Reactions of Metal Ion Complexes

The electron self-exchange rate constants for several encapsulated metal complexes, [MCsar)] (M = Ni, Fe, Mn, and Ru, sar = 3, 6, 10, 13, 16, 19-hexaazabicyclo[6.6.6]eicosane), have been determined from the application of the Marcus cross-relationship to a series of outer-sphere electron transfer crossreactions. The calculated values are 1.7 x 10 (Ni), 6.0 x 10 (Fe), 17 (Mn), and 1.2 x 10 M s (Ru). 

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 (/ i2 = 0.13) was observed. 

The rate constants and activation parameters (including AV ) for electron self-exchange in the [Mn(CNC(CH)3)6]- / - and [Mn(CNC6Hu)6] couples have been determined by Mn NMR line broadening in several pure and binary organic solvent systems. The values of A V cover a range of about 12 cm moP (-9 to -21 cm mol ) with no simple correlation with solvent parameters observed. A self-exchange rate constant of 0.7 0.4 M s has been calculated for the [Mn(edta)(H20)] and [Mn(cdta)(H20)] couples from the application of the Marcus relationship to outer-sphere cross-reactions with a variety of metal complexes in aqueous solution. Deviations from the correlation were observed for the nonadiabatic reactions with osmium tris(polypyridine) complexes. 

The rate constants for electron transfer between a manganese carbonyl cation and anion, and for the disproportionation of the radical products have been detennined from cyclic voltammograms in see Eq. (1). The electron 

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Electron transfer reactions of metal ion complexes in homogeneous solution are understood in considerable detail, in part because spectroscopic methods and other techniques can be used to monitor reactant, intermediate, and product concentrations. Unfavorable characteristics of oxide/water interfaces often restrict or complicate the application of these techniques as a result, fewer direct measurements have been made at oxide/water interfaces. Available evidence indicates that metal ion complexes and metal oxide surface sites share many chemical characteristics, but differ in several important respects. These similarities and differences are used in the following discussions to construct a molecular description of reductive dissolution reactions. 

I spent many years on the study of redox reactions in general before I started to work on the special class of electron transfer reactions. An experiment I did in 1954 — 1 use the personal pronoun because, although I had a coworker, I did most of the laboratory work myself — attracted a great deal of attention to the subject. Ironically, it did not involve electron transfer in the strict sense of the term, but it did introduce a new dimension to the subject of redox reactions of metal ions. It was rather certain at the time that some such reactions of metal-ion complexes do go by simple or overt electron transfer. It was speculated that in other cases, electron transfer could take place by an atom bridging two metal centers in the act of electron transfer, the bridging atom then being transferred from one metal to the other. My contribution was the unequivocal experimental demonstration that it really can occur. 

A rate constant k is assigned to each surface chemical reaction. This is a schematic representation of the mechanism based upon analogous reactions of metal ion complexes in solution (see Purcell and Kotz, 1977, p, 659-669). Experimental determination of dissolved reactant and product concentrations [ArOH(aq), Mn Caq), etc.] can provide indirect information about the surface reaction [discussed in Stone (1986), Stone and Morgan (1987), and Stone (1987)]. Additional detail concerning the stoichiometry and structure of surface species will require the use of spectroscopic or other surface-analytical techniques. 

General Trends in Ligand Replacement Reactions of Metal Ion Complexes... 

Rates of ligand replacement reactions of metal ion complexes are relevant in a number of situations. Assigning a lability to a particular metal based on the rate of a representative ligand replacement reaction is an attractive prospect, but one that has important limitations. In this regard, rates of water-exchange reactions have been examined ... 

R.G. Linck, Rates and Mechanisms of Oxidation-Reduction Reactions of Metal Ion Complexes, in Reaction Mechanisms in Inorganic Chemistry , ed. M.L. Tobe, Butterworths, London, 1972. 

Table 3.3. Rate Constants for Reactions of Metal Ion Complexes with Halides...

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