Electrochemical and Spectroscopic Study of Isomerization, Cross-redox, and Self-Exchange Reactions in the [Cr(CO)₃P₃]^+/0Redox System

Homogeneous and heterogeneous aspects of the redox couple [Cr(CO)₃P₃]^+/°[P = monodentate phosphorus ligand (P(OMe)₃, P(OPh)₃, P(OMe)₂Ph, PMe₂Ph)] have been examined by polarographic, voltammetric, synthetic, magnetic, spectroscopic, and other techniques in dichloromethane solution. The tricarbonyl complexes in each oxidation state can exist in both facial {fac⁺or fac⁰) and meridional {mer⁺or met⁰) forms. Under the electrochemical conditions of polarography or voltammetry, parts of the square reaction scheme in addition to the cross-redox reaction fac⁺+ mer⁰^ fac⁰+ mer⁺, are required to explain the electrode processes at both platinum and mercury electrodes. The heterogeneous rates of electron transfer are relatively fast at either electrode. The homogeneous cross-redox reaction enables generation of mer-Cr(CO)₃P₃to occur as an intermediate under conditions of oxidative controlled-potential electrolysis or chemical oxidation of fac⁰isomers. Ultimately, however, the mer⁺form is the sole product of these oxidation reactions irrespective of whether fac^°- or mer^°-Cr(CO)₃P₃is used as the starting complex. These paramagnetic 17-electron cations can be isolated; they are light and heat sensitive and chemically reactive, readily regenerating the met⁰complexes. The electron-transfer reaction mer⁺+ met⁰= met⁰+ mer⁺has been shown by ESR and NMR measurements to be faster than the cross-redox reaction fac⁺+ mer⁰^ fac⁰+ mer⁺.