UV-VIS, HZ AND EPR SPECTROELECTROCHEMICAL STUDY OF THE EC REDOX TRANSITION [(PR(3))(N)(CO)(3)(R'-PZ)M](+ 0) M=MO, W R'-PZ=N-ALKYLPYRAZINIUMR=ISOPROPYL, CYCLOHEXYL N=1 OR 2/

The title complexes, isolated as trans,mer-configurated salts [(PR(3)) (2)(CO)(3)(R'-pz)M](PF6), display a strong pi interaction between the zerovalent metal center and the excellent pi acceptor ligands R-pz(+). The solvatochromism of the long-wavelength metal-to-ligand charge tra nsfer (MLCT) absorption as analyzed in particular for [(PCy(3))(2)(CO) (3)(CH3(CH2)(17)-pz)W](PF6) correlates not with established MLCT param eters but with the donor numbers of the solvents. The seemingly revers ible one-electron oxidation and reduction processes were studied by sp ectroelectrochemical techniques with regard to EPR, carbonyl vibration al and UV-vis/NIR spectroscopic features. Whereas oxidation leads to l abile M(I) species with 17 valence electrons, the reduction is centere d at the R'-pz(+) ligand and causes a rapid reversible dissociation of one PR(3) group from the metal (EC(rev) process). Both EPR and IR dat a suggest pentacoordination at the 16 + delta valence electron centers in the neutral species [(PR(3)(CO)(3)(R'-pz)M](.).