Ligand-radical complexes

Dithiolenes have been found to stabilize nickel(III) and a number of structural investigations have been performed on nickel(III) dithiolene complexes. Structural data and physical properties of selected compounds are collected in Table 120. The EPR spectra of the [NiS4] unit have been extensively studied in order to decide whether the unpaired electron resides mainly on the metal or on the ligand3202,3203,3210,3212-3217 giving rise to a true nickel(III) complex (422) or to a nickel(II)-stabilized ligand radical complex (423). 

The reaction of terpy with zinc amalgam in 2-methyltetrahydrofuran results in the formation of the ligand radical complex [Zn(terpy)2], which has been characterized by ESR spectroscopy (79). 

III. PERSISTENT LIGAND RADICAL COMPLEXES OBTAINED BY LIGAND OXIDATION... 

For example, in the Fe (d ) coordinated phenoxyl-radical complex (Fe -0 -Ph), the formal oxidation state of the metal is classed as +IV, since a closed shell phenolato anion would have to be removed. However, in many cases spectroscopic measurements, amongst others EPR, have proven the presence of a high-spin d electron configuration at the iron and a phenoxyl ligand in such complexes. In this case, the iron ion has a physical oxidation number of +III even though the formal oxidation state would be classed as -I-IV. As a result of these potential confusions, several research groups have prepared numerous examples of metal-coordinated ligand-radical complexes, particularly coordinated phenoxyl radicals, in order to examine the nature of the metal oxidation states and the extent of spin delocalisation in such complexes. 

The reaction of cobalt(II) salen complexes with -quinones results in the formation of a [cobalt(III)-(salen)-(t7-SQ)] complex, where ( >-SQ) represents the semiquinone radical ion. (Similar reactions are observed with iron(II) and manganese(II)-salen complexes, the reactions with these metal centers being more extensive than those for cobalt(II)). In the case of 3,5-di-rm-butyl-t>-benzoquinone, however, the cobalt(III) ligand radical complex has been isolated, and from hyperfine coupling constant studies which are assumed as diagnostic of the extent of electron transfer, the complex is described best as a low-spin cobalt(III) system with a coordinated semiquinone. 

An optically transparent thin-layer electrode (OTTLE) study18 revealed that the visible spectra of the reduced forms of [Ru(bipy)3]2+ derivatives can be separated into two classes. Type A complexes, such as [Ru(bipy)3]2+, [Ru(L7)3]2+, and [Ru(L )3]2+ show spectra on reduction which contain low-intensity (e< 2,500 dm3 mol-1 cm-1) bands these spectra are similar to those of the reduced free ligand and are clearly associated with ligand radical anions. In contrast, type B complexes such as [Ru(L8)3]2+ and [Ru(L9)3]2+ on reduction exhibit spectra containing broad bands of greater intensity (1,000 [Pg.584]

We are therefore faced here with radical complexes which easily distort depending on the structural arrangement and whose SOMO is different for every crystal structure associated with a given counter-ion, a very original feature in these series. The unfolded d1 complexes can be described as Mo(IV) complexes with a spin density essentially localized on the dithiolene ligand while the more folded complexes have a stronger metal character. This variable spin density delocalization is expected to influence strongly the amplitude and dimensionality of intermolecular interactions between radical species in the solid state, as detailed below in Sect. 3. 

CpMo(S2C2Ph2)2] at 2.0275, 2.0074 and 1.9936 or from oriented single-crystal for [Cp Mo(dmit)2] at 2.027, 2.012 and 1.992, demonstrate an extensive delocalization of the unpaired electron the dithiolene ligands. The solid state properties of these series of radical complexes will be described below in detail in Sect. 3. 

As mentioned above for the [Cp2Mo(dithiolene)]+, the [Cp Mo(dithiolene)2] and the [CpNi(dithiolene)] radical complexes, the spin density is not only partially delocalized on the dithiolene ligand but also on the metal and even the Cp rings. This peculiar feature opens new paths for intermolecular interactions in the solid state besides the direct dithiolene/dithiolene overlaps, since Cp/dithiolene and Cp/Cp contacts are also to be considered. 

Most of the kinetic models predict that the sulfite ion radical is easily oxidized by 02 and/or the oxidized form of the catalyst, but this species was rarely considered as a potential oxidant. In a recent pulse radiolysis study, the oxidation of Ni(II and I) and Cu(II and I) macrocyclic complexes by SO was studied under anaerobic conditions (117). In the reactions with Ni(I) and Cu(I) complexes intermediates could not be detected, and the electron transfer was interpreted in terms of a simple outer-sphere mechanism. In contrast, time resolved spectra confirmed the formation of intermediates with a ligand-radical nature in the reactions of the M(II) ions. The formation of a product with a sulfonated macrocycle and another with an additional double bond in the macrocycle were isolated in the reaction with [NiCR]2+. These results may require the refinement of the kinetic model proposed by Lepentsiotis for the [NiCR]2+ SO/ 02 system (116). 

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