Ligand Substituent Effects on Ruthenium (III/II) Redox Properties: An Advanced Inorganic Laboratory Experiment

A convenient inorganic laboratory exercise is presented that exposes the student to the influences of ligand substituents on a metal centers redox properties. Students prepare a series of compounds with the general formula [Ru(bpy)₂(R¹,R²mal)](PF₆), where bpy is 2,2-bipyridine and R¹,R²mal is an anionic -diketone ligand with various R¹ and R² substituents. Each complex is prepared in a single synthetic step, and the Ru(III)/Ru(II) couple is characterized by cyclic voltammetry. The potential of the Ru(III)/Ru(II) couple shifts upon varying R¹ and R² from 0.64 to 1.10 volts vs. SSCE in the following order: (R¹ = R² = Me) < (R¹ = R² = Ph) < (R¹ = Me, R² = CF₃) < (R¹ = R² = CF₃). The inductive effects of the R substituents on the Ru(III)/Ru(II) couple are consistent with their electron-donating and electron-withdrawing properties. Hammett constants for the substituents provide an approximate measure of these effects. A linear relationship is observed when the Hammett constants are plotted against the Ru(III)/Ru(II) potential. This laboratory exercise applies a versatile electroanalytical method, cyclic voltammetry, to measure the substituent effects on a metal centers electron density. Student results of the syntheses and redox characterizations of a series of [Ru(bpy)₂(R¹,R²mal)]⁺ complexes are presented and discussed.