Metal-to-ligand charge transfer photochemistry: Homolysis of the Mn-Cl bond in the mer-Mn(Cl)(CO)(3)(alpha-diimine) complex and its absence in the fac-isomer

Homolytic breaking of the axial metal-Cl bond is not observed upon irradiat ion at 488 nm of either fac-Mn(Cl)-(CO)(3)(alpha-diimine) or the parent Mn( Cl)(CO)(5). Surprisingly, it does occur for the equatorial Mn-Cl bond in se veral mer-Mn(Cl)(CO)(3)(alpha-diimine). Using DFT calculations it is shown that this photochemical homolysis can only be understood if strong relaxati on of the metal fragment occurs concurrent with the equatorial Cl departure , releasing sufficient energy to make the photodissociation energetically p ossible. The unrelaxed metal fragment with an equatorial vacancy would be v ery unstable (by 116 kJ/mol) with respect to the relaxed fragment with an a xial vacancy. The migration of an axial CO to the equatorial site invoked i n the proposed photodissociation mechanism does not occur on the potential energy surface of the photoactive excited state, which is bound in the Mn-C l dissociation coordinate. It is proposed to occur in a continuum state (ab ove the asymptotic energy) of the ground-state potential energy surface tha t is in resonance with the photoactive excited state. The possible importan ce of this mechanism for TM complex photochemistry, where rearrangement of ligands may often occur upon photodissociation, is stressed.