The results of self-consistent field (SCF) nonlocal density functional molecular orbital calculations are presented for the various spin states and tautomeric forms of a cobalt complex with two o-quinone-derived ligands. In addition, new variable-temperature solution magnetic susceptibility, EPR, and electronic absorption data are presented to characterize the low-spin [Co-III(3,5-DTBSQ)(3,5-DTBCat)(phen)] to high-spin [Co-II(3,5-DTBSQ)(2)-(phen)] valence-tautomeric interconversion, where 3,5-DTBSQ(-) and 3,5-DTBCat(2-) are the semiquinonate and catecholate forms of 3,5-di-tert-butyl-o-benzoquinone, respectively, and phen is 1,10-phenanthroline. The solution magnetic susceptibility data were fitted to give Delta H = 2238 cm(-1) and Delta S = 118.1 J mol(-1) K-1 for the ls-Co-III reversible arrow hs-Co-II equilibrium. Appreciable changes are seen in the electronic absorption spectrum as the complex changes between the two tautomeric forms. Unrestricted SCF calculations gave J = -594 cm(-1) for the parameter characterizing the antiferromagnetic exchange interaction between hs-Co-II ion (S = 3/2) and each of the two coordinated semiquinonate (S = 1/2) ligands in the hs-Co-II tautomer. The calculations indicated that the ls-Co-II tautomer state is the most stable with an energy separation of Delta E = 4428 cm(-1) (0.55 eV) between this ls-Co-III state and the S = 1/2 component of the hs-Co-II spin ladder. This Delta E value compares favorably with the Delta H value evaluated from variable-temperature susceptibility data. The calculations indicate that, while there are still localized electronic structural features reflecting the different metal and ligand oxidation states in the ls-Co-III and hs-Co-II tautomeric forms, appreciable covalent interactions exist between the cobalt ion and the ligands. Finally, the results of the calculations were used to assign the electronic transitions seen for the ls-Co-III and hs-Co-II tautomers.