The energy barrier for CO migration in Mn2(CO)6(H2PCH2PH2)2: a PM3(tm) study

Carbonyl fluxionality in Mn 2 (CO) 6 (dhpm) 2 (dhpm = bis(dihydrophosphino)-methane), serving as a model of Mn 2 (CO) 6 (dppm) 2 (dppm = bis(diphenylphosphino)-methane), was studied using both the PM3(tm) semiempirical method and the DFT//PM3(tm) approach. When the PM3(tm) method is used for both geometry optimization and energy determination, the most stable conformer of Mn 2 (CO) 6 (dppm) 2 is predicted to possess six planar carbonyls, two of which are symmetrically bridging the Mn atoms, in contrast to the usually assumed planar all-terminal structure of Mn 2 (CO) 6 (dppm) 2 . Carbonyl scrambling via the pairwise exchange mechanism in Mn 2 (CO) 6 (dppm) 2 is predicted to occur as follows at the PM3(tm)//PM3(tm) level: lowest energy, planar symmetrically di-bridged conformer -–> planar, all-terminal conformer -–> lowest energy, planar symmetrically di-bridged conformer. The corresponding barrier to the exchange process is 32 kcal/mol, much higher than the experimental value of 10 kcal/mol in the related Mn 2 (CO) 6 (dppm) 2 complex. Calculation of the total energies using density functional theory at the PM3-optimized geometries improves results in two ways. Firstly, the energy barrier to scrambling is reduced to only 15-19 kcal/mol depending on the functional employed. Secondly, the energy ordering of the conformers is reversed, with the planar, all-terminal structure of Mn 2 (CO) 6 (dppm) 2 being more stable than the bridging structure. The resulting mechanism is identical to that found in our earlier work on Mn 2 (CO) 10 . When DFT is used to optimize the conformer geometries, the energy barrier is further reduced to 13 kcal/mol.Key words: PM3(tm) semiempirical method, density functional theory, carbonyl scrambling.