[(ETA-C5H5)M](4)O-6 (M=NB,V) - AB-INITIO CALCULATIONS PREDICT EQUIVALENT METAL ATOMS IN A RECTANGULAR CONFORMATION

Ab initio SCF calculations have been carried out on the organometallic model clusters [(eta-C5H5)M]O-4(6) (M = Nb, V), for the purpose of comparing the energies of the various conformations proposed for the niobium complex [(eta-C(5)Me(5))Nb]O-4(6). 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 D-2h 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 [(eta-C5H5)M]O-4(6) model also predicts a conformation with D-2h symmetry for M = V. This prediction is in contradiction with the reported Td structure of the [(eta-C(5)Me(5))V]O-4(6) complex, because the model does not account for the important steric strain induced by the coplanar position of the centroids of the C(5)Me(5) ligands. A change in the structure of the vanadium complex could therefore be expected from the replacement of C(5)Me(5) by less bulky Cp ligands.