The observation of nearly identical infrared spectra in the diatomic (2000 cm(-1)) region of oxidized forms of [NiFe]hydrogenases, as isolated from Chromatium vinosum (Happe et al. Nature 1997, 385, 126) and Desulfovibrio gigas (Volbeda et al. J. Am. Chem. Sec. 1996, 118, 12989) and the anion (eta(5)-C5H5)Fe(CN)(2)(CO)(-) (Darensbourg et al. J. Am. Chem Sec. 1997, 119, 7903), including isotopic label shifts, has prompted further development of the organometallic model complex as a spectroscopic reference. The vibrational spectroscopy of the pyramidal Fe(CN)(2)(CO) unit found in the salts of (eta(5)-C5H5)Fe(CN)(2)(Co)(-) and (eta(5)-C-5- Me-5)Fe(CN)2(CO)(-) is thoroughly investigated with respect to band positions and intensity ratios as influenced by counterion and solvent. The neutral analogues (eta(5)-C5H5)- and (eta(5)-C5Me5)Fe(CN)(CO)(2) as well as the protonated H[(eta(5)-C5H5)Fe(CN)(2)(CO)] are included for comparison. The X-ray crystal structure of the latter finds short interionic N ... N distances of 2.55 Angstrom indicative of CN-nitrogen protonation and strong II-bonding as similarly seen in the attachment of Fe(CN)(2)(CO) to the protein found in the crystal structure of [NiFe]H-2-ase enzyme isolated from the D. gigas bacteria. For a series of nine complexes that covers a broad range of electronic effects las confirmed by electrochemical studies) within a constant hexacoordinate structure and medium (CH3CN), there is an excellent linearity in the correlation between nu(CO) (or F-CO) and nu(CN) (or F-CN) The enzyme states that are not in the catalytic cycle reasonably fit the model complex correlation and are expected to maintain hexacoordination about iron. The possibile source(s) of deviations from this correlation both in the model tin aqueous media) and in the enzyme system are discussed.