Cation Catalysis of Anion−Anion Electron Transfer in Aqueous Solution: Self-Exchange Reaction Kinetics of Some Hexa- and Octacyanometalate Couples at Variable Pressure

Abstract

Rate constants kex for the Os(CN)63-/4-, Mo(CN)83-/4-, and W(CN)83-/4- self-exchange reactions in aqueous solution have been measured by 13C NMR as functions of pressure, temperature, and added electrolyte concentrations. Detailed interpretation of kex values in the absence of added electrolytes is complicated by anion−cation association, but kex is strongly influenced by the nature of added cations (Li+ < Na+ < K+ < Rb+ < Et4N+ < Cs+ ≤ Me4N+) and is linearly dependent on their concentrations. For K+ as the counterion, ΔVex⧧ for Os(CN)63-/4-, Mo(CN)83-/4-, and W(CN)83-/4- is +19.0 ± 0.9, +14.7 ± 0.6, and +22.5 ± 1.1 cm3 mol-1, respectively; these values are completely inconsistent with a simple two-sphere Marcus-type model of the electron-transfer process, which predicts ΔVex⧧ on the order of −6 cm3 mol-1. With Et4N+ as the counterion, however, ΔVex⧧ for Mo(CN)83-/4- is −8.2 ± 0.6 cm3 mol-1, and for W(CN)83-/4- with Me4N+ ΔVex⧧ is −7.4 ± 0.5 cm3 mol-1. The results are interpreted in terms of a mechanism which is outer-sphere as far as the cyanometalate ions are concerned but in which a partially desolvated cation bridges the reacting anions. In the case of alkali metal cations, removal of some (probably two) of the coordinated water molecules precedes electron transfer, whereas the tetraalkylammonium ions have no coordinated water. Anion−anion electron transfer is much more sensitive to cation catalysis than cation−cation exchange is to anion effects, probably because a positively charged bridge provides a much more favorable tunneling path for the negatively charged electron than would an anionic mediator.