Chemical and electrochemical investigation of Cs-4 [Ru-2(SO4)(2)(mu-SO4)(2)(mu-O)(2)]center dot 2H(2)O

The title compound was prepared by a modification of the method of Fomina et al. and characterized by UV-Vis and IR spectroscopy. The [Ru-2(SO4)(2)(mu-SO4)(2)(mu-O)(2)](4-) anion is quite stable in dilute sulfuric acid solutions (pH approximate to 2; 0 degrees C), but its absorption spectrum changes dramatically in more concentrated solutions. In less acidic solutions (pH greater than or equal to 4) the complex anion is rapidly aquated to give a neutral species, supposedly [Ru-2(H2(O))(4)(mu SO4)(2)(mu-O)(2)], and finally precipitates as a yet not well defined ruthenium-oxosulfato compound. Voltammetry at a platinum rotating disk electrode in sulfate/bisulfate solution (pH=1.90; mu=0.5) at 0 degrees C reveals two cathodic waves for the stepwise reduction of the ruthenium(IV)- to a ruthenium(III)-sulfato complex with the formation of a ruthenium(3.5) species as the intermediate product. However, these steps give rise to two separate waves only because the second reduction step is shifted to more cathodic values by the effect of slow electron transfer kinetics. Potentiometry indicates the existence of a dismutation equilibrium and yields the following potential scheme A + e reversible arrow B E(A/B)(0)=0.718(+/-0.008) V B + e reversible arrow C E(B/C)(0)=0.686(+/-0.008) V where A, B and C represent the species [Ru-2(SO4)(2)(mu-SO4>(2)(mu-O)(2)](4-), [Ru-2(SO4)(2)(mu-SO4)(2)(mu-O)(2)](5-) and [Ru-2(SO4)(2)(mu-SO4)(2)(mu-O)(2)](6-), respectively. The ruthenium(III)-sulfato complex prepared by electrolytic reduction undergoes a very slow chemical follow-up reaction involving the release of all the sulfate groups: this reaction probably involves several steps on going from an anionic to a cationic form but the kinetics of this process has not been studied in detail.