From the reaction mixture of [M-II(bpy)Cl-2], the ligand 2-anilino-4,6-di-tertbutylphenol, H[L-AP], and 2 equiv of a base (NaOCH3) in CH3CN under anaerobic conditions were obtained the blue-green neutral complexes [M-II(LAP-H)(bpy)] (M = Pd (1), Pt (2)). (L-AP-H)(2-) represents the o-amidophenolato dianion, (L-AP)(1-) is the o-aminophenolate(1-), (L-ISQ)(1-) is its one-electron-oxidized, pi-radical o-iminobenzosemiquinonate(1-), and (L-IBQ)(0) is the neutral quinone. Complexes 1 and 2 can be oxidized by ferrocenium hexafluorophosphate, yielding the paramagnetic salts [M-II(L-ISQ)(-1)(L's)(bpy)]PF6 (S = 1/2) (M = Pd (1a), Pt (2a)). The reaction of PtCl2, 2 equiv of H[L-AP], and 4 equiv of base in CH3CN in the presence of air yields diamagnetic [Pt(L-ISQ)(2)] (3), which is shown to possess an electronic structure that is best described as a singlet diradical. Complexes 1, la, 2, 2a, and 3 have been structurally characterized by X-ray crystallography at 100 K. It is clearly established that O,N-coordinated (L-AP-H)(2-) ligands have a distinctly different structure than the corresponding 0,N-coordinated (L-ISQ)(1-) radicals. It is therefore possible to unambiguously assign the protonation and oxidation level of o-aminophenol derived ligands in coordination compounds. All complexes have been investigated by cyclic voltammetry, spectroelectrochemistry, EPR, and UV-vis spectroscopy. Complexes 1 and 2 can be reversibly oxidized to the [M-II(L-ISQ)(bpy)](+) and [M-II(L-IBQ)(pby)](2+) mono- and dications, respectively, and reduced to the [M(L-AP-H)(bpy*)](-) anion, where (bpy*)(1-) is the radical anion of 2,2'-bipyridine. Complex 3 exhibits four reversible one-electron-transfer waves (two oxidations and two reductions) which are all shown to be ligand centered. The EPR spectra of the one-electron-reduced species [Pt(L-AP-H)(L-ISQ)](-) (S = 1/2) and of the one-electron-oxidized species [Pt(L-ISQ)(L-IBQ)](+) (S = 1/2) in CH2Cl2 solutions have been recorded. To gain a better understanding of the electronic structure of 3 and its monooxidized and reduced forms, relativistic DFT calculations have been carried out. Magnetic coupling parameters and hyperfine couplings were calculated and found to be in very good agreement with experiment. It is shown that both the one-electron oxidation and reduction of 3 are ligand centered. A simple MO model is developed in order to understand the EPR properties of the monocation and monoanion of 3.