The estimation of van der Waals radii for transition-metal complexes is critical for comparison of the electron-transfer reactivity of these molecules with theoretical predictions. Presently, these radii are usually estimated with X-ray crystallography and/or molecular models. When the complexes are not spherical or when the ligand structure has large voids, these estimates may not accurately represent the true average radius. We report the diffusion coefficients for a variety of transition-metal complexes in aqueous solutions containing low concentrations (0.05-0.12 M) of 1:1 electrolytes. The diffusion coefficients, D, were measured electrochemically by utilizing a rotating glassy-carbon-disk electrode. Most of the complexes examined constitute structurally related redox series in which the complexes differ only in the size of the organic substituents bound to the ligands. The uncorrected diffusion coefficients range from 2.2 × 10-6 to 8.3 × 10-6 cm2 s-1, while the average radii, 〈r〉, estimated from models range from 3.4 to 9.1 Å. The use of the diffusion coefficients to calculate limiting ionic conductivities and average radii is discussed.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry