Spectroscopic and density functional theory (DFT) electronic structure computational studies on a binuclear bis-mu-oxo bridged (Ni3+)(2) complex, [(PhTt(tBu))(2)Ni-2(mu-O)(2)] (1), where (PhTt(tBu)) represents phenyltris((tert-butylthio)methyl) borate, are presented and discussed. These studies afford a detailed description of the Ni2O2 core electronic structure in bis-mu-oxo (Ni3+)(2) dimers and provide insight into the possible role of the (PhTtBu) thioether ligand in the formation of 1 from a Ni1+ precursor by O-2 activation. From a normal coordinate analysis of resonance Raman data, a value of k(NI-O) = 2.64 mdyn/Angstrom is obtained for the Ni-O stretch force constant for 1. This value is smaller than k(cu-O) = 2.82-2.90 mdyn/Angstrom obtained for bis-mu-oxo (Cu3+)(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 similar to7000 cm-1 relative to the corresponding transition in bis-mu-oxo (Ni3+)(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 Ni2O2 core MOs. This unique property of the thioether ligand likely contributes to the reactivity of the Ni center in the precursor [(PhTtBu)Ni1+CO] toward O-2. DFT computations reveal that conversion of a hypothetical side-on peroxo (Ni2+)(2) dimer, [(PhTtBu)(2)Ni-2(mu-eta(2):eta(2)-O-2)], to the bis-mu-oxo (Ni3+)(2) dimer 1 is energetically favorable by 32 kcal/mol and occurs without a significant activation energy barrier (DeltaH(double dagger) = 2 kcal/mol).