The structures and the OOO and MOO bonding characters of a series of reported side-on ( 2 ) 1:1 metal complexes of O2 are analyzed by using density functional theory calculations. Comparison of the calculated and experimental systems with respect to OOO bond distance, OOO stretching frequency, and OOO and MOO bond orders provides new insights into subtle influences relevant to O2 activation processes in biology and catalysis. The degree of charge transfer from the generally electronrich metals to the dioxygen fragment is found to be variable, such that there are species well described as superoxides, others well described as peroxides, and several cases having intermediate character. Increased charge transfer to dioxygen takes place via overlap of the metal dxy orbital with the in-plane * orbital of O2 and results in increased MOO bond orders and decreased OOO bond orders. Comparison of theory and experiment over the full range of compounds studied suggests that reevaluation of the OOO bond lengths determined from certain x-ray crystal structures is warranted; in one instance, an x-ray crystal structure redetermination was performed at low temperature, confirming the theoretical prediction. Librational motion of the coordinated O2 is identified as a basis for significant underestimation of the OOO distance at high temperature. With the longstanding goal of understanding in detail how dioxygen binds and is activated at metal centers in biological and catalytic systems, great effort has been expended to characterize the structures, physicochemical properties, and reactivity of metal-dioxygen complexes (1-8). As the initially formed species in most oxidation processes, 1:1 metal-O 2 adducts are of special interest, particularly in view of their postulated involvement in enzymes that react with dioxygen at an isolated monometallic active site, such as Cu in dopamine ␤-monooxygenase or galactose oxidase (9). Two binding modes have been identified in 1:1 metal-O 2 complexes, end-on ( 1 ) and side-on ( 2 ). These adducts have been further defined as superoxo or peroxo complexes, primarily on the basis of x-ray structural data (OOO bond distance) and vibrational spectroscopy (OOO stretching frequency, OO ) (1-4). Thus, compounds with an OOO bond length of Ϸ1.4-1.5 Å and OO between Ϸ800 and 930 cm Ϫ1 are designated as peroxides, whereas those with OOO Ϸ1.2-1.3 Å and OO Ϸ1,050-1,200 cm Ϫ1 fall into the superoxide category. On this basis, and keeping in mind the caveat that it is often difficult to assess experimentally the actual amount of charge transferred to dioxygen on complexation, the majority of the known 2 1:1 compounds are generally agreed to be best described as peroxo complexes (1,5,8).Reports of , Tris(3,5-dimethylpyrazol-1-yl)hydroborate) (13) (Scheme 1). In the course of the investigation of 6, for which intermediate character of the dioxygen unit was found (14, 15), we noted puzzling incongruities among the data reported for these complexes (Table 1). For example, whereas 1 and 2 (as reported in ref. 11, d...

Scheme 5. Hydrogen Atom Transfer from Toluene to Permanganate Oxo Ligand Scheme 6. C-H Bond Oxidation of Dihydroanthracene by a Dinuclear (µ-Oxo)Manganese Complex (L ) 1,10-phenanthroline) Scheme 7. C-H Bond Oxidation of Toluene via Initial Hydride Abstraction by Manganese(IV) Dimer [(phen) 2 Mn(µ-O) 2 Mn(phen) 2 ](ClO 4 ) 4 (L ) 1,10-Phenanthroline)

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Reduction of a diazadiene chromium halide complex, [(HLiPr)Cr(μ-Cl)]2 (1, HLiPr = bis(2,6-diisopropylphenyl)diazadiene), with KC8 gave a diamagnetic, bimetallic complex, (HLiPr)2Cr2 (2). Complex 2 has been structurally characterized by X-ray crystallography and consists of a Cr2 unit spanned by two diazadiene ligands. The very short Cr−Cr distance (1.8028(9) Å) and low formal oxidation state of the Cr atoms suggest that the Cr−Cr bond order may be greater than 4. Spin-restricted and spin-unrestricted DFT calculations on a model complex both converge on a singlet spin state and accurately reproduce the metric parameters of 2. The calculations show high-order metal−metal bonding along with extensive delocalization over the diazadiene ligands. Natural resonance theory analysis shows population of resonance structures that contain quintuple bonds, and natural bond order analysis gives a bond order of 4.28 for the Cr−Cr bond.

In this Communication, the imido complex [Tp tBu,Me Co = NAd] (1) was incorrectly characterized as a paramagnetic compound with a spin ground state of S = 1. Further investigation has now revealed that this compound, like all other known Co(III) imido complexes, has a spin ground state of S = 0 and is diamagnetic at low temperatures. However, at T > 280 K, 1 undergoes a spin crossover to a thermally accessible open-shell state. All the experimental observations reported in the original Communication remain unchanged, but their interpretation is more complicated than originally realized.

Calcium is required for the substrate binding and for the chemical step of the interfacial catalytic turnover cycle of pancreatic phospholipase A2 (PLA2), but not for the binding of the enzyme to the interface. The role of calcium and other divalent cations (C) is analyzed for the effect on the substrate binding and kcat* for the chemical step. The cofactor role of 3d-cations(II) (C) for the hydrolysis of dimyristoylphosphatidylmethanol (DMPM) vesicles is characterized as an equilibrium dissociation constant for the interfacial binary (E*C) and ternary (E*CL) complexes of PLA2 and substrate mimics (L). Of the cations(II) that promote the binding of a mimic to the enzyme at the interface (E*), only a subgroup supports the chemical step. For example, Cd, Zn, and Cu form ternary E*CL complexes with kcat* of <1 s-1, compared to the rate of >100 s-1 with Ca, Fe, Mn, Co, and Ni. Oxygen exchange from H218O to the products of hydrolysis of DMPM incorporates one 18O in myristate. Incorporation of the first and second 18O occurs during the incubation of both the products of hydrolysis in H218O with PLA2 and Ca, but not with Zn. The cation-dependent changes in the UV difference spectrum, associated with the formation of E*C and E*CL, suggest that the changes are mainly due to catalytic His-48, and possibly Tyr-52 and Tyr-73, and are different with Ca as opposed to Zn. These results and simulations suggest considerable plasticity in the calcium binding and catalytic site environment. It is proposed that the higher ground state stability of the E*CS complex with the inhibitory cations increases the effective activation energy. For the chemical step, calcium coordinated with a nucleophilic water and the ester carbonyl oxygen facilitates the near-attack geometry in the E*CaS, and the His-48.Asp-99 pair acts as a proton acceptor. As a prelude to establishing the catalytic mechanism, factors controlling the energetically demanding transition state are also discussed.

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1c)) were prepared and their structures determined by X-ray diffraction. In the presence of excess methylaluminoxane (MAO), 1a-c catalyzed the homopolymerization of ethylene and the copolymerization of ethylene with R-olefins.