OUTER-SPHERE METAL-METAL CHARGE-TRANSFER TRANSITIONS IN ENCOUNTER COMPLEXES OF FREE MOBILE CYANOMETALATE IONS - DIRECT OBSERVATIONS OF THE REACTIVE ASSOCIATE OF ELECTRON-TRANSFER REACTIONS

Outer-sphere metal-metal charge-transfer (MMCT) transitions are observed in the visible and near-infrared spectral regions for encounter complexes of free mobile hexacyanoferrate(III) and hexacyanometalate(II) (M = Fe, Os) and octacyanometalate(IV) (M = Mo, W) anions, respectively, in aqueous solution at high concentrations of the reactants. The assignment of the observed bands to MMCT transitions is supported by the similarity of both anion-dependent absorption maxima shifts and relative bandwidths when compared to the MMCT transitions in ion pairs of [Ru(NH3)6]3+ with the same donor anions. Molar extinction coefficients epsilon(max) = 28 (M = Fe), 45 (Os), 13 (Mo), and 24 M-1 cm-1 (W), respectively, as well as pre-equilibrium constants K(A) = (5.5 +/- 0.6) x 10(-2) M-1 in each case are obtained for the [M(CN)x]4-/Fe(CN)6]3- systems. The electronic coupling between the metal centers, estimated from the intensities of the MMCT absorptions, is sufficiently strong for a nearly adiabatic nature of the corresponding thermal reactions.