In situ FT-IR measurements and electronic structure calculations are reported for the reduction of CO_2 catalyzed by the macrocyclic complex [Co^(II)N_4H]^(2+) (N_4H = 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(17),2,11,13,15-pentaene). Beginning from the [Co^(II)N_4H]^(2+) resting state of the complex in wet acetonitrile solution, two different visible light sensitizers with substantially different reducing power are employed to access reduced states. Accessing reduced states of the complex with a [Ru(bpy)_3]^(2+) sensitizer yields an infrared band at 1670 cm^(–1) attributed to carboxylate, which is also observed for an authentic sample of the one-electron reduced complex [CoN_4H(MeCN)]^+ in CO_2 saturated acetonitrile solution. The results are interpreted based on calculations using the pure BP86 functional that correctly reproduces experimental geometries. Continuum solvation effects are also included. The calculations show that Co is reduced to Co^I in the first reduction, which is consistent with experimental d–d spectra of square Co(I) macrocycle complexes. The energy of the CO_2 adduct of the one-electron reduced catalyst complex is essentially the same as for [CoN_4H(MeCN)]^+, which implies that only a fraction of the latter forms an adduct with CO_2. By contrast, the calculations indicate a crucial role for redox noninnocence of the macrocyclic ligand in the doubly reduced state, [Co^I(N_4H) –•], and show that [Co^I(N_4H) –•] binds partially reduced CO_2 fairly strongly. Experimentally accessing [Co^I(N_4H) –•] with an Ir(bpy)_3 sensitizer with greater reducing power closes the catalytic cycle as FT-IR spectroscopy shows CO production. Use of isotopically substituted C^(18)O_2 also shows clear evidence for ^(18)O-substituted byproducts from CO_2 reduction to CO
