Quantum‐Chemical Investigations of Stabilizing Interactions in μ‐Diborylcarbene Dicobalt Complexes with a Planar Tetracoordinate Carbon Atom

Quantum-chemical methods have been employed to study the nature of stabilization in dinuclear cobalt complexes of the general formula [{(C5 H5 )Co}2 (μ-CR(1) 2 BCBR(2) R(3) )] (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 Hückel and ab initio Hartree-Fock calculations have been carried out for the model compounds 6b (R(1) = R(2) = R(3) = H) and 6c (R(1) = R(2) = H; R(3) = C6 H5 ). Ab initio electron deformation density maps and natural population analysis calculations show that complexes 6 are stabilized through push-pull effects by which the ptC experiences π electron density delocalization and σ electron density accumulation. The calculated electronic configuration of the ptC in the free ligand 9b is σ(2.978) π(1.501) , and in 6b σ(3.944) π(1.356) . Electron density donation from one cobalt atom to an aryl group on the bridging ligand further contributes to the stabilization of the complexes 6.