CNDO/2 study of the isoelectronic series of complexes dicarbonyl(dinitrogen)- and dicarbonyl-η-benzenechromium, -η-cyclopentadienyl-manganese, -η-cyclobutadieneiron, and -trismethylenemethane-iron obtained by the matrix–isolation technique

Abstract

The electronic structure of the isoelectronic series of [ML(CO)₂(N₂)] and [ML(CO)₂]complexes [ML = Cr^–(η-C₆H₆), Mn(η-C₅H₅), Fe(η-C₄H₄), or Fe{C(CH₂)₃}] has been investigated within a CNDO/2 formalism, using both experimental and standard geometries. The computed trends for CO and N₂ bond strengths in the complexes, as measured by Wiberg indices, charge distributions, and orbital populations, are well correlated with the experimental CO and N₂ stretching frequencies and energy-factored stretching force constants, i.e. Cr < Mn < Fe and [ML(CO)₂] < [ML(CO)₂(N₂)] < [ML(CO)₃]. The computed total energies of the [ML(CO)₂] complexes suggest that in all cases a tilted geometry is preferred, i.e. the plane of the L ligand is not perpendicular to the M(CO)₂ plane. New CNDO/2 calculations have been carried out for [Fe{C(CH₂)₃}(CO)₃] which is shown to fit in well with other [ML(CO)₃] complexes.