Transition-metal complexes, CoHMD 2 + (HMD=5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene) and Ru(bpy) 2 (CO)X n + (bpy=2,2'-bipyridine, X=CO, Cl, H, etc.), mediate electron transfer in the photochemical reduction of CO 2 . The thermodynamics and kinetics of CO 2 binding to CoHMD + , and spectroscopic characterization of the CO 2 adducts of CoHMD + and [Ru I (bpy)(bpy - )(CO)] are described.