Mononuclear complexes trans,mer-(PR(3))(2)(CO)(3)M(L) (M = Mo, W; R = Pr-i, Cy; L = THF, eta(2)-H-2) and dinuclear complexes trans,mer-[(PR(3))(2)(CO)(3)M](2)(mu-L) with the symmetrically bridging ligands mu-L = pyrazine (pz), 4,4’-bipyridine (bp) and 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine (4,4’-bptz) were studied by cyclic voltammetry and by IR, UV/vis/near-IR, and EPR spectroscopy. Oxidation of the electron-rich and dissociatively labile (Mo > W) systems yields M(I) species, including stable (K-c > 10(6)) mixed-valent d(5)/d(6) cations {[((PPr3)-Pr-i)(2)(CO)(3)M](2)(pz)}(+). These mixed-valent complex ions exhibit complete delocalization on the vibrational time scale and show similar spectroscopic features as the structurally related Creutz-Taube ion {[H3N)(5)Ru](2)(mu-pz)}(5+). Reversible two-election oxidation processes and thus no mixed-valent states were observed for the bp- and 4,4’-bptz-bridged ditungsten systems.;Whereas the oxidation of (PCy(3))(2)(CO)(3)W(eta(2)-H-2) is irreversible with the assumed loss of a proton, the solvates (PR(3))(2)(CO)(3)M(THF) are reversibly oxidized and reduced, the latter process requiring rather negative potentials. In nonpolar solvents the neutral dinuclear complexes display very intense and strikingly narrow charge transfer bands in the near-infrared region, suggesting very little geometrical change between ground and MLCT excited states. One-electron reduction of the dinuclear complexes produces EPR-detectable radical anion complexes which show the loss of one PR(3) Ligand per metal center, i.e. the preference for a 16 + delta rather than an 18 + delta valence electron configuration. The tungsten-centered oxidation : of complexes (PR(3))(2)(CO)(3)W(L) is facilitated in the order L = mpz(+), pz, H-2, 4,4’-bptz, bp, and THF, illustrating quantitatively the well-balanced sigma donor and sigma* acceptor characteristics bf the H-2 ligand. From the results of this study it appears that the (PR(3))(2)(CO)(3)M fragments are suited to bind H-2 because of a very finely tuned combination of sigma-acceptor and fairly strong but not excessive pi-donor characteristics, in addition to the proper amount of steric shielding.