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 [{(C5H5)Co}2(μ‐CR12BCBR2R3)] (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 (R1 = R2 = R3 = H) and 6c (R1 = R2 = H; R3 = C6H5). 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 σ2.978π1.501, and in 6b σ3.944π1.356. Electron density donation from one cobalt atom to an aryl group on the bridging ligand further contributes to the stabilization of the complexes 6. The stability of the planar environment of the tetraco‐ordinate carbon C2 in the diborylcarbene in di‐ cobalt complex 1 is achieved in a manner similar to that suggested by R. Hoffmann for planar methane. The results of extended Hückel and ab initio calculations indicate that C2 accepts σ and donates π electron density with respect to the two cobalt atoms.