A new approach to measurement of the donor strength and co-ordination chemistry of various solvents by oxidation of metal amalgam electrodes in dichloromethane

Electrochemical oxidation of thallium, lead, and cadmium amalgam electrodes in dichloromethane containing 0.2 mol dm^-3 [NBu₄][PF₆] produces highly activated metal cations. The cations generated in this non-co-ordinating medium react rapidly with added donor solvents (solv) such as dimethyl sulphoxide, dimethylformamide, tetrahydrofuran, methanol, ethanol, and acetonitrile but not benzene to produce solvated metal ions. The kinetics of solvation are more rapid than precipitation of the metal salts which occurs in longer-time-scale bulk electrolysis experiments. The co-ordination number and equilibrium constants have been calculated for many of the solvated metal complexes, and the equilibrium constants compared with the solvent donor strengths. In the case of the dropping thallium amalgam electrode, the oxidation process is a well defined, strictly reversible one-electron step. From the data obtained for the thallium oxidation process, the value of the equilibrium constant, β₁, may be calculated for the reaction TI⁺ + solv ⇌ β₁ [TI(solv)]⁺ and correlated with the Gutmann donor number. Measurement of β₁ values calculated in this way is proposed as a simple method of estimating the donor strengths of co-ordinating solvents.