The radical anion (tmbp)(.-), where tmbp = 4,4',5,5'-tetramethyl-2,2'-biphosphinine was generated by reduction of tmbp on a potassium mirror. EPR/ENDOR spectra and DFT calculations show that, in contrast to the neutral species, this anion is planar and that the unpaired electron is mainly delocalized on the PCCP fragment with a large participation of the phosphorus p(pi) orbitals. This planar structure was confirmed by the first crystal structure of an anionic biphosphinine: [tmbp][Li(2.2.1)]. Reduction of [Ni(tmbp)(2)] led to the 19-electron complex whose g and P-31 hyperfine tensors were obtained from EPR in liquid and frozen solutions. These results, together with DFT calculations on [Ni(bp)(2)] and [Ni(bp)(2)](.-), indicate that, by accepting an extra electron, the neutral nickel complex distorts toward a mon planar geometry and that the dihedral angle between the two phosphinine rings of each ligand slightly increases. In the reduced Ni complex, the unpaired electron is mainly delocalized on the ligands, in a molecular orbital which retains the characteristics of the SOMO found for the reduced isolated ligand. A charge decomposition analysis (CDA) shows that, in [Ni(bp)(2)], metal-ligand back-donation strongly contributes to the metal-ligand bonding.