Homoleptic organocobalt(ɪɪɪ) compounds with formula [NBu₄][Co^ɪɪɪ(C₆X₅)₄] [X = F (3), Cl (4)] were obtained in reasonable yields by chemical oxidation of the corresponding divalent species [NBu₄]₂[Co^ɪɪ(C₆X₅)₄] [X = F (1), Cl (2)]. The [Co^ɪɪɪ(C₆X₅)₄]⁻/[Co^ɪɪ(C₆X₅)₄]²⁻ couples are electrochemically related by quasi-reversible, one-electron exchange processes at moderate potential: E_½ = −0.29 (X = F) and −0.36 V (X = Cl) versus saturated calomel electrode. The [Co^ɪɪɪ(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^ɪɪɪ derivatives (d⁶) 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 d⁷ parent species [NBu₄]₂[Co^ɪɪ(C₆X₅)₄] were also thoroughly studied both at microscopic (EPR) and macroscopic levels (alternating current and direct current magnetization measurements). They behave as S = ½ (low spin) systems with mainly (d_z²)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.