Envelope-Flip Dynamics in CpCo(Diene) Complexes: An ab Initio Quantum Mechanical Study

Abstract

Ab initio quantum mechanical methods are brought to bear on the problem of syn/anti isomerization of the terminal hydrogens in (η5-C5H5)Co(η 4-1,3-cis-butadiene) by an envelope-flip process. The first experimentally determined barrier to pairwise syn/anti interconversion in a group VIII highly substituted metal - diene complex is reported. Remarkable agreement between computational and experimental barriers is obtained. A path of stereoisomerization involving two discrete steps, ring flipping and inversion of pyramidality at cobalt, is elucidated in preference to a planar envelope-flip transition state. These results clarify the stereospecific rearrangement observed in two substituted diene derivatives.