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 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.