UV-vis, IR and EPR spectroelectrochemical study of the EC redox transition [(PR3)n(CO)3(R′-pz)M]+/0; M = Mo, W; R′-pz = N-alkylpyrazinium; R = isopropyl, cyclohexyl; n = 1 or 2

The title complexes, isolated as trans,mer -configurated salts [(PR 3 ) 2 (CO) 3 (R′-pz)M](PF 6 ), display a strong π interaction between the zerovalent metal center and the excellent π acceptor ligands R-pz + . The solvatochromism of the long-wavelength metal-to-ligand charge transfer (MLCT) absorption as analyzed in particular for [(PCy 3 ) 2 (CO) 3 (CH 3 (CH 2 ) 17 -pz)W](PF 6 ) correlates not with established MLCT parameters but with the donor numbers of the solvents. The seemingly reversible one-electron oxidation and reduction processes were studied by spectroelectrochemical techniques with regard to EPR, carbonyl vibrational and UV-vis/NIR spectroscopic features. Whereas oxidation leads to labile M(I) species with 17 valence electrons, the reduction is centered 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 data suggest pentacoordination at the 16 + δ valence electron centers in the neutral species [(PR 3 )(CO) 3 (R′-pz)M].