Photolysis at ca. 350 nm of Mn 2(CO) 10 in the presence of each of I 2, CH 3I, SnI 4, CuCl 2 - 2 H 2O, HgX 2 (X Cl, Br or I), C 6H 5HgI, η 5-C 5H 5Cr(NO) 2Cl, (η 5-C 5H 5) 2Cr 2(NO) 4 and [Co 2(CNCH 3) 10](BF 4) 4, generally under N 2 or CO in several organic solvents (mainly cyclohexane and THF), was investigated. The observed photoreactions are best rationalized in terms of initial homolytic cleavage of the MnMn bond. In the presence of a halogen (X)-containing compound, the resultant Mn(CO) 5· radical abstracts X to yield Mn(CO) 5X. These reactions are characterized by high quantum efficiencies (generally, φ-Mn 2(CO) 10 ⪆0.36). Following the abstraction, the remaining metal-containing species (usually a radical), SnCl 3·, CuCl, HgX·, C 6H 5Hg· or η 5-C 5H 5Cr(NO) 2·, undergoes further abstraction of halogen by Mn(CO) 5·, coupling with the Mn(CO) 5· or ligand substitution. Isolated metal-containing products include SnI 2, Cu, Mn(CO) 5HgX, Hg, [Mn(CO) 5] 2Hg and η 5-C 5H 5Cr(CO) 2NO. Photolysis of Mn 2(CO) 10 in the presence of each of the metal-metal bonded compounds, (η 5-C 5H 5) 2Cr 2(NO) 4 and [Co 2(CNCH 3) 10](BF 4) 4, does not give the respective heterodinuclear combinations, (η 5-C 5H 5)(NO) 2CrMn(CO) 5 and [(CH 3NC) 5CoMn(CO) 5] 2+, as detectable or isolable species; instead, η 5-C 5H 5Cr(CO) 2NO, Mn(CO) 4NO, Mn 2(CO) 9CNCH 3, and [Mn(CO)(CNCH 3) 5] + are among the isolated products. The photoreaction between Mn 2(CO) 10 and CH 3I provides a convenient synthesis of Mn(CO) 5I.