How realistic are alternating C–C-bond lengths in s-cis-1,3-butadiene transition metal complexes?

Abstract

A crystallographic redetermination of bis(s-cis-1,3-butadiene)monocarbonylmanganese showed that the earlier reported C–C bond lengths of the butadiene fragments are probably artefacts caused by pseudo symmetry problems. The problems of pseudo symmetry have been solved and the correct absolute structure has been assigned during the refinement of the structure in the non-centrosymmetric tetragonal space group P2₁m. This redetermination yielded balanced C–C distances for the complexed butadiene fragments. The molecular structure of the title compound has also been investigated by DFT-quantum chemical calculations [Becke3LYP/6-31+G(d) (C, H, O); 6-31G(f) (Mn)]. The typical understanding of bonding of butadienes to transition metal complexes and the experimental geometric parameters are in impressive agreement with the quantum chemical calculations. The problems discussed can also be expanded to molecular structures of isotypical complexes and similar pentamethylcyclopentadienyl transition metal complexes that crystallise in the space group P2₁m with the molecules located on a C_2v (mm2) site.