We report the reactivity of a new Mn-I(cyclam) complex (cyclam: 1,4,8,11-tetraazacyclotetradecane) toward O-2 and H2O as a model for the photoinhibited species of oxygen-evolving complex (OEC). The reactivity varies according to the number of CO ligands. A Mn-I dicarbonyl complex, [Mn-I(cyclam)(CO)(2)](+) reacts with O-2, but not with H2O, to form a bis(mu-oxo) Mn-2(III,IV) complex, though a Mn-I tricarbonyl complex, [Mn-I(cyclam)(CO)(3)](+) does not react with either O-2 or H2O. Newly synthesized Mn-I(cyclam) dicarbonyl complex was characterized by ESI mass spectrometry, UV-vis absorption spectroscopy, IR spectroscopy, and X-ray analysis.