Sa. Decker et al., A CONTRIBUTION TO THE UNDERSTANDING OF CARBONYL MIGRATION IN MN-2(CO)(10) VIA THE PAIRWISE EXCHANGE MECHANISM, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 101(46), 1997, pp. 8734-8740
Density functional theory using flexible Gaussian basis sets was emplo
yed 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 symme
try) 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) conf
ormer, 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 pr
esent estimate of 5 kcal/mol for the Mn-Mn rotation barrier is much lo
wer than the previously reported value of 34 kcal/mol, and eliminates
the rotational barrier as the sole origin of the absence of carbonyl m
igration in Mn-2(CO)(10). The present estimate of 19-21 kcal/mol for t
he 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.