Abstract Photolysis of [M(CO)5M′(CO)3(dab)] (M, M′ = Mn, Re; dab = 1,4-diazabuta-1,3-diene, RNCHCHNR) in 2-Me-THF leads to both homolytic and heterolytic splitting of the metalmetal bond depending on the solution temperature. In a rigid medium such as a CH4-matrix no breaking of the metalmetal bond is observed but instead formation of [M(CO)3M′(CO)3(dab)] in which compound the dab-ligand is σ