelectronic (7)Bottomley, F.;Boyle, P. D.; Karslioglu, S.; Thompson, R. C. Organometallics 1993,12, 4090. (8) Hu, W. J.; Barton, D.; Lippard, S. J. J.Am. Chem. SOC.1973, 95,1170. (9)Fawcett, T. G.;Ou, C.-C.; Potenza, J. A.; Schugar, H. J. J . Am. Chem. SOC.1978,100,2058.
0276-733319512314-5665$09.00/0 0 1995 American Chemical Society
5666 Organometallics, Vol. 14,No. 12, 1995
Sarasa et al.
complex of vanadium, for which the adamantane-like structure has been confirmed,2and the influence of the steric strain induced by the bulky C5Me5 ligands will be discussed. [(q-C5H5)Nb]406: Computational Details and Results
a
b
C
Figure 1. XMOL representation of the four possible conformations considered for [(pC5Me5)Nb]406: (a) adamantanelike form; (b)D 2 h conformation; (c) pseudo-C3, form with three p3-0; (d) pseudo-czh conformation with two ,us0.
The geometry optimizations have been carried out by means of the TURBOMOLE program.1° The final calculations including polarization functions on the niobium and oxygen atoms were carried out with the ASTERIX program.ll Triplet and singlet coupling of the four metal electrons, carried out at the optimal geometries, were performed by means of the internally contracted CI program written by Siegbahn and interfaced to ASTERIX.12 The energy balance between the two most stable conformations, namely the adamantane-like one (DM symmetry, conformation a) and the one with D 2 h symmetry (conformation b) was found to be extremely sensitive to the quality of the basis set. Four sets of atomic bases were used, corresponding to increasing accuracy in the valence-shell description of the metaloxygen core or of the carbon atoms (Table 1). Basis set I uses the core potential derived by Hay and Wadt13 to represent the Ar core of niobium. All-electron, double-5 basis sets are used for the other atoms. Basis set I1 (496 contracted Gaussians (CGTOs)) represents the metal atoms with an all-electron basis, minimal for the inner shells but triple-5 for the valence d shell. The valence shell of the oxygen atoms, formally 02-,is also described with an extended basis set composed of 4 s-type and 4 p-type CGTOs. The carbon and hydrogen atoms of the Cp rings are described with a split-valence basis. Basis set I11 differs from basis set I1 by giving more flexibility to the set of Gaussian functions describing the p shell of carbons. Finally, basis set IV (708 CGTOs) adds two polarization functions to each atomic Gaussian set of the metal oxygen core. Geometry optimizations have been carried out with basis set I assuming the four geometric conformations postulated by Bottomley et al. for [(~-C5Me5)Nb]406. In those optimization processes, the four metal electrons were accommodated into separate, singly occupied orbitals, giving rise to quintet states. The optimization process carried out with basis set I yielded the lowest energy for the adamantane-like conformation with D 2 d symmetry (Table 1). The open-shell quintet electronic configuration is a11b21e2. This state correlates with (10) (a) TURBOMOLE: a direct SCF program from the Quantum Chemistry Group of the University at Karlsruhe under the directorship of Prof. R. Ahlrichs. (b) Ahlrichs, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, C. Chem. Phys. Lett. 1989, 162, 165. (11)(a) Ernenwein, R.; Rohmer, M.-M.; Benard, M. Comput. Phys. Comm. 1990,58,305. (b) Rohmer, M.-M.; Demuynck, J.; Benard, M.; Wiest, R.; Bachmann, C.; Henriet, C.; Ernenwein, R. Comp. Phys. Cpmm. 1990, 60, 127. (c) Wiest, R.; Demuynck, J.; B h a r d , M.; Rohmer, M.-M.; Ernenwein, R. Comp. Phys. Commun. 1991,62, 107. (d) Rohmer, M.-M.; Ernenwein, R.; Ulmschneider, M.; Wiest, R.; Benard, M. Int. J. Quantum Chem. 1991,40, 723. (12) Siegbahn, P. E. M. Int. J. Quantum Chem. 1983,23,1869. The CI program has been interfaced with ASTERIX by C. Daniel, M.-M. Rohmer, and M. Speri. (13) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985,82, 270, 299. (14)(a) Huzinaga, S. Tech. Rep. Uniu. Alberta, Dep. Chem., Diu. Theor. Chem. 1971. (b) Huzinaga, S.; McWilliams, D.; Domsky, B. J. Chem. Phys. 1971,54,2283. (15)Veillard, A.; Dedieu, A. Theor. Chim. Acta 1984, 65, 215. (16)Huzinaga, S. J. Chem. Phys. 1965,42, 1293. The contraction into 6s, 4p is carried out according to Dunning's procedure.17 (17)Dunning, T. H. J. Chem. Phys. 1971, 55, 716.
Organometallics, Vol. 14, No. 12, 1995 5667
[(q -c5HdMl40 6
Table 1. [(CsH~)MIrOe:Gaussian Basis Setsa Used for the Open-Shell RHF Calculation and Energies (hartrees) Obtained at the Optimal Geometry for the Open-Shell Quintet States Corresponding to the Most Probable Conformations of the Clusters with Relative Energies (kcalemol-9 in Parentheses basis set no. I Nb V 0 C H Dzh
core potential
--
+ (8,6,4) - [3,3,2113
(8, 4) [4, 2Il4 (8, 4) [4, 2Il4 (4) ~2114
-
(b)
Dw (a)
cs (c)
Cs (d)
-1440.939 45 ($14.2) -1440.962 06 (0) -1440.824 07 (+86.5) no local minimumd
I1
I11
Basis Sets (15,10,8) [6,4,4315 (13, 7, 5) [5,3,3Iz4 (11,7) [6, 4116 ~ 5 ~3,2114 ) same as I
--
--
IV
-
same as I1
(15,10,8,2Ib [6,4,4,21 (13,7,5,2)b [5,3,3,21 (11,7, 2Ib [ 6 , 4 , 21 same as I11 same as I
-
same as I1 (9, 5) [3, 3114 same as I
[(C5Hs)Nbl406: EnergiesC -16202.114 66 (0) -16202.160 00 (0) -16202,111 93 (+1.7) -16202.153 71 (+3.9)
-16202.632 34 (0) -16202.588 44 (f27.5)
[ ( C B H ~ ) V ] ~EnergiesC O~: -4982.000 15 (+6.2) -4982.010 10 (0)
-4982.309 39 (0) -4982.267 72 (+26.1)
a Figures in parentheses represent the number of s-, p-, d-, and f-type primitive Gaussians. The size of the contracted basis set is given within brackets. Exponents of the polarization functions: for niobium and vanadium (f-type), 0.8 and 0.25; for oxygen (d-type), 1.2 and 0.4. The geometries have been fully optimized with basis sets I and I1 for M = Nb and with basis set I1 for M = V. Calculations with basis sets I11 and IV (with basis set IV only for M = V) have been carried out using the geometries optimized with basis set 11. The gradient optimization converged toward the geometry obtained for conformation b.
an a11tz3state in the Td point group. Then comes the quintet state associated with conformation b, 14 kcabmol-l higher in energy. This conformation belongs to symmetry point group D2h and the electronic configuration is alg1b2glb~u1b3u1. Conformation c corresponds to the coordination pattern [(r-Cp)Nbl4@2-0)3@3-0)3. The CsVsymmetry is lowered to C, by the top Cp ring (Figure 1). An energy minimum was characterized for this conformation at +86.5 kcal-mol-l with respect to the lowest state. Finally, no minimum could be characterized for conformation d, the gradient calculation leading back to the optimal geometry of conformation
b. Geometry optimization carried out with basis set 11, which includes diffuse orbitals on all atoms of the metal-oxygen core, reverses the energy order of conformations a and b. The D2h form is now found more stable by 1.7 kcal-mol-l. No further calculation has been carried out on form c. The geometries optimized with basis set I1 have been used for single point calculations carried out with basis sets I11 and IV.Basis set I11 increases to 4 kcal-mol-l the energy gap in favor of conformation b. Finally, the addition of polarization functions to the atoms of the Nb4O6 core leads to an unambiguous assignment of conformation b as the most stable one. The energy difference with conformation a reaches 27.5 kcal-mol-' at this level of calculation. The most important geometrical parameters of conformations b and a as optimized with basis set I1 are displayed in Table 2. The niobium-oxygen distances optimized for the D2d conformation (1.924 and 1.935 A) are quite similar to the Nb-0 distances reported for [(r-C5Me5)Nb@-Cl)@0113' (1.937 Alls and for [(q-CsMes)NbC1212@-OH)2@0 ) (1.942 The Nb-0 distances optimized for the D2h structure (1.885 A for the double Nb-0 bridges, 1.961 A for the single Nb-0 bridges) correspond to the same order of magnitude. The distances from the metal to the centroid of the Cp ring are computed to be larger by 0.10-0.15 A than the currently reported values. This lengthening of the metal-Cp distances optimized at the SCF level of calculation is a well-documented artifact due to the neglect of electron c~rrelation.'~ (18)Bottomley, F.; Karslioglu, S. organometallics 1992, 11, 326.
Table 2. [(q-CsHs)M1406 (M = Nb, V): Selected Interatomic Distances (A) of Conformations a (Dw Symmetry) and b (D2h Symmetry) from Gradient Optimization of the Quintet Electronic States conformation a (Dw)
conformation b
dist M1-M3
Mi-MZ Mi-01 Mi-03 M1-Qlb H...Hc
(Dzh)
dist
M=Nb
M=V
n"
M=Nb
M=V
na
3.431 3.635 1.924 1.935 2.27 4.74
3.251 3.234 1.778 1.795 2.04 3.92
2 4 4 8 4
3.012 3.758 1.885 1.961 2.26 3.40
2.711 3.419 1.767 1.803 2.06 2.65
2 2 4 8 4
a Number of equivalent parameters. Centroid of the C5H5 ring. Closest contact between hydrogen atoms belonging to different C5H5 rings.
An important information is provided by the Nb-Nb distances. In the conformation with D2d symmetry, It should be noticed those distances are 3.43 and 3.63 first that those distances differ one from another by 0.2 A, which means that the Nb406 core significantly departs from its postulated Td symmetry. This may be interpreted as a trend of the Nb406 core toward fluxionality due to the lack of direct metal-metal interaction. Metal-metal distances of -3.5 A are usually thought to preclude such interactions,20an assumption which is substantiated by the orbital energy sequence of the four open-shell metal orbitals. Those orbitals belong to the al, b2, and e representations of the D u point group. They are made of different combinations of the same d atomic orbitals. Those four-lobe d orbitals are contained in planes approximately parallel to the Cp cycles. This orientation allows for more efficient donation interactions from both the oxygen and the Cp
A.
(19) (a) Liithi, H. P.; Ammeter, J. H.; Almlof, J.; Faegri, K., Jr. J . Chem. Phys. 1982, 77, 2002. (b) Almlof, J.; Faegri, K., Jr.; Schilling, B. E.; Liithi, H. P. Chem. Phys. Lett. 1984,106, 266. (c) Luthi, H. P.; Siegbahn, P. E. M.; Almlof, J.; Faegri, K., Jr.; Heiberg, A. Chem. Phys. Lett. 1984, 111, 1. (d) Liithi, H. P.; Siegbahn, P. E. M.; Almlof, J. J . Phys. Chem. 1986,89, 2156. (20) Unusually long metal-metal bonds were however characterized in relation with steric strain in some dinuclear complexes of Zr(II1): (a) Rohmer, M.-M.; Benard, M. Organometallics 1991, 10, 157. (b) Benard, M.; Rohmer, M.-M. J . Am. Chem. SOC.1992,114, 4785. (c) DeKock, R. L.; Peterson, M. A,; Reynolds, L. E. L.; Chen, L. H.; Baerends, E. J.; Vernoijs, P. Organometallics 1993, 13, 2794.
Sarasa et al.
5668 Organometallics, Vol. 14,No. 12, 1995
Table 3. [(q-CaHs)NblaOe:Computed Energies (hartrees;Shifted by 16 202) of the Lowest Quintet, Triplet, and Singlet States Resulting from the Appropriate Coupling of the Four Metal Electrons in the Adamantane-like (Dw) and in the D2h Conformations with Relative Energies (cm-') in Parentheses conformation b (Dzh) conformation a (Du) state
5B1
3E 3Az 'A1
energy -0.588 -0.587 -0.586 -0.587
444 (0) 324 (+246) 222 (+488) 315 (+248)
state
5AP
3Blu 3Bzg 'A,
energy -0.632 -0.632 -0.631 -0.632
357 (0) 335 ( f 5 ) 128 (+270) 323 (+7)
ligands, but it drastically reduces the possibility for a through-space coupling between the metal electrons. Such a coupling could be effective through the orbital combination with a1 symmetry, which is in phase and formally bonding with respect to the four metal atoms of the core.21s22 The three other metal orbitals are nonbonding with respect to the core of metal atoms and correlate with the triply degenerate I t 2 orbital reported ~ ~ , orbital ~~ energy obtained for the Td ~ y m m e t r y . The for the formally bonding la1 orbital is -2.34 eV, to be compared with -1.99 eV (lbe) and -2.01 eV (le) for the set of nonbonding orbitals. The in-phase character of the la1 orbital does not provide it with much stabilization, indicating that the through-space metal-metal interaction is not far from being negligible. The geometry optimized for the D2h conformation displays a rectangular arrangement for the metal atoms. The large Nb-Nb distances (3.758 A) are obtained along the single oxygen bridges, due t o a widely open NbO-Nb angle (171"). The metal atoms connected by a double bridge are separated by a relatively short distance of 3.012 A. This distance is compatible with the presence of a metal-metal bond, but it could also be induced by the interactions with the bridging ligands as in the do dimer [(rl-CsMes)NbC1212Cu-OH)2~-0)for which the Nb-Nb distance is 3.027 Assuming the metal framework to lie in the xy plane (x collinear to the short side of the rectangle), the four open shell metal orbitals are made of approximate dx2-zzatomic orbitals. The lobe oriented along the x axis is expected t o ensure some through-space coupling between the doubly bridged metal atoms. As a matter of fact, the two combinations that are in phase with respect to the short Nb-Nb distances have energies of -2.75 and -2.59 eV, to be compared with -2.12 and -2.23 eV for the out-of-phase combinations. The energy gap is larger than for the D u conformation but cannot be interpreted as characteristic of a strong metal-metal interaction. This weak metalmetal bonding that emerges from the analysis of the quintet wave functions obtained for both geometric conformations has direct consequences on the coupling of the four metal electrons. Table 3 reports for each (21) Such a coupling involving four metal d electrons in a complex with Td symmetry has been evidenced for metallocarbohedrenes M8C12. The four d orbitals taking part in this interaction are dplike orbitals defined in a local reference system attached to each metal atom with the z axis pointing toward the center of the tetrahedral molecule. The in-phase combination with a1 symmetry was in this case strongly stabilized with respect to the nonbonding combination with t z symmetry.22 In the present niobium complex, the singly occupied orbital combinations are made of d,z,z-like orbitals (defined in the same local reference system) with lobes directed outside the Nb40.a core. (22) (a) Rohmer, M.-M.; BBnard, M.; Henriet, C.; Bo, C.; Poblet, J.M. J . Chem. Soc., Chem. Commun. 1993,1182. (b) Rohmer, M.-M.; 1996,117, BBnard, M.; Henriet, C.; Poblet, J.-M. J . Am. Chem. SOC. 508. (23) Bottomley, F. Polyhedron 1992,11, 1707.
conformation the energy computed using basis set IV for coupled states, respectively 5Ap, 3Blu,3B2g, and lAg for the D2h conformation and 5B1, 3A2, 3E, and lA1 for the conformation with D 2 d symmetry. Singlet and triplet coupling of the four metal d electrons starting from the SCF orbitals of the quintet state leads for both conformations to intermediate and low-spin states energetically less stable than the quintets (Table 3). For the D 2 d conformation, this energy difference is 248 cm-l with respect to the lA1 state and about the same with respect to the lowest triplet (Table 3). For the Dah conformation, the 5Agstate is computed to be almost degenerate with the two coupled states 3Blu and '4, with energy differences less than 10 cm-l (Table 3). The triplet state next in energy (3B2g)is higher by -270 cm-l. A near degeneracy of states with different spin multiplicities could explain the observed magnetic behavior, i.e. the small temperature-independent paramagnetism and the increase of the magnetic moment with temperature. The singlet state would have been expected however t o be lowest in energy. In view of the very small energy gaps obtained from the couplings of the four d electrons, a change in the sequence of states is not excluded from further improvement of the basis set quality or from a proper account of electron correlation. Even if a change in the energy sequence of the coupled states was to be obtained for the D2h conformation at a higher level of approximation, the energy gaps are not expected to change much and the three states of lowest energy will remain quasi degenerate.
Vanadium Complex: Results and Discussion SCF calculations and geometry optimization have been carried out on the isoelectronic complex of vanadium, [(~]-C5HdVI406, with basis sets similar to those referred to as I1 and IV for the calculations on the niobium complex (Table 1). The basis set used for vanadium is taken from the (13s 7p 5d) basis optimized by Hyla-Kryspin et al.,24augmented with one p-type function ( 5 = 0.15) and contracted to [5, 3, 31. In the large molecular basis equivalent t o IV,polarization functions were added to the vanadium and oxygen atoms with the same exponents as those used for the niobium complex. The results are reported in Table 1. The effect of polarization functions appears still more dramatic than for niobium, since they induce a reversal of the relative energies obtained for the D2h and D a conformations. The crucial importance of polarization functions and, more generally, of the valence basis set extension in this specific problem could be explained by the different occupation of space characteristic of the competing conformations. The conformation with D a symmetry isotropically extends in the three dimensions of space due to the quasi-tetrahedral pattern of the metal atoms and Cp centroids. At variance from that, the D2h conformation is almost planar when the c p ligands are restricted t o their centroids. The doubly bridging oxygens only depart from planarity. In the tetrahedral conformation, the need for angular flexibility is taken care of, at least partly, by the unoccupied basis functions of the surrounding atoms, in a classical "basis set superposition effect" (BSSE). This BSSE is obviously less efficient when the molecular conformation (24) Hyla-Kryspin, I.; Demuynck, J.;Strich, A.; BBnard, M. J . Chem. Phys. 1981,75,3954.
[(q-Cd&dM/406
does not equally extend in the three dimensionsof space. The extreme case corresponds to diatomic molecules, the energy of which is very slowly convergent with respect to the angular quantum number of the polarization f~nctions.2~It is therefore not surprising that the extension of the basis set is consistently more favorable to the quasi-planar conformation of the niobium complex and of its vanadium equivalent. The result found for the vanadium complex raises a problem, however, since the structure of the parent complex [(q-C5Me5)Vl406has been characterized and is reported to have Td symmetry within experimental error.2 Since the energy difference computed in favor of the quasi-planar, D2h conformation reaches 26 kcalemol-l with the large basis set (Table l), the contradiction is expected to lie in the model chosen for the calculations, i.e. the complex with unsubstituted Cp rings [(tpCgH5)V1406. It has been proved already that the accumulation of four C5H5 ligands around a small organometallic core is susceptible t o induce an important distortion of the core because of interligand H..-H repulsive contacts.20 Such repulsions are not expected to develop in either configuration of the [(q-C5H5)Ml406 complexes (M = Nb or V) since the shortest H*..H distance, obtained in the D2h conformation of the vanadium complex, reaches 2.65 8, (Table 2). H.-H distances are significantly larger for the D2h conformation of the niobium complex (3.40 A) and particularly for the Dad conformations of both complexes (3.92 8, for M = V; 4.74 %, for M = Nb). This situation is likely to change when C5H5 is replaced by the bulky C5(CH3)5, as in the complexes of niobium and vanadium reported experimentally. If a strict rectangular arrangement is maintained for the centroids of the C5Me5 rin s in the D2h conformation, H.*-H contacts of -1.8 if should develop in the niobium complex. The repulsion generated by such contacts is not anymore negligible.20 However, the flexibility of the CsMe5 ring provides several possibilitiesto relax this small strain at low cost: rotation of the methyl groups; bending of the C-CH3 bonds out of the cycle plane; slight deviation of the Cp centroids out of the plane defined by the rectangle of metal atoms. A combination of those slight deformations apparently allows the Nb406 core to accommodate the bulky CsMes ligands without departing from its preferred D2h conformation. It should be noted that the adamantane-like conformation can accommodate CsMe5 ligands without generating H-*H contacts shorter than -3.2 A. Replacing niobium by vanadium yields a contraction of all interatomic distances by -10% (Table 2). This is sufficient for the steric strain to become dramatic for the planar conformation of [(q-C5Me5)V]406with H.--H repulsive contacts of -1 A. Even though this conformation is electronically favored as shown in the present work, the centroids of the C5Me5 rings cannot remain coplanar. They are expected to undergo a displacement toward a tetrahedral conformation which would most efficiently reduce the steric strain. This displacement (25) See for instance: (a) McLean, A. D.; Liu, B. Chem. Phys. Lett. 1983,101, 144. (b) Walch, P.; Bauschlicher, C. W., Jr.; Roos, B. 0.; Nelin. C. J. Chem. Phvs. Lett. 1983.103.175.(c) Bauschlicher. C. W.. Jr.; Partridge, H.; LaGghoff, S. R.; Taylor, P. R.; Walch, S. P. J.'Chem:
Phys. 1987,86,7007.
Organometallics, Vol. 14, No. 12, 1995 5669
would be the driving force leading the vanadiumoxygen core toward the adamantane-like stable conformation.
Summary and Conclusion Ab initio SCF and electron-couplingcalculations have been carried out on [(rj+,H5)Ml406(M = Nb and V) taken as models for the recently synthesized [(q-C5Me5)M]406 complexes. Two main conclusions emerge from this work. The first one is of theoretical interest and concerns the crucial importance of basis set extension and, more specifically, of including polarization functions to decide between the relative stabilities of the two most probable conformations. Those conformations are (i) an adamantane-like structure close to tetrahedral symmetry and (ii) a structure with D2h symmetry characterized by a planar arrangement of the metal atoms and of the Cp centroids. The use of a nonpolarized basis set artificially favors the conformation that extends in the three dimensions of space and can take advantage from the unoccupied basis functions of the surrounding atoms (basis set superposition error). A geometry optimization carried out with a basis set including diffuse functions but devoid of polarization functions gives a slight energetic advantage to the D2h form with Nb and t o the D2d form with V. Including polarization functions on all atoms of the metal-oxygen core clearly shows that the D2h conformation is more stable with vanadium as with niobium. The energy difference is comprised between 25 and 30 kcal*mol-l in both cases. This result is in agreement with the NMR spectra and magnetic behavior reported for the real niobium complex [(q-C5Me5)Nb]406. The four d electrons of the niobium cluster undergo a very weak coupling leading to the presence of three states with different spin multiplicities (l&, 3Blu,and 5&) within a few cm-'. No such coupling exists for the D2d conformation, and the quintet state 5B1 is found lowest by -250 cm-' with respect to quasidegenerate singlet and triplet states. A contradiction appears however between our calculations on the model complex of vanadium and the X-ray characterization of a tetrahedral conformation for the real molecule. The theoretical result is easily reconciled with the observed structure when considering the steric strain induced by the four bulky CsMe5 ligands in the D2h form. The present calculations eventually suggest that a vanadium complex incorporating four unsubstituted C5H5 cycles, would it be feasible, would probably adopt the conformation with D2h symmetry as the [(qC5Me5)Nbl406 complex.
Acknowledgment. Calculations have been carried out in part on the Cray C98 computer of the Institut de DBveloppement et de Ressources en Informatique Scientifique (IDRIS) and IBM RS6000 workstations purchased with funds provided by the DGICYT of the Government of Spain and by the CIRIT of Generalitat de Catalunva (Grant Nos. PB92-0766-C02-02 and GR9Q93-7063). The cooperation between the Strasbourg andTamagona POUPS has been supported by the HCM Contract ERBCHRXCT-930156. '
OM950538R
