A contribution to the understanding of carbonyl migration in Mn-2(CO)(10) via the pairwise exchange mechanism

Density functional theory using flexible Gaussian basis sets was employed in an all-electron ab initio study of Mn-2(CO)(10) that focused on the origins of the absence of carbonyl fluxionality in this compound. Calculations predict a staggered arrangement of carbonyls (D-4d symmetry) to be the most stable conformation of Mn-2-(CO)(10), in agreement with experiment. Carbonyl migration then proceeds from the staggered conformer via rotation about the Mn-Mn bond to an eclipsed (D-4h) conformer, with a barrier of only 5 kcal/mol, and then to a symmetrical di-bridged (D-2h) conformer, through an additional barrier of 14-15 kcal/mol, and finally back to the staggered through the eclipsed conformer. The eclipsed conformer was found to be a transition state connecting two staggered conformers in the rotation about the Mn-Mn bond. The present estimate of 5 kcal/mol for the Mn-Mn rotation barrier is much lower than the previously reported value of 34 kcal/mol, and eliminates the rotational barrier as the sole origin of the absence of carbonyl migration in Mn-2(CO)(10). The present estimate of 19-21 kcal/mol for the overall activation energy for carbonyl scrambling in Mn-2-(CO)(10) is fairly close to the upper limit of 25 kcal/mol for processes which may be followed using NMR spectroscopy.