METAL-TO-LIGAND CHARGE-TRANSFER (MLCT) PHOTOCHEMISTRY OF FAC-MN(CL)(CO)(3)(H-DAB) - A DENSITY-FUNCTIONAL STUDY

The title compound has low-energy Mn-3d to 1,4-diaza-1,3-butadiene (H- DAB),pi metal-to-ligand charge transfer (MLCT) excited states, which are not, by their electronic nature, Mn-CO dissociative. Their potenti al energy curves (PEG) exhibit Mn-COeq and Mn-COax bonding minima arou nd R(e). Loss of an equatorial CO ligand upon MLCT elicitation is expl ained by a radiationless transition from the MLCT states to the dissoc iative continuum of the electronic ground state. According to the calc ulated PEC of the ground state, the complex will undergo a strong stru ctural rearrangement upon equatorial CO dissociation, during which the chloride shifts to the equatorial open site. This rearrangement expla ins the experimentally found formation of mer-Mn(Cl)(CO)(3)(alpha-diim ine) complexes upon back-reaction of their CO-loss product with CO. Th is mechanism of equatorial CO dissociation is very different from the usual photochemical dissociation directly from a dissociative ligand-f ield state or through crossing of the photoactive excited state by suc h a ligand field state. In contrast, axial CO dissociation, which does not occur readily, does not give rise to structural rearrangement and is predicted to produce the fac complex upon back-reaction with CO.