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), [Co(2)(PhCCR)(CO)(4)(eta-BMA)](-), R = Ph, H, [Co(2)(PhCCPh)(CO)(4)(&mgr;-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 (31)P nuclei with coupling to two equivalent (1)H nuclei for [BPCD](-) and unresolved coupling to one or two (59)Co nuclei for the Co complexes with chelating or bridging BMA, respectively. The (31)P 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 PPh(2) rotational conformers, related by the thermodynamic parameters DeltaH degrees = -0.8 +/- 0.2 kJ mol(-)(1) and DeltaS degrees = 4 +/- 1 J mol(-)(1) K(-)(1) and interconverted with activation parameters DeltaH() = 18.2 +/- 0.4 kJ mol(-)(1), DeltaS() = -30 +/- 2 J mol(-)(1) K(-)(1). The temperature dependence of the (31)P couplings is explained by a negative spin-polarization contribution to and a positive contribution due to P 3s character; the latter increases with the asymmetry of the PPh(2) conformations. The range of conformations accessible to the metal complexes is less than for the ligand radical anions, and accordingly the temperature dependence is significantly smaller.