SALT EFFECTS ON THE ELECTRON-TRANSFER EQUILIBRIA OF REDOX-ACTIVE ION-PAIRS

Electron-transfer equilibrium is achieved in the ion-pair annihilation of the organometallic anion TpM(CO)3- [Tp = hydrido-tris(3,5-dimethylpyrazolyl)borate] with the triarylpyrylium cation (P+) to a mixture of the 17-electron radical TpM(CO)3. and the pyranyl radical P., respectively. The equilibrium constants K(ET) for the electron-transfer equilibria of the ion pairs P+ TpM(CO)3- are evaluated for TpM(CO)3- with M = Mo, W and Cr and for a series of 2,4,6-triarylpyrylium cations with varying values of the oxidation potential E(ox)0 of the anionic redox couple (TpM(CO)3- half-arrow-right-over-half-arrow-left TpM(CO)3. + e) and the reduction potential E(red)0 for the cationic redox couple (P+ + e half-arrow-right-over-half-arrow-left P.), by specifically taking into account the contribution from the reversible dimer P2. The importance of the primary salt effect on the modulation of the electron-transfer equilibrium is identified with the exchange of contact ion pairs, i.e., the "special salt effect" discussed by Tchoubar and coworkers. Structural insight into contact ion pairs extant in solution is obtained from the charge-transfer salt of N-methyl-4-cyanopyridinium, i.e., NCP+TpMo(CO)3-, as revealed by single-crystal X-ray crystallography.