An EPR Study of 2,3-Bis(diphenylphosphino)maleic Anhydride (BMA) Complexes and the BMA Radical Anion.

EPR spectra are reported for four metal complexes of 2,3-bis(diphenylphosphino)maleic anhydride (BMA), [Co2(PhCCR)(CO)4(η-BMA)]-, R = Ph, H, [Co2(PhCCPh)(CO)4(μ-BMA)]-, and [PhCW(CO)2(BMA)Cl]-, as well as the radical anions, [BMA]- and [BPCD]-, BPCD = 4,5-bis(diphenylphosphino)cyclopentene-1,3-dione. At room temperature, all spectra are 1:2:1 triplets due to hyperfine coupling to two equivalent 31P nuclei with coupling to two equivalent 1H nuclei for [BPCD]- and unresolved coupling to one or two 59Co nuclei for the Co complexes with chelating or bridging BMA, respectively. The 31P couplings are temperature dependent, ca. −3 and −13 mG K-1 for the metal complexes and ligand radical anions, respectively. At low temperature, the spectrum of [BMA]- shows the presence of symmetric and asymmetric PPh2 rotational conformers, related by the thermodynamic parameters ΔH° = −0.8 ± 0.2 kJ mol-1 and ΔS° = 4 ± 1 J mol-1 K-1 and interconverted with activation parameters ΔH⧧ = 18.2 ± 0.4 kJ mol-1, ΔS⧧ = −30 ± 2 J mol-1 K...