Spectroscopic and density functional theory (DFT) electronic structure computational studies on a binuclear bis-mu-oxo bridged (Ni(3+))(2) complex, [(PhTt(t(Bu))(2)Ni(2)(mu-O)(2)] (1), where (PhTt(t(Bu)) represents phenyl-tris((tert-butylthio)methyl)borate, are presented and discussed. These studies afford a detailed description of the Ni(2)O(2) core electronic structure in bis-mu-oxo (Ni(3+))(2) dimers and provide insight into the possible role of the (PhTt(t(Bu)) thioether ligand in the formation of 1 from a Ni(1+) precursor by O(2) activation. From a normal coordinate analysis of resonance Raman data, a value of k(Ni)(-)(O) = 2.64 mdyn/A is obtained for the Ni-O stretch force constant for 1. This value is smaller than k(Cu)(-)(O) = 2.82-2.90 mdyn/A obtained for bis-mu-oxo (Cu(3+))(2) dimers possessing nitrogen donor ligands, indicating a reduced metal-oxo bond strength in 1. Electronic absorption and magnetic circular dichroism spectroscopic techniques permit identification of several O-->Ni and S-->Ni charge transfer (CT) transitions that are assigned on the basis of DFT calculations. The dominant O-->Ni CT transition of 1 occurs at 17 700 cm(-)(1), red-shifted by approximately 7000 cm(-)(1) relative to the corresponding transition in bis-mu-oxo (Ni(3+))(2) dimers with nitrogen donor ligands. This red-shift along with the relatively low value of k(Ni)(-)(O) are due primarily to the presence of the thioether ligands in 1 that greatly affect the compositions of the Ni(2)O(2) core MOs. This unique property of the thioether ligand likely contributes to the reactivity of the Ni center in the precursor [(PhTt(t(Bu))Ni(1+)CO] toward O(2). DFT computations reveal that conversion of a hypothetical side-on peroxo (Ni(2+))(2) dimer, [(PhTt(t(Bu))(2)Ni(2)(mu-eta(2):eta(2)-O(2))], to the bis-mu-oxo (Ni(3+))(2) dimer 1 is energetically favorable by 32 kcal/mol and occurs without a significant activation energy barrier (DeltaH++) = 2 kcal/mol).