Homoleptic organocobalt(III) compounds with formula [NBu4][Co(III)(C6X5)4] [X = F (3), Cl (4)] were obtained in reasonable yields by chemical oxidation of the corresponding divalent species [NBu4]2[Co(II)(C6X5)4] [X = F (1), Cl (2)]. The [Co(III)(C6X5)4](-)/[Co(II)(C6X5)4](2-) couples are electrochemically related by quasi-reversible, one-electron exchange processes at moderate potential: E1/2 = -0.29 (X = F) and -0.36 V (X = Cl) versus saturated calomel electrode. The [Co(III)(C6X5)4](-) anions in salts 3 and 4 show an unusual square-planar geometry as established by single-crystal X-ray diffraction methods. According to their stereochemistry, these Co(III) derivatives (d(6)) are paramagnetic non-Kramers systems with a large zero-field splitting contribution and no observable electron paramagnetic resonance (EPR) spectrum. The thermal dependence of their magnetic susceptibilities can be explained in terms of a spin-Hamiltonian formalism with S = 1 ground state (intermediate spin) and substantial spin-orbit contribution. The magnetic properties of the square-planar d(7) parent species [NBu4]2[Co(II)(C6X5)4] were also thoroughly studied both at microscopic (EPR) and macroscopic levels (alternating current and direct current magnetization measurements). They behave as S = 1/2 (low spin) systems with mainly (dz(2))(1) electron configuration and a certain degree of s-orbital admixture that has been quantified. The electronic structures of all four open-shell [Co(C6X5)4](q-) compounds (q = 1, 2) accounting for their respective magnetic properties are based on a common orbital energy-level diagram.