A new potentially tetradentate redox-active o-aminophenol-based ligand, H2L = 2-(2-ethylthio)pyridine-anilino-4,6-di-tert-butylphenol, reacts with Pd-II(O2CCH3)(2) in CH3OH in the presence of air and Et3N affording isolation of a green crystalline solid of composition [Pd(L)] 1. When examined by cyclic voltammetry (CV), 1 exhibits two quasireversible oxidative responses at E-1/2 = 0.16 (peak-to-peak separation, Delta E-p = 100 mV) and 0.89 V (Delta E-p = 90 mV) vs SCE (saturated calomel electrode). Chemical oxidation of 1 by [Fe-III(eta(5)-C5H5)(2)][PF6] and AgBF4 in CH2Cl2 led to the isolation of two crystalline solids, red [Pd(L)][PF6]center dot CH2Cl2 2 and dark green [Pd(L)][BF4](2)center dot 2CH(2)Cl(2) 3, respectively. Single-crystal X-ray crystallography at 100(2) K unambiguously established that the O,N,S,N-coordinated ligand is present in the square-planar complexes [Pd-II{(L-AP)(2-)}] 1, [Pd-II{(L-ISQ)(-)}][PF6]center dot CH2Cl2 2, and [Pd-II{(L-IBQ)(0))][BF4](2)does not satisfy center dot CH2Cl2 3, as dianionic (LAP2-), monoanionic o-iminobenzosemiquinonate(1-) p-radical (Srad = (1)/2) (LISQ-), and neutral o-iminobenzoquinone (L-IBQ)(0) redox level. Reaction of 1 and 2 with PPh3 afforded isolation of two crystalline complexes: dark green [Pd-II(L)(PPh3)] 4 and red [Pd-II{(LISQ-}(PPh3)][PF6]center dot CH2Cl2 5. X-ray structure determination of 5 at 100(2) K revealed (PdON2P)-O-II coordination environment. The square-planar complexes 1-5 possess an S = 0, (1)/2, 0, 0, and (1)/2 ground-state, respectively, as was established by H-1 NMR and EPR spectroscopy, and room-temperature magnetic moment data. All redox processes are thus shown to be ligand-based. Absorption spectral measurements were done for all complexes. DFT calculations at B3LYP-level of theory adequately describe the electronic structures of 1-3, and 5, containing a spin-paired d(8) Pd-II ion. Time-dependent-DFT calculations on 1-3 and 5 shed light on the origin of UV-vis-NIR spectral absorptions.