The second example of a low-spin, square-planar Co(II)L(4) species (L = unidentate ligand), (Bu(4)N)(2)Co(CN)(4) (1), and a new family of tetrabutylammonium tetracyanocobaltate(II) complexes have been isolated and characterized. All of the solid compounds react reversibly with dioxygen to form 1:1 cobalt(III)-superoxo complexes. Since a variety of neutral bases react readily with 1 and the complexes can be obtained as solids, the effects of different axial bases on the dioxygen-binding behavior of the solids could also be studied. Interestingly, reversible phase changes occur on oxygenation/deoxygenation for (Bu(4)N)(2)[Co(CN)(4).C(5)H(5)N] (2) and [(Bu(4)N)(2)Co(CN)(4)](2).(C(10)H(8)N(2)) (3). Presumably this happens because the crystal lattices of the unoxygenated materials do not contain enough room for the dioxygen. Complex 2 crystallizes in the orthorhombic space group Pbcm with the following cell parameters at 25 degrees C: a = 9.024(3) Å, b = 21.886(8) Å, c = 23.184(6) Å, V = 4578.6(27) Å(3), and Z = 4. From the molecular packing, it is apparent that the [Co(CN)(4).C(5)H(5)N](2)(-) anions are aligned parallel to the a axis, with the pyridine of each anionic unit pointing toward the vacant site on the cobalt of the adjacent anion. The tetrabutylammonium cations completely surround the anions, forming hydrophobic casings that also run parallel to the a axis. Thus, it is clear that the bulky tetrabutylammonium cations are capable of separating the cobalt centers so that &mgr;-peroxo dimers cannot form.