ELECTRON-TRANSFER .124. OXIDATIONS OF TRINUCLEAR TUNGSTEN CATIONS

Aqueous solutions of the trinuclear dioxo-capped acetato-bridged complex [W3(mu3-O)2(mu-OAc)6(H2O)3]2+, ''-(W(IV))3'' (I), and the related monoxo-capped cation [W3(mu3-O)(mu-OAc)6(H2O)3]2+, ''(W(III)2W(IV)'' (II), are resistant to the action of all but very strong oxidants. Oxidations with Ce(IV), Cr(VI), MnO4- and HIO4 yield preponderantly W(VI). Although these conversions require several steps, the initial act of electron transfer to the oxidant is rate-determining in each case. Rate laws for oxidation of (W(IV))3 with MnO4- and HCrO4- at low pH values contain [H+]- and [H+]2-proportional terms associated with partial conversion of these oxidants to more reactive mono- and diprotonated forms. In addition, the (W(IV))3-HCrO4- system features a 1:1 Ox.Red complex (Q(assn) = 51 +/- 8 M-1 at 21-degrees-C). The (W(IV))3-I(VII) reaction is accelerated by lowering [H+], possibly reflecting deprotonation of a W(IV)-bound aqua ligand of the reductant. In the (W(IV))3-Ce(IV) reaction, the supply of active oxidant is partially depleted by conversion of monomeric Ce(IV) to an unreactive dimer and also by transformation of the monomer to a negligibly reactive protonated species. The 2e- oxidant, HIO4, reacts with (W(IV))3 substantially more slowly than either the le- acceptor, Ce(IV), or the oxidants Mn(VII) and Cr(VI), which can utilize combinations of paths, suggesting that the overall conversion is best initiated by a single-electron transfer. Such reactions must be predominantly outer-sphere since they proceed many orders of magnitude more rapidly that ligand substitution at the W(IV) center. Only the (W(III))2W(IV)-IrCl6(2-) reaction exhibits a simple bimolecular rate expression. The observed rate constant, 3 M-1 s-1, when considered in the framework of the Marcus model, corresponds to an electron self-exchange rate for the couple W334/344 between 10(-6.0) and 10(-4.3) M-1 s-1, thus falling well below reported values for several 4d- and 5d-based systems but approaching the values typifying 3d-based couples. It is suggested that interconversions between members of redox series of trinuclear acetato-bridged complexes of the type at hand entail changes in the bond connectivities about the transition metal centers, a feature increasing the Franck-Condon barrier to electron transfer.