The bidentate coordination of an CL-keto acid to an iron(II) center via the keto group and the carboxylate gives rise to metal-to-ligand charge-transfer transitions between 400 and 600 nm in model complexes and in a-ketoglutarate-dependent dioxygenases. Excitation into these absorption bands of the Fe(II)TauD(alpha -KG) complex (TauD = taurine/alpha -ketoglutarate dioxygenase, alpha -KG = alpha -ketoglutarate) elicits two resonance Raman features at 460 and 1686 cm(-1), both of which are sensitive to O-18 labeling. Corresponding studies of model complexes, the six-coordinate [Fe(II)(6-Me-3-TPA)(alpha -keto acid)](+) and the five-coordinate [Fe(II)(Tp(Ph2))- (alpha -keto acid)] (6-Me-3-TPA = tris[(6-methyl-2-pyridyl)methyl]amine, Tp(Ph2) = hydrotris(3,5-diphenylpyrazol-1-yl)borate), lead to the assignment of these two features to the Fe(II)(a-keto acid) chelate mode and the v(C=O) of the keto carbonyl group, respectively. Furthermore, the chelate mode is sensitive to the coordination number of the metal center: binding of a sixth ligand to the five-coordinate [Fe(II)(Tp(Ph2)) (benzoylformate)] elicits a 9-20 cm(-1) downshift. Thus, the 10 cm(-1) upshift of the chelate mode observed for Fe(II)TauD(alpha -KG) upon the addition of the substrate, taurine, is associated with the conversion of the six-coordinate metal center to a five-coordinate center. as observed for the iron center of clavaminate synthase from X-ray crystallography (Zhang, Z.: et al. Nat. Struct. Biol. 2000, 7. 127-133) and MCD studies (Zhou, J.; et al. J. Am. Chem. Sec. 1998, 120, 13539-13540). These studies provide useful insights into the initial steps of the oxygen activation mechanism of alpha -ketoglutarate-dependent dioxygenases.