A matched set of dinuclear cobalt complexes with II-II, II-III, and III-III oxidation states have been prepared and structurally characterized. In [(bpbp)CO2(O2P(OPh)(2))(2)]n(+) (n = 1, 2, or 3; bpbp(-) = 2,6-bis((N,N'-bis-(2-picolyl)amino)-methyl)-4-tertbutylphenolato), the nonbonded Co center dot center dot center dot Co separations are within the range 3.5906(17) to 3.7081 (11) angstrom, and the metal ions are triply bridged by the phenolate oxygen atom of the heptadentate dinucleating ligand and by two diphenylphosphate groups. The overall structures and geometries of the complexes are very similar, with minor variations in metal-ligand bond distances consistent with oxidation state assignments. The (CoCoIII)-Co-II compound is a valence-trapped Robin-Day class II complex. Solid state P-31 NMR spectra of the diamagnetic (CoCoIII)-Co-III (3) and paramagnetic (CoCoIII)-Co-II (2) and (CoCoII)-Co-II (1) complexes show that P-31 isotropic shifts broaden and move downfield by about 3000 ppm for each increment in oxidation state. Cyclic voltammetry corroborates the existence of the (CoCoII)-Co-II, (CoCoII)-Co-II, and (CoCoIII)-Co-III species in solution. The redox changes are not reversible in the applied scanning timescales, indicating that chemical changes are associated with oxidation and reduction of the cobalt centers. An investigation of the spectroscopic properties of this series has been carried out for its potential usefulness in analyses of the related spectroscopic properties of the dicobalt metallohydrolases. Principally, magnetic circular dichroism (MCD) has been used to determine the strength of the magnetic exchange coupling in the (CoCoII)-Co-II complex by analysis of the variable-temperature variable-field (VTVH) intensity behavior of the MCD signal. The series is ideal for the spectroscopic determination of magnetic coupling since it can occur only in the (CoCoII)-Co-II complex. The (CoCoIII)-Co-II complex contains a nearly isostructural Co-II ion, but since Co-III is diamagnetic, the magnetic coupling is switched off, while the spectral features of the Co-II ion remain. Analysis of the MCD data from the (CoCoIII)-Co-II complex has been undertaken in the theoretical context of a T-1(1g) ground-state of the Co-II ion, initially in an octahedral ligand field that is split by both geometric distortion and zoro-field splitting to form an isolated doublet ground state. The MCD data for the (CoCoII)-Co-II pair in the [(bpbp)Co-2(O2P(OPh)(2))(2)](+) complex were fitted to a model based on weak antiferromagnetic coupling with J = -1.6 cm(-1). The interpretation is confirmed by solid state magnetic susceptibility measurements.