Interactions of hexacyano ions with [Ruen3]3+ and [Ruen3]2+ in solution

The intensity of outer-sphere charge transfer absorption ([Ru(CN)6]4−→[Ruen3]3+) in aqueous solution containing [Ru(CN)6]4− and [Ruen3]3+ at a 1:1 anion to cation ratio is almost the same as it is at a 2:1 ratio, but there is a shift to lower energy at the latter ratio, which is not altered when the ratio increases to 4:1. In 1.5×10−2 M HO3SCF3 a significant increase in intensity accompanies the change from the 1:1 to the 2:1 ratio, a shift also being observed, limiting values again being reached at the 2:1 ratio. We infer that at the 1:1 ratio, in the absence of extraneous electrolyte, hyper clustering of the putative ion pairs takes place, which is diminished when the anion is mono-protonated. Increasing the ratio of anion to cation causes declustering and we infer that at the 2:1 ratio anion–cation–anion triplets are formed consistent with the full occupation by anions of the trigonal faces of cation octahedra, with the expectation that for steric reasons they will have a much higher affinity for anions than equatorial sites. Inner-sphere substitution is unexpectedly rapid, very much more rapid than with [Ru(NH3)6]3+, where no evidence of substitution even over extended periods of time has been observed. The observations made in the kinetic studies, which include following the rate as a function of anion–cation ratio, are consistent with the view that clustering of ion pairs occurs, and that on increasing the ratio to 2:1 discrete ion triplets result. When, as in the present work, kinetics are done in the first-order mode — cations fully assembled into association with anions — to understand results, it must be taken into account that interactions between the anion and the cation can take a number of different forms. Many of these are unproductive for substitution, among them solvent separated structures, thus providing a basis for explaining why substitution in [Ruen3]3+ is more rapid than in [Ruen3]2+.