CHEMICAL AND ELECTROCHEMICAL OXIDATION OF MER FAC-CR(CO)3(ETA-1-L-L)(ETA-2-L-L) CONTAINING A PENDANT DONOR ATOM - ESR STUDIES OF THE CATIONS MER-[CR(CO)3(ETA-1-L-L)(ETA-2-L-L)]+ AND TRANS-[CR(CO)2(ETA-2-L-L)2]+ (L-L = BIDENTATE GROUP-15 LIGAND)

Reactions between Cr(CO)3(C7H8) and Ph2PCH2CH2AsPh2 (ape) and Ph2AsCH2CH2AsPh2 (dae) give mer-Cr(CO)3(eta-1-ape)(eta-2-ape) and fac-Cr(CO)3(eta-1-dae)(eta-2-dae), respectively. Phosphorus-31 NMR studies show that the monodentate ape ligand is coordinated through phosphorus. Upon electrochemical oxidation in dichloromethane (0.1 M Bu4NClO4) at platinum electrodes, mer-Cr(CO)3(eta-1-ape)(eta-2-ape) is reversibly oxidized to mer-[Cr(CO)3(eta-1-ape)(eta-2-ape)]+ whilst fac-Cr(CO)3(eta-1-dae)(eta-2-dae) gives fac-[Cr(CO)3(eta-1-dae)(eta-2-dae)]+, which rapidly isomerizes to mer-[Cr(CO)3(eta-1-dae)(eta-2-dae)]+. In addation, for each complex a further irreversible 1-electron process is observed at very positive potentials. ESR studies on several mer-[Cr(CO)3(eta-1-L-L)(eta-2-L-L)]+ cations (L-L = bidentate group 15 ligand), electrochemically generated in a sealed tube within the ESR microwave cavity, show that they decompose very slowly to trans-[Cr-(CO)2(eta-2-L-L)2]+. The rate of decomposition to trans-[Cr(CO)2(eta-2-L-L)2]+ is faster in a nonsealed tube with nitrogen purging. The ESR spectra of the tricarbonyl and dicarbonyl 17-electron cations show both phosphorus and arsenic hyperfine structure with very similar values for the coupling constants. Chemical oxidation of mer/fac-Cr(CO)3(eta-1-L-L)(eta-2-L-L) with an equimolar (or greater) quantity of AgClO4 also gives mer-[Cr(CO)3(eta-1-L-L)(eta-2-L-L)]+. In one case, (L-L = Ph2PCH2PPh2(dpm)), when 0.5 molar equiv of AgClO4 is used, a diamagnetic intermediate silver complex is identified by phosphorus-31 NMR spectroscopy. In the presence of excess of AgClO4, the rate of decomposition of mer-[Cr(CO)3(eta-1-L-L)(eta-2-L-L)]+ to trans-[Cr(CO)2(eta-2-L-L)2]+ is greatly enhanced, further confirming that the silver ion is a noninnocent oxidant when a pendant donor atom is presented.