EPR, Electronic Spectra, and Electron Transfer Properties of the 17 Electron Carbonylhydrotris(triphenylphosphine)rhodium(II) Cation

Abstract

An unusually stable five-coordinate monomeric divalent rhodium complex, [RhII(H)(CO)(PPh3)3]+, is produced by bulk oxidative electrolysis or chemical oxidation of RhI(H)(CO)(PPh3)3 in dichloromethane. Consequently, odd as well as even electronic configurations are available in this well-known catalytic system. The EPR and electronic spectra of electrogenerated paramagnetic 17-electron cation [RhII(H)(CO)(PPh3)3]+ have been obtained at low temperatures as has the EPR spectrum of the deuterated analogue. Computer simulation of the EPR spectra of the hydride and deuteride complexes reveals three g-values and anisotropic coupling constants for hydrogen, phosphorus, and rhodium. One of the phosphorus coupling constants is very large (A1 = 175.0 G; A2 = 176.0 G; A3 = 230.0 G). This may be accounted for if [RhII(H)(CO)(PPh3)3]+ has the square pyramidal structure, and substantial mixing of the singly occupied metal orbital and the apical phosphorus s-orbital are considered. NMR measurements on mixtures of RhI(H)(CO)(PPh3)3 and [RhII(H)(CO)(PPh3)]+ are consistent with a very fast electron self-exchange reaction and the heterogeneous charge-transfer rate constant for the [RhII/I(H)(CO)(PPh3)3]+/0 redox couple also is very fast. One electron electrochemical oxidation of [RhII(H)(CO)(PPh)3]+ to [RhIII(H)(CO)(PPh3)3]2+ is followed by a very fast reductive elimination reaction (loss of proton) which generates [RhI(CO)(PPh3)3]+.