A nickel(II) enediolate cluster (2) forms upon treatment of [(6-Ph-2-TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (1) with Me4NOH center dot 5H(2)O in CH3CN. Crystallographic studies of 2 revealed a hexanuclear structure of S-6 symmetry with a formula of {[Ni(PhC(O)C(O)C(O)Ph)(CH3OH)] center dot 1.33CH(3)OH}(6). Because isolation of bulk amounts of 2 from the reaction involving 1 proved impossible, a solvation analogue of 2 (labeled 5) was generated from admixture of Ni(ClO4)(2)center dot 6H(2)O, 2-hydroxy-1,3-diphenylpropane-1,3-dione, and Me4ON center dot 5H(2)O in CH3OH/CH3CN. Complex 5 is formulated as {[Ni(PhC(O)C(O)C(O)Ph)(H2O) 0.25CH(3)CN}(6) based on elemental analysis, a molecular weight determination, UV-vis, and a magnetic moment measurement. Treatment of 5 with O-2 and 6-Ph(2)TPA (6 equiv) results in the formation of CO and [(6-Ph(2)TPA)Ni(O(2)Cph)(2) (H2O)] (3), the latter of which was isolated in 69% yield. The level of O-18 incorporation in this reaction matches that for a reaction wherein 2 is generated from 1. These results provide evidence that a nickel(II) enediolate cluster is the O-2 reactive species in a previously reported model reaction for nickel(II)-containing acireductone dioxygenase.