The preparation and properties are described of complexes resulting from the mono- and diprotonation of the [Co(4)O(4)](4+) unit to give products containing [Co(4)O(3)(OH)](5+) and [Co(4)O(2)(OH)(2)](6+), respectively. Treatment of [Co(4)O(4)(O(2)CC(6)H(4)-p-OMe)(2)(bpy)(4)](2+) (bpy = 2,2'-bipyridine) in MeCN with 70% perchloric acid leads to isolation of [Co(4)O(2)(OH)(2)(O(2)CC(6)H(4)-p-OMe)(2)(bpy)(4)](ClO(4))(4) (5) in 61% yield. Treatment of [Co(4)O(4)(O(2)CC(6)H(4)-p-Me)(2)(bpy)(4)](2+) in MeCN with an acidic solution of (NH(4))(3)[Ce(NO(3))(6)] leads to isolation of [Co(4)O(3)(OH)(O(2)CC(6)H(4)-p-Me)(2)(bpy)(4)][Ce(NO(3))(6)] (6) in 96% yield. Complex 5.3MeCN.H(2)O crystallizes in monoclinic space group C2/c with (at -155 degrees C) a = 33.838(8) Å, b = 13.826(3) Å, c = 29.944(7) Å, beta = 98.84(1) degrees, and Z = 8. Complex 6.PhCN.H(2)O crystallizes in orthorhombic space group Pbca with (at -154 degrees C) a = 22.603(4) Å, b = 34.759(6) Å, c = 18.167(3) Å, and Z = 8. The complexes contain [Co(4)O(2)(OH)(2)](6+) (5) and [Co(4)O(3)(OH)](5+) (6) distorted-cubane cores, the sites of protonation being apparent by (i) the approach of ClO(4)(-) or [Ce(NO(3))(6)](3)(-) anions to within hydrogen-bonding distances (O.O approximately 2.7 Å) and (ii) the longer Co-OH(-) bond lengths compared with Co-O(2)(-) bond lengths. Peripheral ligation is provided by chelating bpy and syn,syn-bridging RCO(2)(-) groups. The Co(III) atoms are six-coordinate and approximately octahedral. Solution studies show that the diprotonated species is a strong diprotic acid analogous to H(2)SO(4); the first deprotonation goes to completion, but the monoprotonated species is a weak acid and is in equilibirium with the nonprotonated species. pH studies in aqueous solution yield a pK(a2) value of 3.15 for the second deprotonation. Electronic and (1)H NMR spectra of the diprotonated complexes in various solvents are consistent with the monoprotonated complex being the major species in solution. The combined results demonstrate that the [Co(4)O(4)](4+) core is capable of acting as a Brønsted base, undergoing either one or two protonation reactions.