QUANTUM-CHEMICAL INVESTIGATIONS OF STABILIZING INTERACTIONS IN MU-DIBORYLCARBENE DICOBALT COMPLEXES WITH A PLANAR TETRACOORDINATE CARBON-ATOM

Quantum-chemical methods have been employed to study the nature of sta bilization in dinuclear cobalt complexes of the general formula [{(C5H 5)Co}(2)(mu-(CR2BCBRR3)-B-1-R-2)] (6) as well as the ''antivan't Hoff- Le Bel'' configuration of the planar tetracoordinate carbon (ptC) atom of the bridging diborylcarbene ligand 9. Extended Huckel and ab initi o Hartree-Fock calculations have been carried out for the model compou nds 6b (R-1 = R-2 = R-3 = H) and 6c (R-1 = R-2 = H; R-3 = C6H5). Ab in itio electron deformation density maps and natural population analysis calculations show that complexes 6 are stabilized through push-pull e ffects by which the ptC experiences pi electron density delocalization and sigma electron density accumulation. The calculated electronic co nfiguration of the ptC in the free ligand 9b is sigma(2.978)pi(1.501), and in 6b sigma(3.944)pi(1.356). Electron density donation from one c obalt atom to an aryl group on the bridging ligand further contributes to the stabilization of the complexes 6.