Ligand-centered redox processes

Since most Ni1 species with simple N-donor ligands are prone to disproportionation into Ni° and Ni11, relatively few Ni1 complexes with nonmacrocyclic N-donor ligands have been reported. Formation of Ni1 species is in most cases proposed on the basis of electrochemical data, although ligand-centered redox processes have to be considered. The ligands usually contain imine donor atoms or aromatic N-heterocycles, which because of their 7r-acceptor ability favor stabilization of lower oxidation states. 

The potentially sexadentate macrocyclic ligands l,4,7-tris(3,5-dimethyl-2-hydroxybenzyl)-l,4,7-triazacyclononane (L H3), l,4,7-tris(3,5-di-t-butyl-2-hydroxybenzyl)-l,4,7-triazacyclononane (L"H3), and l,4,7-tris(3-t-butyl-5-methoxy-2-hydroxybenzyl)-l,4,7-triazacyclononane (L" H3) form the stable complexes of type [Mn L"]PF(5, [Mn L" ]PF(5 [M L "] [Mn L ]2(C104)3-(H30)(H20)3. These complexes were investigated by spectroelectrochemistry and were shown to undergo metal- and ligand-centered redox processes a phenoxyl radical Mn complex, [Mn L" ] +, was found to be accessible. 

The vast majority of the coordination compounds of Os that have been prepared are in the oxidation states 11 and III. Moreover, many of these compounds show reversible or well defined Os / couples in which the electronic and redox properties at the metal are controlled by the a-donor, 7r-acceptor, and r-donor properties of the ligands. Indeed, the study of the redox behavior in Os / and Ru / species, metal ions in which octahedral coordination is almost universally retained in both redox partners, has been central in recent developments to parameterize metal centered redox processes as a function of ligand donor and acceptor capacity. The chemistries of Os and Os are, therefore, intimately linked, and have been extended to studies of important mixed valence Os / binuclear and polynuclear species (see Mixed Valence Compounds). For the purposes of brevity and convenience, this section will deal with Os and Os complexes together. The extensive literature on Os / complexes has been developed with a very wide range of donor ligands a comprehensive assessment of this work is beyond the scope of this article, and the reader is directed to published comprehensive reviews. " ... 

The redox processes observed might involve either metal-centered reactions (d or s orbitals) or electron transfer to the organic ligand [n orbitals of the dpp subunits). In some cases, the assignment can be made via EPR measurements and redox potential values. If the Cu +/Cu+ couple always involves a metal-centered redox process, the electron transfer to Li.5+ clearly occurs on the ligand moiety. No distinction can be made between both pathways for many of the complexes studied. 

Complexes of porphyrins90,102-108 and related macrocycles, such as phthalocyanines,109 chlor-ins,103 and tetraazaannulenes110,111 are popular candidates for study, in part due to their intense Raman bands. Typically, the ligand spectra change dramatically if the redox process is centered on the ligand, whereas metal-centered redox processes cause shifts in a limited number of... 

The Lever model has typically been applied to octahedral-type six-coordinate complexes, with metal-centered redox processes, but extensions to other types of complexes have also been proposed, namely, to square-planar four-coordinate and five-coordinate Rh /Rh complexes [39, 76, 77], to sandwich complexes [23, 24, 31, 32], to Ru clusters [78-80], and also to complexes with ligand-centered reduction processes [24]. Relevant or representative cases are discussed in the following sections. 

Finally, we consider the alternative mechanism for electron transfer reactions -the inner-sphere process in which a bridge is formed between the two metal centers. The J-electron configurations of the metal ions involved have a number of profound consequences for this reaction, both for the mechanism itself and for our investigation of the reaction. The key step involves the formation of a complex in which a ligand bridges the two metal centers involved in the redox process. For this to be a low energy process, at least one of the metal centers must be labile. 

The first reduction in the cobalt-based polymer is metal-centered, resulting in the appearance of a new MLCT transition, with the second reduction being ligand-centered. For the nickel-based polymer, in contrast, both redox processes are ligand-based. 

This process is catalyzed by a variety of catalase enzymes, the most common being the heme catalases, which accomplish the two-electron chemistry of Eq. (12) at a mononuclear heme center. Here, both the iron and its surrounding porphyrin ligand participate to the extent of one electron each in the redox process. Manganese catalases contain a binuclear Mn center and cycle between Mn2(II,II) and Mn2(III,III) oxidation states while carrying out the disproportionation of H2O2. The enzyme can... 

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