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 potential energy curves (PEG) exhibit Mn-COeq and Mn-COax bonding minima around R(e). Loss of an equatorial CO ligand upon MLCT elicitation is explained by a radiationless transition from the MLCT states to the dissociative continuum of the electronic ground state. According to the calculated PEC of the ground state, the complex will undergo a strong structural rearrangement upon equatorial CO dissociation, during which the chloride shifts to the equatorial open site. This rearrangement explains the experimentally found formation of mer-Mn(Cl)(CO)(3)(alpha-diimine) complexes upon back-reaction of their CO-loss product with CO. This mechanism of equatorial CO dissociation is very different from the usual photochemical dissociation directly from a dissociative ligand-field state or through crossing of the photoactive excited state by such 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.