Aqueous solution electron exchange reactions

Accordingly, evaluation of k from line-width measurements gives a bimolecular rate constant of 0.5 X 1081 sec-1 M-1. This is one of the fastest electron exchange reactions studied in aqueous solution. Transfer rate here may not describe direct electron transfer between Cu1 and Cu11 Stranks has suggested (129) that the rate may refer to transfer within a bridged activated complex, e.g.,... 

Larsson S. Electron-exchange reaction in aqueous solution. J. 

Note that from an electrode kinetic point of view, the Nemst equation does not give any information as to the actual species that establishes the electrode potential. In the case of a fluoride-containing solution, it is very possible that one of the fluoro-iron(III) complexes rather than the Fe " ion participates in the electron-exchange reaction at the electrode. Complexation usually stabilizes a system against reduction. In the example just considered, because complexation is stronger with Fe(III) than with Fe(II), the tendency for reduction of Fe(III) to Fe(II) is decreased. It is apparent that coordination with a donor group, in general, decreases the redox potential. In the relatively rare instances where the lower oxidation state is favored (e.g., complexation of aqueous iron with phenanthroline), the redox potential is increased as a result of coordination. 

The electron self-exchange rate constant for the [Cr(CNdipp)6] couple (CNdipp = 2,6-diisopropylphenyl isocyanide) in CD2CI2 has been measured between -89 and +22 °C using H NMR line-broadening techniques, with an extrapolated value of 1.8 x 10 M s determined for 25 The kinetics of the outer-sphere oxidations of tris(polypyridine)chromium(II) complexes by a series of tris(chelate)cobalt(III) species have been studied in aqueous solution. " The cross-reaction rate constants obey the Marcus relationship, with the exception of [Co(bpy)3] " and [Co(phen)3] ", for which mild nonadiabaticity (/[Pg.18]

A metal ion in solution, typically MLg , is envisaged as surrounded by a first coordination shell of ligands (L) which is relatively long-lived L may be H2O, CN , Cl , bipyridyl, etc. Before reaction can occur, the metal-ligand bond lengths (M-L) must be adjusted. Consider, for example, the electron-exchange reaction between ferrous and ferric ions in aqueous solution [2,a,c] ... 

Figure 9.9 Correlation of rate constants for electron-exchange reactions with molecular dimensions (1). The plot shows an inverse correlation of rate constant (k ) with diameter (2a) of reactant ions MLg, for electron-exchange reactions of a series of ruthenium complex ions in aqueous solution at 298 K. The plot is of [InCtobs/ a) (i-e-, hiiet) against l/2a. See text. Bond lengths ate assumed equal to those in crystals, determined by X-ray diffraction. The systems ate (1) Ru(NH3)g, (2) Ru(NH3)5(py) +/ +, (3) Ru(NH3)4(bpy) +/2+ (4) Ru(NH3)2(bpy)2 , (5) Rufbpylj. The figure is teptoduced from Ref. [16].

Figure 9.10 Correlation of rate constants for electron-exchange reactions with molecular dimensions (2). The plot is of -[ln(lTobs/ a) + AgJs/IJT ] against (Ado). for a series of electron-exchange reactions in aqueous solutions at 298 K. Values of Ado were determined by EXAFS. The classical model predicts a nearly linear plot See text. The systems are (1) Cr(H20)g" + (2) Fe(H20)g (3) FeCphenlj (4) Ru(H20)g + (5) Ru(NH3) + + (6) Ru(en) +/ + (7) Ru(bpy) + + (8) Co(H20)g+ + (9) Co(NHj) +/ + (10)...

Phys. 74 6746 (1981) b) G. L. Gloss, L. T. Calcaterra, N. J. Green, K. W. Penfield, and J. R. Miller, Distance, stereoelectronic effects, and the Marcus inverted region in intramolecular electron transfer in organic radical anions, J. Phys. Chem. 90 3673 (1986). a) S. Larsson, Electron transfer in chemical and biological systems. Orbital rules for nonadiabatic transfer, J. Am. Chem. Soc. 103 4034 (1981) b) S. Larsson, n Systems as bridges for electron transfer between transition metal ions, Chem. Phys. Lett. 90 136 (1982) c) S. Larsson, Electron transfer in proteins, J. Chem. Soc., Faraday Trans. 2 79 1375 (1983) d) S. Larsson, Electron-exchange reaction in aqueous solution, J. Phys. Chem. 88 1321 (1984) e) S. Larsson,... 

As an example, Fig. 12.6 shows Tafel plots for the exchange of the acetylcholine ion between an aqueous solution and 1,2-DCE. The two branches were obtained under conditions in which the ion was initially present in one phase only. This reaction obeys the Butler-Volmer law surprisingly well, even though a microscopic interpretation faces the same difficulty that we have discussed for electron-transfer reactions. 

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