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 k ex for the Os(CN)6 3-/4-, Mo(CN)8 3-/4-, and W(CN)8 3-/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 k ex values in the absence of added electrolytes is complicated by anion−cation association, but k ex 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, ΔV ex ⧧ for Os(CN)6 3-/4-, Mo(CN)8 3-/4-, and W(CN)8 3-/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 ΔV ex ⧧ on the order of −6 cm3 mol-1. With Et4N+ as the counterion, however, ΔV ex ⧧ for Mo(CN)8 3-/4- is −8.2 ± 0.6 cm3 mol-1, and for W(CN)8 3-/4- with Me4N+ ΔV ex ⧧ 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.

Ionic-strength dependence of electron-transfer reactions of Keggin heteropolytungstates: Mechanistic probes of O2 activation in water

“…z 1 z 2 = 30 (where z 1 and z 2 are 6 − and 5 − , the charges of ␣-2 1e and ␣-2 ox ), shows compliance with the extended Debye-Hückel equation, a prerequisite to the defensible application of theoretical models, such as those of Marcus and Sutin, to outer-sphere electron-transfer reactions between charged donors and acceptors. The parameters α and β used in the extended Debye-Hückel equation, are standard values for ions in water at 25 • C. The charge product of 29 ± 2 thus indicates that: (1) ␣-2 1e and ␣-2 ox behave in aqueous NaCl as charged, 6 − and 5 − , spheres, each with of radius 5.6Å [11], (2) ion pairing with Na + is negligible at NaCl concentrations as large as 1.1 M, and (3) that cation (Na + ) catalysis [21] of electron exchange between the two ions, is not kinetically significant.…”

Application of High Pressure in the Elucidation of Inorganic and Bioinorganic Reaction Mechanisms

“…These findings support a reaction mechanism previously proposed for other anion-anion electron transfer reactions, catalyzed by alkali metal ions, that catalysis is facilitated by partially dehydrated M þ . 141 …”

Polyallylammonium Ferrocyanide Films for Trace Water Detection in Halogenated Solvents

“…The mechanism of water-dependent electroactivity is rooted in the different e À transfer pathways exhibited by cyanometallates [21,57]. The cation-dependent e À transfer pathway is orders of magnitude faster than the cationindependent pathway [23]. We postulate that the Fe(CN) 6 4À/ 3À can exist as ion pairs with NH 3 R þ when water is not present.…”

“…The uncatalyzed selfexchange reaction is 4 orders of magnitude slower [22]. Swaddle has suggested that volumes of activation are consistent with partial deaquation of the cation during the electron-transfer reaction [20,23,24]. Swaddle showed a linear dependence of the magnitude of the self-exchange reaction on the concentration of uncomplexed, aquated K at an electrode or between microelectrodes would be necessary.…”