Using a new technique, which combines pulse radiolysis with nanosecond time-resolved infrared (TRIR) spectroscopy in the condensed phase, we have conducted a detailed kinetic and mechanistic investigation of the formation of a Mn-based CO2 reduction electrocatalyst, [Mn(Bu-t(2)-bpy)(CO)(3)](2) (Bu-t(2)-bpy = 4,4'-Bu-t(2)-2,2'-bipyridine), in acetonitrile. The use of TRIR allowed, for the first time, direct observation of all the intermediates involved in this process. Addition of excess [(Bu4N)-Bu-n][HCO2] to an acetonitrile solution of fac-MnBr(Bu-t(2)-bpy)(CO)(3) results in its quantitative conversion to the Mn-formate complex, fac-Mn(OCHO)(Bu-t(2)-bpy)(CO)(3), which is a precatalyst for the electrocatalytic reduction of CO2. Formation of the catalyst is initiated by one-electron reduction of the Mn-formate precatalyst, which produces the bpy ligand-based radical. This radical undergoes extremely rapid (tau = 77 ns) formate dissociation accompanied by a free valence shift to yield the five-coordinate Mn-based radical, Mn-center dot(Bu-t(2)-bpy)(CO)(3). TRIR data also provide evidence that the Mn-centered radical does not bind acetonitrile prior to its dimerization. This reaction occurs with a characteristically high radical-radical recombination rate (2k(dim) = (1.3 +/- 0.1) x 10(9) M-1 s(-1)), generating the catalytically active Mn-Mn bound dimer.