Ligand-centered oxidation

Complexes (690) undergo two one-electron reversible reductions and two oxidations, all of which appeared ligand centered. Thus, these ligands behave electrochemically much like... 

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. 

Larger dendrimers based on a Ru(bpy)2+ core and containing up to 54 peripheral methylester units (12) have recently been obtained [29a]. Both the metal-centered oxidation and ligand-centered reduction processes become less reversible on increasing dendrimer size [29b]. 

The dinuclear [Fe(C5H5)(C0)2(CgH4S8-C8H4Sg)Fe(C5H5)C02] organometallic complex exhibits ligand-centered oxidation at a low potential [94]. The crystalline material that forms upon chemical oxidation with iodine contains both I3 and I5 anions and has a room temperature compressed pellet conductivity of 1.7 x 1CT4 S cm-1. 

The synthesis of Au(ppy)(CxH4Sg) and Au(ppy)(C 10-C6SX), including the crystal structure of the former, have been reported [97]. The triiodide and TCNQ salts of both complexes have also been prepared by chemical oxidation. Analysis of the EPR spectra indicates that the oxidation is centered on the dithiolate ligands. High room temperature conductivities of 2-4 x 10-2 S cm-1 were measured on compacted polycrystalline samples for the oxidized complexes. 

A redox activation mechanism usually involves a change in oxidation state of the metal (but could also be ligand-centered). Typically, the oxidation state changes from a state in which... 

Table IV lists a series of octahedral (phenolato)chromium(III) precursor complexes that contain one or three oxidizable coordinated phenolato pendent arms (146, 154). These complexes display characteristic electrochemistry Each coordinated phenolato ligand can undergo a reversible one-electron oxidation. Thus complexes with one phenolato moiety exhibit in the C V one reversible electron-transfer process, whereas those having three display three closely spaced (AE1/2 250 mV) ligand-centered one-electron transfer processes, Eqs. (7) and (8).

The coordination chemistry of (phenoxyl)manganese complexes is rather more complicated because both metal- and ligand-centered electron-transfer processes are accessible in the normal potential range. The phenolato precursors are known to exist with manganesc(II), (III), and even (IV). In fact, three phenolato groups strongly stabilize the Mn(IV) oxidation state. 

Only [MnIV(LBuMet)]+ can be reversibly one-electron oxidized yielding a phenoxyl species [MnIV(LButMet )]2+ at Em = +0.56 V, Eq. (11). Two further irreversible oxidation steps may also be ligand-centered processes. 

Table VII lists three precursor complexes of the type tris(phenolato)cobalt(III) and four compounds containing only one coordinated phenolate (146, 152). In the cyclic voltammograms of [Com(LBu2)] and [Com(LBuMet)] three reversible, ligand-centered, one-electron oxidation waves are observed. On the time scale of a Cou-lometric experiment, only the monocations [Com(LBu2 )]+ and [Com(LBuMet )]+ are stable and were spectroscopically characterized. In contrast, [Conl( L,Bu2)(acac)]+ undergoes a reversible one-electron oxidation yielding the dication [Com(T/Bu2 )(acac)]2+, which is fairly stable in solution.

Wang and Stack (211) reported seven four-coordinate bis(phenolato)copper(H) complexes that were chemically one-electron oxidized with tris(4-bromo-phenyl)aminium hexachloroantiomonate to the corresponding EPR silent (phe-noxyl)copper(II) species. These compounds are schematically shown in Fig. 28. In a subsequent paper (212), these authors showed by Cu K-edge XAS that oxidation of the neutral species to the monocation is ligand centered with formation of (phe-noxyl)copper(II) species, in excellent agreement with similar experiments on GO. 

The first two pathways (a) and (b) show, respectively, the influence of H+ and of surface complex forming ligands on the non-reductive dissolution. These pathways were discussed in Chapter 5. Reductive dissolution mechanisms are illustrated in pathways (c) - (e) (Fig. 9.3). Reductants adsorbed to the hydrous oxide surface can readily exchange electrons with an Fe(III) surface center. Those reductants, such as ascorbate, that form inner-sphere surface complexes are especially efficient. The electron transfer leads to an oxidized reactant (often a radical) and a surface Fe(II) atom. The Fe(II)-0 bond in the surface of the crystalline lattice is more labile than the Fe(III)-0 bond and thus, the reduced metal center is more easily detached from the surface than the original oxidized metal center (see Eqs. 9.4a - 9.4c). 

Kinetic and mechanistic studies of nucleophilic substitution at metal(IV) centers are fairly rare (263). Platinum(IV) has the substitution-inert low-spin d configuration, and presumably undergoes nucleophilic substitution by an associative mechanism thanks to its high charge and large size. However there are actually very few data, probably thanks to the tendency for platinum(IV) to oxidize ligands. Substitution kinetics at metal(IV) centers may be more conveniently studied for complexes of the type ML2X2, where M — e.g., Sn, Ti, V, or... 

It is remarkable that the oxidized states of the cytochromes cdi from P pantotrophus and P. aeruginosa have different structures. It is not clear at present whether one of these structures is superior for catalyzing nitrite reduction. Certainly in the P. pantotrophus enzyme the ligand switching at both ligand centers upon changing oxidation state is... 

Tetracyano ligands have been used to bridge between four Ru(NH3)5 moieties. The complexes [ Ru(NH3)5 4(/i-L)] + (L = tetracyanoethene, tetracyano-p-quinodimethane, 1,2,4,5-tetracyano-benzene, 2,3,5,6-tetracyanopyrazine) exhibit intense, long-wavelength electronic absorptions. Oxidation to [ Ru(NH3)5 4(yU-L)] °" " and reduction to [ Ru(NH3)5 4(//-L)] + and [ Ru(NH3)5 4-(/i-L)] + can readily be achieved. The species are fully delocalized with partially reduced ligands or partially oxidized Ru centers. Treatment of [5,10,15,20-tetrakis(4-cyanophenyl)porphyrinato] cobalt(II) or [5,10,15,20-tetrakis(4-cyano-2,6,-dimethylphenyl)porphyrinato]cobalt(II) with [Ru-(NH3)5(0S02CF3)] introduces cyano-bound pendant Ru (NH3)5 groups to the porphyrinato complexes. ... 

The synthesis and properties of [ Os(bpy)2 3(/r-181)] have been reported the formal oxidation state of the Os centers is +3. In contrast, [ Ru py)2 3(/i-181)] + contains Ru° with the bridging ligand possessing three semiquinone domains. The complex undergoes three reversible ligand-centered oxidations taking it ultimately to [ Ru(bpy)2 3(/i-181)]°" with (181) in a tris(quinone) form. The wavelength of the intense NIR absorption (MLCT) that the complex exhibits varies with oxidation state. " ... 

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