EPR, ELECTRONIC-SPECTRA, AND ELECTRON-TRANSFER PROPERTIES OF THE 17 ELECTRON CARBONYLHYDROTRIS(TRIPHENYLPHOSPHINE)RHODIUM(II) CATION

An unusually stable five-coordinate monomeric divalent rhodium complex , [Rh-II(H)(CO)(PPh3)(3)](+), is produced by bulk oxidative electrolys is or chemical oxidation of Rh-I(H)(CO)(PPh3)(3) in dichloromethane, C onsequently, odd as well as even electronic configurations are availab le in this well-known catalytic system, The EPR and electronic spectra of electrogenerated paramagnetic 17-electron cation [Rh-II(H)(CO)(PPh 3)(3)](+) have been obtained at low temperatures as has the EPR spectr um of the deuterated analogue. Computer simulation of the EPR spectra of the hydride and deuteride complexes reveals three g-values and anis otropic coupling constants for hydrogen, phosphorus, and rhodium. One of the phosphorus coupling constants is very large (A(1) = 175.0 G; A( 2) = 176.0 G A(3) = 230.0 G), This may be accounted for if [Rh-II(H)(C O)(PPh3)(3)](+) has the square pyramidal structure, and substantial mi xing of the singly occupied metal orbital and the apical phosphorus s- orbital are considered, NMR measurements on mixtures of Rh-I(H)(CO)(PP h3)(3) and [Rh-II(H)(CO)(PPh3)](+) are consistent with a very fast ele ctron self-exchange reaction and the heterogeneous charge-transfer rat e constant for the [Rh-II/I(H)(CO)(PPh3)(3)](+/o) redox couple also is very fast. One electron electrochemical oxidation of [Rh-II(H)(CO)(PP h)(3)](+) to [Rh-III(H)(CO)(PPh3)(3)](2+) is followed by a very fast r eductive elimination reaction (loss of proton) which generates [Rh-I(C O)(PPh3)(3)](+).