Radicals ligand-centered

The redox properties elicited for Rh(bpy)3 + and its congeners are thus entirely consistent with the description of these species as bound-ligand radicals. On the other hand, the disproportionation reactions eq 2-6 are not known to be characteristic of ligand-centered radicals, but are consistent with behavior expected for rhodium(II). Furthermore the substitution lability deduced for Rh(bpy)3 + and Rh(bpy)2 +> while consistent with that expected for Rh(II), is orders of magnitude too great for Rh(lII). Finally the spectrum observed for the intermediate Rh(bpy)3 + is not that expected for [RhIII(bpy)2(bpy")]2+. The spectrum measured has an absorption maximum at 350 nm with e 4 x 10 M 1 cm l and a broad maximum at 500 nm with e = 1 x 1()3 M 1 cm l. The spectra of free and bound bpy radical anions are quite distinctive (23.35-38) very intense absorption maxima (e 1 x 10 to 4 x 10 M - cm l) are found at 350-390 nm and are accompanied by less intense maxima (e 5 x 10 cm ) at 400 to 600 nm. While the Rh(bpy)3 +... 

In a related experiment by the same authors,119 ESR data were found consistent with a ligand-centered radical, probably the easily oxidized tdt ligand. [Pt(tdt)(bipy)]+ decomposed into unidentified products. Photolysis of [Ni(S2C2Ph2)(phen)] in CHC13 at room temperature promoted an analogous photooxidation. Here, subsequent reaction of the unstable cationic complex resulted in the formation of the symmetric complexes [Ni(S2C2Ph2)2] and [Ni(phen)2]2+. 

Fig. 5 Generation of valence tautomeric ligand-centered radicals from paramagnetic transition metal complexes...

A reverse mode of radical addition reactions is also possible, in which a ligand-centered radical would add to a radical acceptor [475]. Catalytic applications of such a process are so far missing. 

VII. LIGAND-CENTERED RADICALS A. Radical Carbon a to an Arsenic Atom... 

Furthermore, steric bulk increase around the metal center has been proven useful to promote ligand-centered radical reactivity. Several examples of ligand-centered reactivity in metalloporphyrin systems have been previously discussed. The (TMP)RhCO (TMP = tetramesitylporphyrin) couples through the CO ligand to form the diketone dimer.90... 

While many of the species discussed in this chapter are metal-centered radicals, it is clear that formation of ligand-centered radicals by oxidation of redox noninnocent radicals can also play an important role. This includes oxidation of the known redox noninnocent dithiole-thionate-, indole-, and catechol-based ligands, as well as the rather unexpected formation of aminyl radicals upon oxidation of Rh NR2) amido species. 

Reactions of tris(3,5-di-tert-butylcatecholato)manganate(IV) and the manganese(III) complex with O2 and O2 in aprotic media produce superoxo and peroxo adducts, ligand-centered radicals, and disproportionation products, as shown by magnetic, spectroscopic, and electrochemical studies. 

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