Thermal stability of Group 6 bis(cyclopentadienyl) and ethylene bridged bis(cyclopentadienyl) monocarbonyl complexes; a theoretical study

Abstract

Density functional calculations have been used to estimate the energy change involved in dissociation of CO from [M(η-C₅H₅)₂(CO)] (M = Cr, Mo or W) and [Cr{C₂H₄(η-C₅H₄)₂}(CO)] to give, in each case, free CO and a triplet metallocene product. The dissociation energy for [Mη(-C₅H₅)₂(CO)] is calculated to be 99 (Cr), 186 (Mo) and 194 kJ mol^–1 (W) showing [Cr(η-C₅H₅)₂(CO)] to be significantly less thermally stable than the Mo or W analogues consistent with the experimental findings. For the ansa-bridged compound [Cr{C₂H₄(η-C₅H₄)₂}(CO)] the calculated dissociation energy was 112 kJ mol^–1, significantly more endothermic than for [Cr(η-C₅H₅)₂(CO)], consistent with the enhanced thermal stability found for [Cr{C₂Me₄(η-C₅H₄)₂}(CO)] as a result of introduction of an ansa-bridge. Theoretical analysis shows that the cause of the differences in thermal stability lies in the stability of the triplet metallocene product. The high spin-pairing energy for Cr favours formation of a triplet state more than is the case for Mo and W. Formation of a triplet state is of less advantage in the ansa-bridged free chromocene as the rings are unable to relax to a near parallel structure, thereby lowering the energy of the d electrons. Such relaxation is prevented by the presence of an ansa-bridge.