IR spectroscopy in concert with DFT calculations and resonance Raman spectroelectrochemistry has been used to identify the molecular orbital nature of the singly occupied molecular orbital (SOMO) in reduced [Re(CO)(3)CI(L)] and [Re(CO)(3)(4-Mepy)(L)](+) complexes, where L = 1,10-phenanthroline and its 4,7-diphenyl and 3,4,7,8-tetramethyl-substituted analogues. The SOMO of each reduced species considered was found to be of b(1) symmetry, rather than the close lying orbital of a(2) symmetry (within a C-2v symmetry description of the phenanthroline moiety). This was deduced in a number of ways. First, the average carbonyl band force constants (Ak(av) = k(av){reduced complex} - k(av){parent complex}) range from -57 to -41 N m(-1) for the series of compounds studied. The value of Delta k(av), relates to the extent of orbital overlap between the ligand MO and the metal d pi MO. These values are consistent with population of a b(1) MO because the wave function amplitude at the chelating nitrogens for this MO is significantly greater than that for a(2) MO. Second, calculations on singly reduced [Re(CO)(3)(4-Mepy)(phen)](+) and [Re(CO)(3)(4-Mepy)(tem)](+) predict population of a b(2) SOMO. The spectra predicted for these species are in close agreement with the vibrational spectroscopic data; for the IR data the shifts in the CO bands are predicted to 6 cm(-1) and the mean absolute deviation between calculated and measured Raman bands was found to be 10 cm(-1).