A Contribution to the Understanding of Carbonyl Migration in Mn₂(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.