ELECTRONIC-STRUCTURE OF THE 16 VALENCE ELECTRON FRAGMENTS M(CO)3(PR(3))(2) (M=MO, W, R=PR-I, CY) IN THEIR COMPLEXES WITH H-2, THF, AND 3 PI-CONJUGATED DINUCLEATING LIGANDS - ELECTROCHEMISTRY AND SPECTROSCOPY OF DIFFERENT OXIDATION-STATES

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-t etrazine (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 spectro scopic features as the structurally related Creutz-Taube ion {[H3N)(5) Ru](2)(mu-pz)}(5+). Reversible two-election oxidation processes and th us no mixed-valent states were observed for the bp- and 4,4'-bptz-brid ged ditungsten systems.;Whereas the oxidation of (PCy(3))(2)(CO)(3)W(e ta(2)-H-2) is irreversible with the assumed loss of a proton, the solv ates (PR(3))(2)(CO)(3)M(THF) are reversibly oxidized and reduced, the latter process requiring rather negative potentials. In nonpolar solve nts the neutral dinuclear complexes display very intense and strikingl y 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-detec table 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 oxida tion: 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 quantitativel y the well-balanced sigma donor and sigma acceptor characteristics bf the H-2 ligand. From the results of this study it appears that the (P R(3))(2)(CO)(3)M fragments are suited to bind H-2 because of a very fi nely tuned combination of sigma-acceptor and fairly strong but not exc essive pi-donor characteristics, in addition to the proper amount of s teric shielding.