Resonance Raman spectra have been obtained for (PPh(4))(2)[(MoO2)-O-VI(1,2-dicyanoethylene-1,2-dithiolato)(2)]. 2MeOH (1), (NEt(4))(2)[M(VI)O(2)(1,2-benzenedithiolato)(2)] (M = W (3a) and Mo (4a)), (NEt(4))(2)[(MoO2)-O-VI(3,4-toluenedithiolato)(2)] (5), in situ formed (NEt(4))(2)[(MoO2)-O-VI(1,2-bis(methoxycarbonyl)ethylene-1,2-dithiolato)(2)] (2), (PPh(4))(2)[(MoO)-O-IV(1,2-dicyanoethylene-1,2-dithiolato)(2)] (6), and (NEt(4))(2)[M(IV)O(1,2-benzenedithiolato)(2)] (M = W (8) and Mo (9)) which are related to the active site of molybdenum and tungsten oxidoreductases. For 1, 3a, 4a, and 5, nu(s) and nu(as) M(VI)=O bands are observed in the 885 - 858 and 851 - 835 cm(-1) ranges, respectively, which appear at a quite low wavenumber region, due to the M(VI)=O bond activation by the mutual trans influence with the thiolato. The excitation profiles of the nu(s)(M(VI)=O) band for 1, 3a, 4a, and 5 show clear enhancement at 700 - 600 nm in the lowest-energy region of LMCT bands. The cause of the enhancement is attributable to the vibronic coupling of similar geometric changes between the symmetric M(VI)=O stretching and the substantial elongation of the M(VI)=O bond when an excited electron half occupies the LUMO (M(VI)=O antibonding). For the dithiolene complexes, 1, 2, 6, and 7, the nu(C=C) bands (range 1472 - 1535 cm(-1)) are observed at a lower region than those of dimethyl sulfoxide reductase (1568 (oxidized state) and 1575 cm(-1) (reduced state)). Although the frequency shift upon oxidation of dimethyl sulfoxide reductase is +7 cm(-1), those of the model complexes are -19 (for 1 and 6) and -32 cm(-1) (for 2 and 7). These dithiolene frequencies of the enzyme are very anomalous compared with the various dithiolene compounds.