Homoleptic perhalophenyl derivatives of divalent nickel complexes with the general formula [NBu(4)](2)[Ni(II)(C(6)X(5))(4)] [X=F (1), Cl (2)] have been prepared by low-temperature treatment of the halo-complex precursor [NBu(4)](2)[NiBr(4)] with the corresponding organolithium reagent LiC(6)X(5). Compounds 1 and 2 are electrochemically related by reversible one-electron exchange processes with the corresponding organometallate(III) compounds [NBu(4)][Ni(III)(C(6)X(5))(4)] [X=F (3), Cl (4)]. The potentials of the [Ni(III)(C(6)X(5))(4)](-)/[Ni(II)(C(6)X(5))(4)](2-) couples are +0.07 and -0.11 V for X=F or Cl, respectively. Compounds 3 and 4 have also been prepared and isolated in good yield by chemical oxidation of 1 or 2 with bromine or the amminium salt [N(C(6)H(4)Br-4)(3)][SbCl(6)]. The [Ni(III)(C(6)X(5))(4)](-) species have SP-4 structures in the salts 3 and 4, as established by single-crystal X-ray diffraction methods. The [Ni(II)(C(6)F(5))(4)](2-) ion in the parent compound 1 has also been found to exhibit a rather similar SP-4 structure. According to their SP-4 geometry, the Ni(III) compounds (d(7)) behave as S=1/2 systems both at microscopic (EPR) and macroscopic levels (ac and dc magnetization measurements). The spin Hamiltonian parameters obtained from the analysis of the magnetic behavior of 3 and 4 within the framework of ligand field theory show that the unpaired electron is centered mainly on the metal atom, with >97 % estimated d(z(2) ) contribution. Thermal decomposition of 3 and 4 proceeds with formation of the corresponding C(6)X(5)--C(6)X(5) coupling compounds.