Ni redox couple

Figure 30 A two-component fluorescent sensor of the redox potential. The Ni" derivative quenches the emission of the nearby dansyl subunit via a fluorophore-to-metal eT process, the Ni" derivative does not. The Ni /Ni redox couple potential is 0.08 V vs. Fc /Fc. When containing a redox agent with a potential lower than 0.08 V, the solution will be fluorescent If an oxidizing agent with a potential higher than 0.08 V is present, the solution will not emit any more.

However, the redox potentials of the Ni(II) complexes of the aza-cyclam (3b-3g) containing carboxamide or sulfonamide functional groups are reported to be influenced by the nature of the functional group. In particular, the amide fragment controls the reduction potential for the Ni /Ni and Ni /Ni redox couples, which may be attributed to the IT interaction between the nickel ion and the amido group (14). 

The relatively low standard potential of the Ns /Ni redox couple in aqueous solution (+1.33 0.01 V vs. NHE) is even more reduced in organic solvents, and therefore the generation of azidyl radicals by electrochemical methods is perfectly feasible. The multigram scale dimerization of styrene represents an early synthetic application of the electrochemical process (Scheme 8.15). " However, the scope of this reaction is so far limited since other substrates give poor yields and/or significant amounts of by-products. 

Ni" " redox couple while the plateau at 4.1 V is due to the Mn V Mn" redox couple. In contrast to the tetragonal structure formation upon insertion of LF into undoped spinel, the spinel LiNio.5Mn1.5O4 remains cubic to form Li2Nio.5Mni 5O4. 

Five-coordinate Ni111 complexes (89) have been prepared by oxidation of the square planar Ni11 precursor complexes [Ni(L)X] with either X2 or CuX2, and the crystal structure of the iodo derivative has been determined. The geometry at Ni is best described as square pyramidal, with the Ni atom displaced approximately 0.34 A out of the basal plane towards the apical I atom. EPR confirms the Ni111 oxidation state, in which the unpaired electron of the low-spin d1 system is situated in the dz2 orbital.308,309 In aqueous solution full dissociation of both X anions occurs, while in acetone solution dissociation is not significant. The redox couple [Nin NCN (H20)]+/ [Ni111 NCN (H20)ra]2+ in water is +0.14V (vs. SCE). 

In a new twist on this subject, electrochemical activation parameters have been obtained for two series of redox couples that undergo coupled spin-state change and electron transfer (28). One series is [M(tacn)2]3+/2+ where M = Fe, Co, Ni, and Ru, and tacn = 1,4,7-triazacyclononane. The other is [Fe(pzb)2]+/0, where pzb-= hydrotris(pyrazol-l-yl)borate... 

Nickel(III) peptide complexes have a tetragonally-distorted octahedral geometry as shown by electron spin resonance studies (19) and by reaction entropies for the Ni(III,II) redox couple (17). Axial substitutions for Ni(III)-peptide complexes are very fast with formation rate constants for imidazole greater... 

To evaluate the structural consequences that accompany the Ni(II)/ Ni(III) oxidation in solution we can compare the molecular structures of the redox couple [NiI1Cl2(Me6[14]aneN4)]/[NiII1Cl2(Me6[14]aneN4)] +. ... 

Figure 5.9 Example of a redox titration of nickel of hydrogenase from M. marburgensis. The amplitude of the Ni EPR signal is plotted against the measured redox potential. Half of the active sites in the enzyme solution is reduced at a redox potential (midpoint potential) of — 140 mV (at pH 6).The 2H /H2 redox couple has an of —354mV at this pH.The line through the points is a theoretical line assuming a midpoint potential of — 140 mV (Coremans et al. 1989).

The other simple peptide complex e.g. [Fe(Z-Cys-Ala-OMe)4]2- did not exhibit such a reversible redox couple under similar conditions. The Fe(lll) complexes of simple peptide thiolates or cysteine alkyl esters are found to be thermally quite unstable and decompose by oxidaticxi at the thiolate ligand by intramolecular electron transfer. Thus the macro-ring chelation of the Cys-Pro-Leu-Cys ligand appears to stabilize the Fe(in) state. The stability of the Fe(ni) form as indicated by the cyclic voltamnoogram measurements and by the visible spectra of the Fe(in) peptide complexes suggests that the peptide prevents thermal and hydrolytic decomposition of the Fe-S bond because of the hydrophobicity and steric bulk of the Pro and Leu residues (3,4). 

The photoelectrochemical converter is conqposed of a semiconductor in contact with a solution containing a suitable redox couple. CdSe with a band gap (Eg = l.T eV) utilizes a large part of the solar spectrum. These electrodes can be prepared by cathodic codeposition (i) on Ni substrates. Until recently such photoelectrochemical converters were impractical because the action of light on the surface of the photoelectrode produced dissolution. 

Each active mass contains a redox couple. For example, the negative mass (Reaction 1.1) of a Ni/MH battery contains a metallic hydride MH ... 

Since some ILs have excellent electrochemical stability, as shown in Table 3.14, they are favorable for application as electrolyte materials. Recently, ionic liquids have been investigated as conductive and redox media for lithium ions. Stable electrochemical deposition and dissolution of Li metal (Li/Li+) was observed for the lithium salt solution of [Nni3][TFSI], [N 122,201][TFSI], and [PPi4][TFSI] [101-103]. In order to observe the redox couple of lithium metal, Ni should be used as working electrode because it does not form alloys with lithium metal. In addition to this, the atmosphere must be pure Ar, because Li metal reacts rapidly with N2 to form conductive LiN. 

Nil2 and Sml2 are premixed before addition of the substrate. Evaluation of the E° value of the Ni(II)/Ni(0) redox couple clearly shows that Sml2 is capable of readily reducing Ni(II) to Ni(0). As a result, it is possible that Ni(0) intermediates may be responsible for the unique chemistry initiated by the addition of catalytic amounts Nil2 to Sml2. 

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