Marcus Theory and Standard Electrochemical Rate Constants

11 Marcus Theory and Standard Electrochemical Rate Constants 

Describe the basic principles involved in Marcus theory and hence rationalise qualitatively the heterogeneous standard electrochemical rate constants (k°) for the following one-electron reductions of metal ions in aqueous solution  

From Marcus theory we know that electron transfer proceeds through the thermal activation of a reactant to a transition state . This transition state (4) is depicted in Fig. 2.6 and shows it to be the point at which the two potential energy curves for the reactant (R) and product (P) cross over. 

Once the molecule is at the cross-over point it is possible for electron tunnelling to take place and the product is formed in a highly excited vibrational level. The amount of energy required to reach this transition state (A G(t)) controls the rate of electron transfer as given by Eq. 2.14, 

The ion exhibits a high electrochemical rate constant, hence suggesting that there is little rearrangement of either the outer (water) or inner (NH3) coordination spheres on reduction. 

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