The rates of electron exchange between bis(cyclopentadienyl)iron(II) and -(III) (ferrocene and ferrocenium ion) and between oxidized and reduced forms of several derivatives of ferrocene have been measured in a number of different solvents by the NMR line broadening method over a temperature range of 0-30 °C. It was found that the rates did not vary with the dielectric properties of the solvents as predicted by the Marcus theoretical model for electron exchange between neutral and singly charged spherical reactants with similar structures, reactions for which solvent reorganization is the principal deterent to exchange. Also, the product of the collision number (Z) and the transmission coefficient (k) was found to be an order of magnitude smaller than the generally assumed value of = 1011 M'1 s'1. Addition of NaPF6 or NaC104 to acetonitrile solutions of ferrocene and ferrocenium ion reduced the exchange rate by about a factor of 2 at high (0.1-0.5 M) salt concentrations. The presence of substituents on the cyclopentadienyl rings affected the rate of electron exchange only moderately; the largest effect of about a 10-fold increase in rate was observed for the decamethyl derivative. The presence of the methylenetrimethylamine group on one ring, resulting in 1+ and 2+ charged reactants, caused a reduction in rate by a factor of ~5, much less than the factor of ~60 estimated for Coulombic repulsion between uniformly charged spheres 7 Á in diameter, an indication that the charged quaternary amine groups are widely separated (>~10 Á) in the transition state. Electrical conductivity measurements of cobaltocenium hexafluorophosphate in a variety of solvents indicated that it and very probably the similar ferrocenium salt are essentially completely dissociated at the low concentrations used for measurement of electron-exchange rates in most solvents investigated.

Publication costs assisted by the National Science FoundationThe rates of a number of electron-exchange reactions between M L F and ML?+ complex ions have been measured by the NMR line broadening method in the temperature range -5 to 35 "C, the symbols M and L representing, respectively, Fe, Ru, or Os and 1,lO-phenant,hroline (phen), 2,2'-bipyridyl (bpy), or their derivatives. Most measurements were made with acetonitrile as the solvent, but some were made with formic acid, and a few were made with water; ionic strengths were usually close to 0.1 M. The exchange reactions followed a second-order rate law, first order in each reactant concentration. The rate constants determined were all of the same large magnitude ( -IO6 M s-'), and their temperature dependences were small (e N 2 kcal/mol). Exchange rates increased with anion concentration, and were 2 to 3 times larger when Clod-was the anion than when PF6was the anion. The dependence of exchange rates on anion concentration and other data indicate that exchange occurred mainly between reactants that were associated to some extent with anions to form ion pairs, triplets, etc. The rate of electron exchange was 15 times smaller when cyclohexyl groups were substituted for hydrogen in the 4,7 positions of the I?e(phen)2+t3+ reactants, and the rate with cyclohexyl substitution was 20 times smaller than when phenyl groups were substituted in the 4,7 positions. This latter observation indicates that phenyl groups conduct electrons better than do the cyclohexyl groups, since the groups are of similar size and should have similar steric effects. Substitution of methyl groups on the ligands increased the electron exchange rate by a factor of about 2, and several possible reasons for this effect are discussed. Also, changing metal centers from Fe to Ru to Os increased the exchange rate at 25 "C about a factor of 2 for each change. No difference in rates at 25 "C was observed for electron exchange between racemic R~(bpy)?+,~' mixtures and between the d -R~( b p y ) ,~+ ,~+ isomers. The activation parameters derived from the data are similar to theoretical values calculated from the Marcus theory of electron transfer on the assumption that reactants are spherical, are 14 A in diameter, have 2+ and 3+ charges, and react in a continuous, unsaturated dielectric medium with zero ionic strength. However, the similarity in values is most likely fortuitous, since, as discussed, the reactants are probably associated with anions and thus have charges less than 2+ and 3+, and since the reactants are not spheres, but have sufficient space between the ligands to accomodat,e anions and/or solvent molecules. The method used to derive exchange rate constants from NMR line shapes that were broadened due to spin-spin coupling as well as exchange is discussed, and NMR data are given for the complex ions investigated.

The rate of electron exchange between bis(cyclopentadienyl)iron(II) (ferrocene) and bis(cyclopentadienyl)iron(III) ion (ferricenium ion) has been measured in acetonitrile and in methanol by the NMR linebroadening method over the temperature range 0-30°. The rate in acetonitrile depends on the first power of each reactant concentration; the rate has little dependence on electrolyte concentration in either solvent.For ionic strengths <0.1 M, the rate constants at 25°are (5.7 ± 1.0) X 10® and (5.4 ± 1.0) X 106 M~l sec-1, and the experimental activation energies are 5 ± 1 and 3 ± 1 kcal/mol for acetonitrile and methanol, respectively, as solvents.

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The rate of electron exchange between tris(3,4,7,8-tetramethyl-l,10-phenanthroline)iron(II) and -(III) ions in acetonitrile has been measured by the nmr line-broadening method in the temperature range from -5 to 35°. For a total reactant concentration of 0.035 M, the second-order rate constant at 25°is (1.7 ± 0.1) X 107 M-1 sec-1, and the experimental activation energy is 2 ± 1 kcal/mol.