To gain insight into thermodynamic barriers for reduction of CO into CH3OH, free energies for reduction of [CpRe(PPh3)(NO)(CO)](+) into CpRe(PPh3)(NO)(CH2OH) have been determined from experimental measurements. Using model complexes, the free energies for the transfer of H+, H-, and e(-) have been determined. A pK(a) of 10.6 was estimated for [CpRe(PPh3)(NO)(CHOH)](+) by measuring the pK(a) for the analogous [CpRe(PPh3)(NO)(CMeOH)](+). The hydride donor ability (Delta G(H-)degrees) of CpRe(PPh3)(NO)(CH2OH) was estimated to be 58.0 kcal mol(-1), based on calorimetry measurements of the hydride-transfer reaction between CpRe(PPh3)(NO)(CHO) and [CpRe(PPh3)(NO)(CHOMe)](+) to generate the methylated analogue, CpRe(PPh3)(NO)(CH2OMe). Cyclic voltammograms recorded on CpRe(PPh3)(NO)(CMeO), CpRe(PPh3)(NO)(CH2OMe), and [CpRe(PPh3)(NO)(CHOMe)](+) displayed either a quasireversible oxidation (neutral species) or reduction (cationic species). These potentials were used as estimates for the oxidation of CpRe(PPh3)(NO)(CHO) or CpRe(PPh3)(NO)(CH2OH) or the reduction of [CpRe(PPh3)(NO)(CHOH)](+). Combination of the thermodynamic data permits construction of three-dimensional free energy landscapes under varying conditions of pH and PH2. The free energy for H-2 addition (Delta GH(2)degrees) to [CpRe(PPh3)(NO)(CO)](+) (+15 kcal mol(-1)) was identified as the most significant thermodynamic impediment for the reduction of CO. DFT computations on a series of [(CpM)-M-X(L)(NO)(CO)](+) (M = Re, Mn) complexes indicate that Delta GH(2)degrees can be varied by 11 kcal mol(-1) through variation of both the ancillary ligands and the metal.