The nickel(I) complex [PhTt(Ad)]Ni(CO) (PhTt(Ad), phenyltris((1-adamantylthio)methyl)borate) reacts with O(2) generating a 1:1 species identified as a side-on dioxygen adduct based on its spectroscopic properties as supported by DFT computational results and by its reactivity. The Ni EXAFS data are fit to a S(3)O(2) coordination environment with short Ni-O distances, 1.85 A. The brown complex displays a rhombic EPR signal with g values of 2.24, 2.19, 2.01. DFT and INDO/S-CI computations replicate the EXAFS and EPR features and suggest that 2 is a side-on [NiO(2)](+) complex with geometric and electronic properties that are best rationalized in terms of a highly covalent Ni(II)-superoxo description. [PhTt(Ad)]Ni(O(2)) oxidizes PPh(3) to OPPh(3), NO to NO(3)(-), and [PhTt(tBu)]Ni(CO) to the nonsymmetric [PhTt(Ad)]Ni(micro-O)(2)Ni[PhTt(tBu)] dimer.

DFT computational studies on the A cluster of acetyl-coenzyme A synthase are presented and discussed. They aim at evaluating possible A cluster models to settle the ongoing controversy about the nature of the proximal metal site in the catalytically active form of the cluster, recently proposed to be either Ni or Cu. Two possible models for the NiFeC species are considered, [Fe4S4]2+-Ni+CO-Ni2+ and [Fe4S4]2+-Cu+CO-Ni+. While for the former the computed 57Fe, 61Ni, and 13C hyperfine coupling parameters agree reasonably well with corresponding experimental values, for the latter model this agreement is very poor because the actual charge distribution is [Fe4S4]+-Cu+CO-Ni2+. Together, our results provide compelling evidence that the catalytically active A cluster contains Ni rather than Cu at the proximal metal site. Computations on the Ared2 state proposed to be part of the catalytic cycle (Darnault, C.; Volbeda, A.; Kim, E. J.; Legrand, P.; Vernède, X.; Lindahl, P. A.; Fontecilla-Camps, J. C. Nat. Struct. Biol. 2003, 10, 271-279) yield [Fe4S4]+-Ni+-Ni2+, hinting toward a Ni+/Ni3+ redox couple being involved in the methylation reaction.

The preparative chemistry and reactivity of NiÀO x intermediates has advanced significantly in the past few years with the identification of a number of new structure types. The most common synthetic approach, reaction of a nickel(ii) precursor with H 2 O 2 , has yielded bis-m-oxo [1][2][3][4] and bis-msuperoxo complexes.[4] An attractive alternate route utilizes the reaction of nickel(i) complexes with O 2 . This strategy relies on, and is limited by, the ability to prepare suitable nickel(i) species. We have utilized this latter approach to[5] (PhTt tBu = phenyltris((tertbutylthio)methyl)borate) and the related side-on bound superoxo complex, [{PhTt Ad }Ni(O 2 )] (PhTt Ad = phenyltris((1-adamantylthio)methyl)borate).[6] During the course of these studies, we considered whether a m-peroxo Ni 2 species was a possible intermediate along the reaction trajectory leading to the bis-m-oxo dimer. Lacking experimental evidence, a m-h 2 ,h 2 -peroxo bridged Ni 2 dimer supported by the tris(thioether) borato ligand was evaluated by density functional methods. [7] This hypothetical species was deemed to be of high energy, significantly destabilized (DH8 = 32 kcalmol À1 ) relative to the bis-m-oxo dimer and, therefore, an unlikely intermediate. Consequently, we turned our attention to macrocyclic tetradentate ligands that coordinate in a planar array, reasoning that such an attribute would render access to the m-h 2 ,h 2 -peroxo coordination more difficult. Herein, we report on the successful pursuit of this strategy, which led to the discovery of a Ni 2 (m-O 2 ) complex in which the peroxo moiety spans the metals in the m-1,2 mode.Addition of dry O 2 to [Ni(tmc)]OTf [8,9] (1; tmc = 1, 4,8,4,8, OTf = [CF 3 SO 3 ] À ) in THF or acetonitrile at room temperature resulted in a color change from pale blue to clover green. The FT-IR spectrum of the paramagnetic product (m eff = 2.7(1) m B ) included a prominent new feature at 3628 cm À1 assigned to the ñ(OÀH) mode

The five-coordinate high-spin (S = 1) Ni(2+) complex [Ni(tmc)CH(3)](+) (1) (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) serves as a model for a viable reaction intermediate of the A cluster of acetyl-CoA synthase (ACS) in which the distal nickel center is methylated. Spectroscopic and density functional theory (DFT) computational studies afford a quantitative bonding description for 1 that reveals a highly covalent Ni-CH(3) bond. From a normal coordinate analysis of resonance Raman data obtained for 1, a value of k(Ni-C) = 1.44 mdyn/Angstroms is obtained for the Ni-C stretch force constant of this species. This value is smaller than k(Co)(-C) = 1.85 mdyn/Angstroms, which is reported for the Co-C stretch in the methylcobinamide cofactor (5) that serves as the methyl donor to the A cluster in the ACS catalytic cycle. Experimentally calibrated DFT computations on viable methylated A cluster models reveal that the methyl group binds to the proximal (Ni(p)) rather than the distal (Ni(d)) nickel center and afford a simple electronic argument for this preference. By correlating the experimental force constants with the computed bond orders of the M-C bonds in 1 and 5, the Ni(p)(2+)-CH(3) bond strength for an A cluster model with a square-planar Ni(p) conformation, which is the most probable structure of the methylated A cluster on the basis of steric and energetic considerations, is predicted to be similar to the Co(3+)-CH(3) bond strength in CH(3)-CoFeSP. This similarity could be a crucial thermodynamic prerequisite for the reversibility of the enzymatic transmethylation reaction.