Photochemistry of CH3Mn(CO)5: a multiconfigurational ab initio study

The electronic spectroscopy of CH3Mn(CO)5 has been investigated by means of ab initio multiconfigurational MS-CASPT2/CASSCF calculations. The absorption spectrum is characterized by a series of Metal-Centered (MC) excited states in the UV energy domain (below 290 nm) that could be responsible for the observed photoreactivity starting at 308 nm. The upper part of the spectrum is overcrowded between 264 and 206 nm and dominated by a high density of Metal-to-Ligand-Charge-Transfer (MLCT) states corresponding mainly to 3d(Mn) --> pi*(CO) excitations. A non-negligible contribution of Metal-to-sigma-Bond-Charge-Transfer (MSBCT) states corresponding to 3d(Mn) --> sigma*(Mn-CH3) excitations is also present in the theoretical spectrum of CH3Mn(CO)5. However, in contrast to other transition metal hydrides and methyl substituted (HMn(CO)5, HCo(CO)4, and CH3Co(CO)4) these MSBCT transitions do not participate to the lowest bands of the spectrum as main contributions. The photochemistry of CH3Mn(CO)5, namely the loss of a CO ligand vs. the metal-methyl bond homolysis, is investigated by means of MS-CASPT2 states correlation diagrams. This study illustrates the complexity of the photodissociation mechanism of this class of molecules, which involves a large number of nearly degenerate electronic states with several channels for fragmentation.