The potentials for the ML3"/ML3 couple of MnL3, FeL3, and CoL3 complexes (L = acetylacetonate, 8-quinolinate, picolinate, 2,2'-bipyridyl, 1,10-phenanthroline) occur at substantially less positive values than those for their zinc analogues and are clearly ligand-centered. The negative shift in potential for these ligand oxidations is proportional to their metal-ligand covalent-bond energies. The reductions for the bipyridyl and phenanthroline complexes of these transition metals also are ligand-centered. Electrochemical characterization of tetrakis(2,6-dichlorophenyl)porphine and of its neutral porphinato complexes with Zn, Mn, Fe, and Co indicates that electron transfer occurs within the porphyrin ring and that the metal-porphyrin bonding involves covalent a bonds between d"sp valence electrons of the neutral metal (or hydrogen atoms of porphine) and two pyrrole p electrons of the uncharged porphyrin.

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087ChemInform Abstract In pyridine/acetic acid solvent complexes such as (I) and its µ-oxo dimer (II) catalyze hydrogen peroxide for the selective ketonization of methylenic carbons and the dioxygenation of acetylenes to α-diketones and arylolefins to aldehydes; e.g. cyclohexane is transformed with 72% efficiency to give 95% cyclohexanone and 5% cyclohexanol. On the basis of the relative reaction efficiencies and spectrophotometric, electrochemical and magnetic results a mechanism is discussed. A reactive binuclear dioxygen intermediate via in situ formation of hydrogen peroxide and (II) is proposed.

2645altered v( Fe"-02) value reported for HRP-I11 may be attributable to some other type of distal perturbation.For insight into the nature of this perturbation, we consider the function, in particular the initial reaction steps, of a peroxidase.The peroxidase heme pocket binds hydrogen peroxide in a manner that facilitates the cleavage of the 0-0 bond. This is most likely accomplished by the action of a positively charged arginine group that pulls electron density out of the 0-0 bond of the peroxide?"Removal of electron density from the 0=0 bond of dioxygen, however, strengthens the bond because the HOMO is antibonding.Thus, when O2 rather than HOOH is bound in the peroxide heme crevice, the oxygen-oxygen bond is strengthened and v(O=O) increases while v(Fe"-02) decreases. These distal effects on the electron density in the 0=0 bond of the bound dioxygen presumably overshadow the trans-ligand effects of the proximal histidine and thus explain the Y( Fe"-02) frequency of HRP-111. On the other hand, in an oxidase, cleavage of the dioxygen bond is likely accomplished by adding electron density into the 0 4 bond, as this further populates the x* orbital. Thus, in cytochrome oxidase intermediates, the dioxygen may assume a configuration that tends to build electron density between the oxygen atoms, resulting in a decrease in the v ( O = O ) frequency and a concommitant increase in the v(Fe"-02) frequency. Our time-resolved studies, however, show that the initial Cyt Ox a3-02 complex is relatively unperturbed by distal effect^.^ Thus, weakening and rupture of the 0-0 bond occurs in a subsequent step of the reaction. Acknowledgment.