Ligand-radical species

Oxidation of d V(V) complexes must occur at the ligands. Although many oxidations are irreversible, some reversible processes are known. For example, the square-pyramidal bimetallic complex shown in Fig. 23 has two reversible oxidations Ef" = —0.05 and 0.37 V versus Cp2Fe/MeCN), the first of which results in a cationic ligand radical species, on the basis of its Ef value, its EPR spectrum, and UV-vis data [111]. The complex also shows two closely spaced reductions (Ef =—1.2(> and —1.36 V... 

Ni(III) complexes often exhibit equilibrium with Ni(II) ligand radical species. For example, the Ni(III) complex of lacunar cyclidene is octahedral [NimL(CH3CN)2]3+ at low temperature, but at high temperature it transforms into the Ni(II) complex with an oxidized ligand radical [NinL+]3+, identifiable by the absorption at 590 nm. The Ni(III) complexes and Ni(II) species with an oxidized ligand radical exhibit a thermal equilibrium (Eq. 12) (105). 

The reaction of [OH] with Co(IIl)-polyamine complexes involves hydrogen abstraction at the ligand followed by rapid intramolecular electron transfer to form a Co(II)-coordinated ligand-radical species that then undergoes ligand labilzation. ... 

The [Ni(EDTA)] (EDTA = ethylenediaminetetraacetate) and [Ni(NTA)] ions (NTA = nitrilotriacetate) react with [OH] and other oxidizing radicals to yield transient absorptions arising from the corresponding Ni(IlI) species ". Experiments with [Ni(NTA)] indicate that the reaction with [OH] forms a coordinated ligand-radical species from hydrogen abstraction, which reacts" with [Ni(NTA)] , [IrClJ ... 

Co(II)-coordinated ligand-radical species that then undergoes ligand labilzation . 

The reaction (585, 707) of cupric dithizonate and diphenylcarbazate with alkali is interesting in that the color change and ESR signal indicate that a ligand radical species is formed in solution. The reaction is reversible with acid. 

A different interpretation of the spectral and kinetic data has been put forward, namely that OH abstracts an H atom from the NH groups in (1) and (2) to form a ligand radical species Ni L which absorbs at 540 nm, as well as oxidizing the metal centre directly, and that the pH-dependent spectral changes are due to interconversion of Ni L and Ni L ... 

Gagne RR, Ingle DM (1981) One-electron-reduced nickel(II)-macrocyclic ligand complexes. Four-coordinate nickel(I) species and nickel(II)-ligand radical species which form paramagnetic, five-coordinate nickel(I) adducts. Inorg Chem 20 420-425... 

Nucleophilic reactions with coordinated heterocycles are also represented, and the interaction of [PhNN = NNPh] with [Cu(phen)Cl2) gives the complex (50), formally derived by nucleophilic attack upon the 1,10-phenanthroline. It is proposed that the reaction proceeds by the initial formation of (51), which then rearranges to the nitrene (52), and that it is (52) which reacts to form the new C—N bond. If we regard an electron as the simplest possible nucleophile, the reaction of [Pt(phen)L2] (L = NHj, pyridines) with zinc in ethanol has been shown to generate the blue ligand radical species [Pt (phen )L2]". ... 

---