RESPONSES OF THE FE(CN)(2)(CO) UNIT TO ELECTRONIC CHANGES AS RELATED TO ITS ROLE IN [NIFE]HYDROGENASE

The observation of nearly identical infrared spectra in the diatomic ( 2000 cm(-1)) region of oxidized forms of [NiFe]hydrogenases, as isolat ed from Chromatium vinosum (Happe et al. Nature 1997, 385, 126) and De sulfovibrio 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 prom pted further development of the organometallic model complex as a spec troscopic reference. The vibrational spectroscopy of the pyramidal Fe( CN)(2)(CO) unit found in the salts of (eta(5)-C5H5)Fe(CN)(2)(Co)(-) an d (eta(5)-C-5- Me-5)Fe(CN)2(CO)(-) is thoroughly investigated with res pect to band positions and intensity ratios as influenced by counterio n 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 st ructure of [NiFe]H-2-ase enzyme isolated from the D. gigas bacteria. F or a series of nine complexes that covers a broad range of electronic effects las confirmed by electrochemical studies) within a constant he xacoordinate structure and medium (CH3CN), there is an excellent linea rity 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 t he model complex correlation and are expected to maintain hexacoordina tion about iron. The possibile source(s) of deviations from this corre lation both in the model tin aqueous media) and in the enzyme system a re discussed.