Series redox

Isolation and structural characterization of the whole redox series shows that, in agreement with the electrochemical reversibility, the first one-electron removal Ptn/Ptm does not substantially modify the original square planar geometry (the Pt-C bond lengths elongate by less than 0.01 A, whereas the C-C distances remain unaltered).25 By contrast, more severe are the structural consequences of the second electron... 

Applying the concept of isoelectronic compounds and redox series proposed by Kaim (1980), one can point out the existence of analogous redox systems based on BR2/ BR2 instead of -NR2/-NR2 as essential end groups of conjugated n systems. The corresponding redox series then involve anionic charges instead of cationic ones as seen in Scheme 3.59. 

The position of a system within one of these series is established by its redox potential E (see p. 18). The redox potential has a sign it can be more negative or more positive than a reference potential arbitrarily set at zero (the normal potential of the system [2 H /H2]). In addition, E depends on the concentrations of the reactants and on the reaction conditions (see p.l8). In redox series (4), the systems are arranged according to their increasing redox potentials. Spontaneous electron transfers are only possible if the redox potential of the donor is more negative than that of the acceptor (see p.l8). 

The illustration shows the important redox systems involved in mitochondrial electron transport and their approximate redox potentials. These potentials determine the path followed by the electrons, as the members of a redox series have to be arranged in order of increasing redox potential if transport is to occur spontaneously (see p. 32). 

Strong field CN stabilizes low-spin Mn in oxidation states I through IV and is unique in stabilizing a four-member redox series ... 

Several chromium nitrosyls, and complexes of quinones which are members of redox series, can be considered to contain chromium(O) or chromium(I) (see Sections 35.4.2.6 and 35.4.4.8). 

Thioxanthato (alkyl- or aryl-trithiocarbonato) (RSxan) complexes are known for a few metals only. The methods of preparation are mentioned in ref. 1. The ligand field strength does not differ much from that of the xanthates.110 IR spectra give strong carbon-sulfur stretching vibrations in the regions 990-980 and 950-940 cm-1.1 The contribution of the resonance form (98) is low.60 Electrochemical studies show extended redox series, e.g. (99)111 [compare with dithiocarbamates (100)].112... 

The [Co(mnt)2f ( = 1, 2, 3) redox series was reexamined with the aid of ESR by Vlcek and Vlcek.101 Upon chemical or electrochemical oxidation of [Co(mnt)2]2-, an initial spin-triplet monomer [Co(mnt)2] formed. Slow dimerization led to the final oxidation product [Co(mnt)2]2 , which in the solid state or in noncoordinating solvents was diamagnetic. Solution spectra in DMSO showed an ESR signal due to [Co(mnt)2] THF solution spectra were identical but indicated three times less monomer concentration. A THF solution of [Co(mnt)2]2- underwent a reversible one-electron reduction process localized on the metal to yield a green solution of a Co1 complex, [Co(mnt)2]3-, with no ESR signal. 

Vlcek and Vlcek149 have summarized the reactivity of dithiolenes in a general way. The n-delo-calization in the dithiolenes favors an extensive redox series but restricts axial interactions. Reaction in this position can only occur if the incoming ligand can incorporate its own 7t-orbitals into that of the planar complex. The dianions are nucleophilic at sulfur oxidation to the anion or neutral species lets them become electrophilic and Diels-Alder-type additions may occur. 

With the discovery of the extraordinary redox behavior of the dithiolenes and the establishment of a redox series between the neutral dithiolenes and their mono- and di-anionic analogs arose the question why these compounds are able to exist in several stable oxidation states while related species such as their SacSac analogs or even the isomeric 1,1-dithiolenes show only poorly defined and generally irreversible oxidation and reduction behavior. 

Dithio-oxamide (LH4) and dimethyldithio-oxamide (LMH2) form complexes of types Ni4(LH2)5 and Nig(LM)9 which have anomalous magnetic moments 405 The complex of iViV-diethylphenylazothioformamide (143) [NiL2]2+, formed by reaction of nickel perchlorate with the ligand in acetone, is a member of a facile redox series... 

Marcaccio M, Paolucci F, Paradisi C, et al. Electrochemistry of multicomponent systems, redox series comprising up to 26 reversible reduction processes in polynuclear ruthenium(II) bipyridine-type complexes. J Am Chem Soc 1999 121 10081-91. 

In recent years several complexes have been prepared that contain oxidant and reductant ions of the same element, with the same inner-sphere ligand environment and a symmetrical bridging group (Table VI). In the most-studied cases, analogous compounds with one more and one less electron are known, forming a redox series A + XA+(env), A+ X A(env), A X A(env), and the electronic spectrum of the mixed-valence species contains a unique band in addition to the bands characteristic of the separate A+ X and A X chromophores. 

A characteristic feature is that such ligands can form metal complexes in a wide range of oxidation states, as in the redox series... 

TABLE 2. Most significant experimental data for the [Ni(S2C2Me2)2] (16) redox series... 

FIGURE 3. Orbital energy levels for the [Ni(Me2C2S2)2], z = 2—, 1—, 0 redox series and orbital contour plots for z = 0. The MO of the ligands are divided in n (out-of-plane, along the z-axis) and a (in-plane, in the xy-plane) types. Reprinted with permission from Reference 13. Copyright 2003 American Chemical Society... 

In the preceding section, we have shown the electron-reservoir properties of the redox series [Fe(/7 -C5R5)( 7 -C6Me6)] / / on the reduction side with the neutral 19-electron Fe form. The 17-electron dicationic Fe form can be isolated as the SbXfi salts (R = Me, X = F or Cl)) and used as very strong oxidants, with a stan-... 

The well-known metal polypyridine [378, 379] and metal-porphyrin [380-382] and phthalocyanine complexes also have extended redox series which make them good candidates as electron-reservoirs. They are thoroughly reviewed and discussed in other chapters of this volume. 

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