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A Contribution to the Understanding of Carbonyl Migration in Mn 2 (CO) 10 via the Pairwise Exchange Mechanism

1997; American Chemical Society; Volume: 101; Issue: 46 Linguagem: Inglês

10.1021/jp972355j

1520-5215

Stephen A. Decker, Oreola Donini, Mariusz Kłobukowski,

Molecular spectroscopy and chirality

Density functional theory using flexible Gaussian basis sets was employed in an all-electron ab initio study of Mn2(CO)10 that focused on the origins of the absence of carbonyl fluxionality in this compound. Calculations predict a staggered arrangement of carbonyls (D4d symmetry) to be the most stable conformation of Mn2(CO)10, in agreement with experiment. Carbonyl migration then proceeds from the staggered conformer via rotation about the Mn−Mn bond to an eclipsed (D4h) conformer, with a barrier of only 5 kcal/mol, and then to a symmetrical di-bridged (D2h) 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 Mn2(CO)10. The present estimate of 19−21 kcal/mol for the overall activation energy for carbonyl scrambling in Mn2(CO)10 is fairly close to the upper limit of 25 kcal/mol for processes which may be followed using NMR spectroscopy.