MOLECULAR-ORBITAL CALCULATIONS ON TRANSITION-METAL COMPLEXES .22. BIS-(BORABENZENE) COMPOUNDS OF THE 3D SERIES

INDO-SCF molecular orbital calculations have been carried out for the bis(borabenzene) M(C5H5BR)2 (R=H, M=Cr, Mn, Fe, Co; R=CH3, M=Fe, Co) complexes of the 3d series. The major contribution to the d-orbital splitting is found to arise from the interaction between the ligand π orbitals and the metal e1(dxy, dyz) orbitals, which become strongly antibonding, but the ligand σ orbitals play a significant role in the bonding and are substantially mixed with the metal e2(dx2-y2, dxy) and a1(dz2) levels. The ordering of the mainly metal 3d levels corresponds to the one-electron energy sequence [FORMULA OMMITED] similar to that observed for the metallocenes, and the calculated splittings for the Fe derivative agree closely with those deduced from the electronic spectrum of bis(1-methylborabenzene)iron via the ligand field model. As for ferrocene, however, the SCF orbital energies in Fe(C5H5BH)2 indicate the sequence dz2<dx2-y2~dxy, although electron loss from all three levels is calculated, ia the appropriate state energies for the corresponding cation, to be almost equienergetic. For the Co derivatives the splitting of the formerly degenerate e1(dxz, dyz) level, due to the ring heteroatoms, is found to be quite small (~3000 cm-1 or less), contrary to a recent interpretation of ESR data for bis(1-phenyl-borabenzene)cobalt, and a ground-state configuration [FORMULA OMMITED] is always more stable than [FORMULA OMMITED]. Moreover, simple ligand field considerations enable the ESR data to be reconciled with this conclusion. For the Cr system an effectively 3E(e23a1) rather than a 3A2(e22a12) state is calculated to lie lower, with only a small splitting of the e2 level, in accordance with the orbital contribution to the magnetic moment observed for the 1-methyl and 1-phenyl derivatives. Symmetrical Mn(C5H5BR)2 species are as yet unknown, but the results suggest that the 2E2(e23a12) and 2A, (e24a1 states should lie very close together, with the 6(e22a1e12) level slightly higher. The molecular orbital compositions for the M(C5H5BR)2 species show the borabenzene ligand to be intermediate between Cp (π-C5H5) and Bz (π-C6H6) in its bonding characteristics, but with an appreciably greater involvement of the ligand σs levels. © 1979, American Chemical Society. All rights reserved.