The nickel(II) complexes 1(X) supported by bis[(pyridin-2-yl)methyl]benzylamine tridentate ligands carrying m-substituted phenyl groups (X = OMe, Me, H, Cl, NO2) at the 6-position of pyridine donor groups (L-X, N,N-bis [(6-m-substituted-phenylpyridin-2-yl)methyl]benzylamine) have been synthesized and characterized. The X-ray crystallographic analyses have revealed that [Ni-II(L-H)(CH3CN)(H2O)](ClO4)(2) (1(H)), [Ni-II(L-OMe)(CH3CN)(MeOH)](ClO4)(2) (1(OMe)), [Ni-II(L-Me)(CH3CN)(H2O)](ClO4)(2) (1(Me)), and [Ni-II(L-Cl)(CH3CN)(H2O)](ClO4)(2) (1(Cl)) have a five-coordinate square pyramidal geometry, whereas [Ni-II(L-NO2)(CH3CN)(2)(H2O)](ClO4)(2) (1(NO2)) exhibits a six-coordinate octahedral geometry having an additional CH3CN co-ligand. H NMR spectra of the nickel(II) complexes 1(X) in CD3CN have indicated that all the complexes have a high spin ground state. The nickel(II) complexes 1(X) react with hydrogen peroxide (H2O2) in acetone to give bis(mu-oxo)dinickel(III) complexes 2(X) exhibiting a characteristic UV-vis absorption band at similar to 420 nm. In the case of 2(H), a resonance Raman band ascribable to a Ni2O2 core vibration was observed at 611 cm(-1) that shifted to 586 cm(-1) upon (H2O2)-O-18. The bis(mu-oxo)dinickel(III) intermediates 2(X) undergo an efficient aromatic ligand hydroxylation reaction, producing a mononuclear nickel(II)-phenolate complexes 4(X) via a putative intermediate (mu-phenoxo)(mu-hydroxo)dinickel (II) (3(X)). The kinetic studies on the aromatic ligand hydroxylation process including m-substituent effects (Hammett analysis) and kinetic deuterium isotope effects (KIE) have indicated that the reaction of 2(X) to 3(X) involves an electrophilic aromatic substitution mechanism, where C-O bond formation and C-H bond cleavage occur in a concerted manner. Intermediate 3(H) was detected by ESI-MS during the course of the reaction, and the final product 4(H) was characterized by elemental analysis, ESI-MS, and X-ray crystallographic analysis.