Low-temperature flash photolysis of Mn-2(CO)(10), MnRe(CO)(10), and Re-2(CO)(10) in 3-methylpentane glass at 93 K leads to loss of CO as the sole net photochemical process. The initial product upon photolysis of MnRe(CO)(10) is the solvento form MnRe(CO)(9)(S) (S = solvent). In contrast with Mn-2(CO)(9)(S), which readily loses solvent at 93 K to form the semibridge form Mn-2(CO)(8)(mu-eta(1),eta(2)-CO), MnRe(CO)(9)(S) is stable. Upon warming of the glass, it recombines with CO without formation of the semibridge form. Irradiation with visible light at 93 K causes conversion to the semibridge form, MnRe(CO)(8)(mu-eta(1),eta(2),-CO), in which the CO is presumed to be bound in eta(1) fashion to Re and in eta(2) fashion to Mn. MnRe(CO)(8)(mu-eta(1),eta(2)-CO) reacts with CO somewhat more readily than does Mn-2(CO)(8)(mu-eta(1),eta(2)-CO). On irradiation of Re-2(CO)(10), CO loss results in formation of the solvento form Re-2(CO)(9)(S), which upon warming recombines with CO without intermediate formation of the semibridging form. Irradiation with visible light does not result in conversion of the solvento to the semibridging form. Comparisons of IR and reactivity data for dinuclear manganese and rhenium carbonyl semibridging structures suggest that phosphine substitution at the metal stabilizes the semibridge form largely through steric effects. Loss of a CO or other ligand, with subsequent semibridge formation, reduces steric crowding at the metal center. In addition, the semibridge species, once formed, is stabilized by the presence of phosphines, which block access by incoming nucleophiles.