Redox 1 ligand exchange

Shaw, C.F. Ill, Eldridge, J. and Cancro, M.O. (1981) Carbon-13 NMR studies of aurothioglucose ligand exchange and redox disproportionation reactions. Journal of Inorganic Biochemistry, 14, 267-274. 

A major consideration before the ligand exchange equilibria can be considered with reference to biological systems is the stability of a particular oxidation state in the biological medium. Low-spin complexes undergo rapid one-electron oxidation and reduction. As a biological system operates at a low redox potential, say —0.5 to 0.0 volts, reduced, i.e. low valence, states of the metals are to be expected. The metal complexes, Ru, Os, Rh, Ir, Pd, Pt and Au should be reduced to the metallic state in fact but for the slow speed of this reduction. The metals of Fig. 6 will tend to go to the following redox states ... 

Whatever the synthetic procedure adopted, the isolated mono, bis or /rr.v( l, 2-dithiolene) or (1,2-diselenolene) complexes can be further exploited as starting materials for ligand exchange reactions, involving both redox and non-redox... 

Using linear regression, it is possible to estimate the protonation constants of the Fe(II) complexes of siderophore complexes where the redox potentials have been measured over a range of pH values (59). This also explains the variation in reversibility of reduction as the pH changes, as the stability of the ferro-siderophore complex is much lower than the ferric complex, and the increased lability of ligand exchange and increased binding site competition from H+ may result in dissociation of the complex before the iron center can be reoxidized. 

Give examples of photodissociation, ligand exchange and redox processes of d-block complexes. 

The principal photochemical reactions of metal complexes include dissociation, ligand exchange and redox processes. Unlike organic photoreactions (which take place almost exclusively from the S3 or T3 states), the excited state formed on irradiation depends on the wavelength employed. Hence the quantum yield often depends on the wavelength of the irradiating source. The excited-state processes give rise to a reactive intermediate which may find application in the synthesis of new compounds. 

Complete sequences of ca. 50 different plant-type ferredoxins(Fd) are known. The invariant sequences nearest to the 2-Fe core are confirmed to be Pro-Tyr-Ser-Cys-Arg-Ala-Gly-Ala-Cys-Ser-Thr-Cys-Ala-Gly and Leu-Thr-Cys-Val. 2Fe-2S complexes of oligopeptides with the Cys-X-Y-Cys sequence have been synthesized by ligand exchange reactions (7,23). We have examined the redox potentials of these model complexes, and the results are shown in Table I. The reversibility improved remarkably and the potential approached the value of the native proteins as the sequence more closely simulated that of the proteins. It is conjectured that hydrogen bonds from the peptide N-H s to thiolate and/or sulfide groups increase the stability of the reduced cluster. It is likely that peptide sequences like those found in the proteins favor the formation of such hydrogen bonds. 

M. Rizzotto, A. Levina, M. Santoro, S. Garcia, M. Frascaroli, S. Signorella, L. F. Sala, and P. A. Lay, Redox and ligand-exchange chemistry of chromium(VI/V)-methyl glycoside systems, J. Chem. Soc. Dalton Trans. (2002) 3206-3213. 

Catalysts and their effects on chemical reactions aid in efficiency, effectiveness and selectivity. A recent example of current research is redox and ligand exchange reactions of the oxygenation catalyst (N,N -bis(salicylidene)ethylenediaminato)co-balt(II), Co(SALEN)2 (below), and its one-electron oxidation product, Co(salen) 2-These were investigated in DMF, pyridine, and mixtures of these solvents. Solvent effects on the potentials, the thermodynamics of cross reactions, and the distribution of Co(II) and Co(III) species as a function of the solvent composition are important considerations (Eichhorn, 1997). The results in these solvents should be compared with other work with catalysts using more environmentally benign media (Collins et al., 1998). 

The nature of the central metal ion. In principle the redox potential of the Mre+1/re couple and the stabilization of the M" + 1 ion by the ligand R affect the metal-carbon bond strength. The stability of the transient complex is also affected by the rate of ligand exchange of the LmM"+1 R complex. 

Other examples of the EC mechanism include nucleophilic addition of Z (such as H20 or CN") to electrogenerated organic radical cations, ligand exchange reactions in the case of coordination compounds, and redox reactions between R and Z (or O and Z in the case of a reduction). 

Ab initio and semiempirical methods have been applied to the interpretation of many aspects of dithiolene chemistry electronic spectra, ESR, Mossbauer, XPS, charge distributions, redox properties, reaction mechanisms, metal binding in biological systems and ligand-exchange behavior. We shall focus our attention on the theoretical deductions of some representative research groups. For computational details, the reader is referred to the original papers and references therein. 

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