Metal-to-Ligand Charge Transfer Photochemistry: Homolysis of the Mn−Cl Bond in the mer-Mn(Cl)(CO)3(α-diimine) Complex and Its Absence in the fac-Isomer

Homolytic breaking of the axial metal−Cl bond is not observed upon irradiation at 488 nm of either fac-Mn(Cl)(CO)3(α-diimine) or the parent Mn(Cl)(CO)5. Surprisingly, it does occur for the equatorial Mn−Cl bond in several mer-Mn(Cl)(CO)3(α-diimine). Using DFT calculations it is shown that this photochemical homolysis can only be understood if strong relaxation of the metal fragment occurs concurrent with the equatorial Cl departure, releasing sufficient energy to make the photodissociation energetically possible. The unrelaxed metal fragment with an equatorial vacancy would be very unstable (by 116 kJ/mol) with respect to the relaxed fragment with an axial vacancy. The migration of an axial CO to the equatorial site invoked in the proposed photodissociation mechanism does not occur on the potential energy surface of the photoactive excited state, which is bound in the Mn−Cl dissociation coordinate. It is proposed to occur in a continuum state (above the asymptotic energy) of the ground-state potential energy surface that is in resonance with the photoactive excited state. The possible importance of this mechanism for TM complex photochemistry, where rearrangement of ligands may often occur upon photodissociation, is stressed.