Vibrational spectroscopy of reduced Re(I) complexes of 1,10-phenanthroline and substituted analogues

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).