Envelope-flip dynamics in CpCo(diene) complexes: an ab initio quantum mechanical study

Ab initio quantum mechanical methods are brought to bear on the problem of syn/anti isomerization of the terminal hydrogens in (eta(5)-C5H5)Co(eta(4)- 1,3-cis-butadiene) by an envelope-flip process. The first experimentally de termined barrier to pairwise syn/anti interconversion in a group VIII highl y substituted metal-diene complex is reported. Remarkable agreement between computational and experimental barriers is obtained. A path of stereoisome rization involving two discrete steps, ring flipping and inversion of pyram idality at cobalt, is elucidated in preference to a planar envelope-flip tr ansition state. These results clarify the stereospecific rearrangement obse rved in two substituted diene derivatives.