Organometallics 1993, 12, 47-55
47
Theoretical Investigation of the Electronic Structure of the Mixed-Sandwich Complex (q5-Cyclopentadienyl)(q6-benzene)ironand Its Cation A. Le Beuze,**+ R. Lissillour,+and J. Weber$ Laboratoire de Chimie Thborique, U A CNRS 1495, Universitb de Rennes I, Avenue du General Leclerc, 35042 Rennes Cedex, France, and Dbpartement de Chimie Physique, Universitb de Genhe, 30 quai Ernest-Ansermet, 1211 G e n b e 4, Switzerland Received January 21, 1992 SCF M S - X a calculations have been carried out for the d6 [(Cp)Fe(Bz)]+ (1) and d7 (Cp)Fe(Bz) (2) compounds (Cp = q5-cyclopentadienyl,Bz = +benzene), and the results are compared with those previously reported for metallocenes Fe(Cp)2 (3), Co(Cp)z (4), [Ni(Cp)2]+ (6), and d8Ni(Cp)z (6) using the same technique. Analysis of the electronic structure of 1 and 2 shows that whereas Cp and Bz ligands play roughly the same role in d i g a n d to metal donation, they behave differently as far as 3d6 back-donation is concerned, the Bz system having a much more important role in this case. This is confirmed by spin-polarized (SP)calculations performed for 2 and 4, which show that the 3d6 orbitals exhibit a much larger interaction with the Bz ligand than with the Cp group. In addition, a considerable charge relaxation occurs a t the metal upon ionization of 2, as the gross charges on the iron are roughly the same in 1 and 2. When using the MS-Xa results to calculate spectroscopic properties, a good agreement is found between theoretical and experimental results determined for both the electron field gradient (EFG)a t the metal nucleus and Mossbauer isomer shift (IS)for compounds 1-3. Similarly, both MS-Xa ionization and electronic excitation energies calculated for 1 and 2 compare fairly well with photoelectron and UV-visible absorption data.
I. Introduction The electronic structures of metallocene and bis(arene) sandwich complexes have been the subject of numerous theoretical investigations by means of ligand field, semiempirical, and the more sophisiticated SCF ab initio and Xa methods.' The results of these studies have been, in general, directly compared with spectroscopic data, which has largely contributed to a better understanding of the chemical bonding in these complexes. In contrast to these symmetric bis(benzene) and bis(cyclopentadieny1) complexes, few papers have been published concerning the electronic structure of dissymmetric mixed-sandwich species.2 A simple prototype of dissymmetric mixed sandwiches can be found in the 19e- (Cp)Fe(Bz)complex, a reduced form of a stable and totally reversible system (Scheme I) exhibiting a high redox p~tential.~ As several experimental studies3have been performed for this series of compounds, it was of interest to carry out a theoretical investigation de Rennes I. Universite de GenBve.
+ Universite f
(1) (a) Prins, R. Mol. Phys. 1970,19,603. (b) Aderson, S. E.; Drago, R. Inorg. Chem. 1972, 11, 1664. (c) CoutiBre, M. M.; Demuynck, J.; Veillard, A. Theor. Chim. Acta 1972,27,281. (d) RBsch, N.; Johneon, K. H. Chem. Phys. Lett. 1974,24,179. (e) Botrel, A.; Dibout, P.; Lissillour, R. Theor. Chim. Acta 1975,37,37. (0 Bagus,P. S.; Walgren, U. I.; Almlof, J. J. Chem. Phys. 1976,64,2324. (g) Lissillour, R. J.Chim. Phys. 1979, 76,1103. (h) Haaland, A. Acc. Chem. Res. 1979,12,415. (i) Goursot, A.; Penigault,E.; Weber, J.Nouo. J.Chim. 1979,3,675. (i)Weber, J.;Goursot, A.; Penigault, E.; Ammeter, J. H.; Backman, J. J. Am. Chem. SOC. 1982, 104, 1491. (k) Rbch, N.; Streitwieser, A., Jr.; J. Am. Chem. SOC. 1983, 105,7237. (2) (a) Clack, D. W.; Warren, K. D. J. Organomet. Chem. 1978, 152,
C60. (b) Mariot, J. P.; Michaud, P.; Iauer, S.; Astruc, D.; Trautwein, A. X.; Varet, F. J.Phys. 1983,44,1377. (c) Lacoste, M.; Rabaa, H.; Astruc,
D.; Le Beuze, A.; Saillard, J. Y.;Precigoux, G.; Courseille, C.; Ardouin, N.; Bowyer, W. Organometallics 1989,8, 2233. (3) (a) Astruc, D. Acc. Chem. Res. 1986,19,377. (b) Astruc, D. Chem. Rev. 1988,88, 1189 and references therein.
Scheme I
fe+
___)
c -
Fe
@kRI 1 WII)
2 Fe(I)
using the SCF MS-Xa model. Indeed, the study of such transition-metal complexes has turned out to be an ideal field for the application of the MS-Xa method.1dJJs4 As detailed Xa calculations have been carried out for the d6 ferrocene,ld d7 cobaltocene,lj and d8 nickelocene and its cation," it was therefore of interest to perform similar calculations on the d6 and d7 Cp/Bz (Cp = v5-cyclopentadienyl; Bz = $-benzene) mixed compounds in order to extend the theoretical information to these homogeneous series of compounds. The main purpose of the present work is to provide a realistic description of the electronic structure of the 19(4) (a) Case, D. A.; Karplus, M. J.Am. Chem. SOC. 1977,99,6132. (b) Case, D. A.; Huynh, B. H.;Karplus, M. J. Am. Chem. SOC.1979,101,433. (c) Norman, J. R., Jr.; Renzoni, G. E.; Case, D. A. J. Am. Chem. SOC.1979, 101,5265. (d) Lie, S. K.; Taft, C. A. Chem. Phys. Lett. 1982,89,463. (e) Sunil, K. K.; Harrisson, J. F.; Rogers, M. J. Chem. Phys. 1982, 76,3078. (0 Bencini, A.; Gatteschi, D. J. Am. Chem. SOC. 1983, 105, 5535. (g) Maroney, M. J.; Norman, J. G., Jr.; Osborne, H.Inorg. Chem. 1984,23, 2261. (h) Perfield, K. W.; Gewith, A. A.; Solomon, E. I. J. Am. Chem. SOC.1985,107,4519. (i) Noodleman, L.; Norman, J. G., Jr.; Oebome, H.; Aizman, A,; Case, D. A. J. Am. Chem. SOC.1985,107,3418. (i)Gomot, A.; Chermette, H.; Chanon, M.; Waltz, W. L. Inorg. Chem. 1986,24,1042. (k) Weber, J.; K h d i g , E. P.; Goursot, A,; Penigault, E. Can. J. Chem. 1985,63,1734. (1) Bencini, A.; Gatteachi, D. J. Am. Chem. SOC. 1986,108, 5763. (m) Weber, J.; Chermette, H.; Heck, J.; Organometallics 1989,8, 2544. (n) Applications of the Density Functional to the Properties of Inorganic Systems. Presented a t a symposium in Arles, France, Sept 27, 1988; J. Chim. Phys. 1989,86 (4).
0276-7333/93/2312-0047$04.00/00 1993 American Chemical Society
48 Organometallics, Vol. 12, No. 1, 1993 Scheme I1 z
electron mixed sandwich (Cp)Fe(Bz) (d7) and its cation (d6). First, we shall describe the main features of the chemical bonding in [(Cp)Fe(Bz)l+(1) and (Cp)Fe(Bz) (2) and then compare them with those of their d6,d7, and d8symmetric analogues, namely Fe(Cp)z (31, Co(Cp)z (41, [Ni(Cp)zl+(5), and Ni(Cp)z (6). Then, polarization effects of the unpaired electron on bonding in 2 and 4 will be discussed. Moreover, these calculations should give additional information with regard to the total s-electron densities at the nucleus in order to analyze the electronic hyperfine parameters and directly compare with Mossbauer results. Finally, theoretical results concerning ionization and electronic excitation energies of 1 and 2 will be compared with experimental data such as photoelectron5 and UV-visible absorption6 spectroscopy.
11. Computational Details The standard version of the density functional SCF MS-Xa method’ is used and applied to complexes 1 and 2. It is wellknown that this model is based on the local Hartree-Fock-Slater Hamiltonian,“ whose eigensolutions are found by using the multiple-scattering (or scattered-wave (SW)) approximation.’ The method has been described in detail in several articles,7S8 and it does not need further development here. However, some computational details as well as the choice of the calculation parameter deserve comment. A standard C, symmetry is assumed for the complexes, using planar ligand rings and standard C-C (1.40 A) and C-H (1.10 A) bond lengths. The metal distances Fe-Cp = 1.78 8, and Fe-Bz = 1.54 A for the radical complex and Fe+-Cp = 1.68 8, and Fe+Bz = 1.55 8, for the cation have been taken from structural data availablefor related substituted compounds? In the C, symmetry (i.e. the yz plane being the mirror plane), the two conformations I and I1 are possible, differing in rotation of the Bz ring by 90’ (see Scheme 11). Calculations have been carried out for these two conformations. The radii of muffin-tin spheres, characteristic of the MS-Xa model, were chosen as follows: starting from geometry-induced touching spheres, obtained for metal and ligand atoms, the carbon radii were enlarged by 25% to provide a better description of the ring systems and an additional empty sphere (E) was located in the center of each ligand ring. This procedure, which has been shownlij to give reliable results for similar complexes, leads to the muffin-tin radii (au) r(Fe) = 2.2795, r(C) = 1.6535, r(H) = 0.8758, r(E-Bz) = 1.3226, and r(E-Cp) = 1.0843. Finally, the radii of the outer spheres (?(OUT)) of the complexes 2 and 1 are 6.4246 and 6.4336 au, respectively. The exchange scaling parameters a were taken from the tabulation of SchwarzlOafor Fe (0.711 51) and C (0.753 31) and (5)Green, J. C.; Kelly, M. R.; Payne, M. P.; Seydon, E. A.; Astruc, D.; Hamon,J. R.; Michaud, P. Organometallics 1983,2, 211. Hamon,J. R.;Althoff, G.;Roman,E.;Batail, P.;Michaud, (6)Astruc, D.; P.; Mariot, J. P.; Varret, F.; Cozak, D. J.Am. Chem. SOC.1979,101,5445. (7) Johnson, K. H. Ado. Quantum Chem. 1973,7,143. (8) (a) Slater, J. C. Quantum Theory of Molecules and Solids; McGraw-Hill: New York, 1974. (b) Danese, J. B.; Connoly, J. W. D. J. Chem. Phys. 1974,61,3063. (9) Hamon, J. R.; Astruc, D.; Roman, E.; Batail, P.; Mayerle, J. J. J. Am. Chem. SOC.1983,103,2431. (10)(a) Schwarz, K. Phys. Reu. 1972,B6.2466. (b) Slater, J. C. Int. J. Quantum Chem., Quantum Chem. Symp. 1973,7,533.
Le Beuze et al. from an article by Slaterlobfor H (0.777 25). The a parameters of the empty spheres and the outer sphere (a(E-Bz) = a(E-Cp) = a(0UT) = 0.762 90) are obtained from the weighted average of the atomic values. The maximum L values in the partial wave expansion included in the calculation were L = 2 in the metal sphere, L = 1 for carbon and empty spheres, L = 0 for hydrogen spheres, and L = 3 in the extramolecular region. For the cation, a Watson sphere,” having the same radius as the outer sphere and bearing the charge -1, has been introduced in order to mimic the external stabilizing electrostatic field. The transition-state method12 has been used to calculate ionization and excitation energies.
111. Results and Discussion A. Electronic Structure and Chemical Bonding. (i) The Mixed (Cp)Fe(Bz) Radical 2. Non-spinpolarized (NSP) energy levels and charge distributions are presented in Table I for this radical in conformation I, and the corresponding molecular orbital (MO) diagram is depicted in Figure 1. The sequence of MO’s with predominant Fe 3d character is 18a’(x2 - y2), 12a”(xy) [3ds41< 19a’(z2) [3da21
