Redox chemistry of the d⁶ acetato-bridged Clusters [M^IV₃(mu₃-O)_n(mu-O₂CCH₃)₆(H₂O)₃]²⁺ (M = Mo, W, n = 1, 2): reversible redox Between Mono-mu₃-oxo d⁸ M^III,III,IV and d⁹ M^III₃ forms

A cyclic voltammogram of aqueous 0.1 mol dm(-3) triflic acid solutions of the d(6) bioxo-capped M - M bonded cluster [Mo-3(mu(3)-O)(2)(O2CCH3)(6)(H2O)(3)](2+) at a glassy carbon electrode at 25 degrees C gives rise to an irreversible 3e(-) cathodic wave to a d(9) Mo-3(III) species at - 0.8 V vs. SCE which on the return scan gives rise to two anodic waves at +0.05 V vs. SCE (E-1/2, 1e(-) reversible to d(8) (Mo2MoIV)-Mo-III) and +0.48 V vs. SCE (2e(-) irreversible back to d(6) Mo-3(IV)). The number of electrons passed at each redox wave has been confirmed by redox titration and controlled potential electrolysis which resulted in 90% recovery of [Mo-3(mu(3)-O)(2)(O2CCH3)(6)(H2O)(3)](2+) following electrochemical re-oxidation at + 0.8 V. A corresponding CV study of the d(8) monoxo-capped (W2WIV)-W-III cluster [W-3(mu-O)(O2CCH3)(6)(H2O)(3)](2+) gives rise to a reversible 1e(-) cathodic process at -0.92 V vs. SCE to give the d(9) W-3(III) species [W-3(mu-O)(O2CCH3)(6)(H2O)(3)](+); the first authentic example of a WIII complex with coordinated water ligands. However the cluster is too unstable (O-2/water sensitive) to allow isolation. Comparisons with the cv study on [Mo-3(mu-O)(2)(O2CCH3)(6)(H2O)(3)](2+) suggest irreversible reduction of this complex to monoxo- capped [Mo-3(III)(mu-O)(O2CCH3)(6)(H2O)(3)](+) followed by reversible oxidation to its d(8) counterpart [Mo-3(mu-O)(O2CCH3)(6)(H2O)(3)](2+) ((Mo2MoIV)-Mo-III) and finally irreversible oxidation back to the starting bioxo-capped cluster. Exposing the d(9) Mo-3(III) cluster to air (O-2) however gives a different final product with evidence of break up of the acetate bridged framework. Corresponding redox processes on d(6) [W-3(mu-O)(2)(O2CCH3)(6)(H2O)(3)](2+) are too cathodic to allow similar generation of the monoxo-capped W-3(III) and (W2WIV)-W-III clusters at the electrode surface.