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)(mu-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 P-31 nuclei with coupling to two equivalent H-1 nuclei for [BPCD](-) and unresolved coupling to one or two Co-59 nuclei for the Co complexes with chelating or bridging BMA, respectively. The P-31 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 Delta H degrees = -0.8 +/- 0.2 kJ mol(-1) and Delta S degrees = 4 +/- 1 J mol(-1) K-1 and interconverted with activation parameters Delta H-double dagger = 18.2 +/- 0.4 kJ mol(-1), Delta S-double dagger = -30 +/- 2 J mol(-1) K-1. The temperature dependence of the P-31 couplings is explained by a negative spin-polarization contribution to < a(p)> and a positive contribution due to P 3s character; the latter increases with the asymmetry of the PPh2 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.