Technetium electrochemistry. 3. Spectroelectrochemical studies on the mixed-ligand technetium(III) complexes trans-[Tc(PR2R')2L]+ where L is a tetradentate Schiff base ligand and PR2R' is a monodentate tertiary phosphine ligand

Abstract

The redox chemistry of a series of eight well-characterized, robust, cationic technetium(III) complexes has been investigated in propylene carbonate by using spectroelectrochemistry with a gold minigrid optically transparent thin-layer electrode. The complexes undergo a reversible 1-equiv reduction of Tc(III) to Tc(II) and a reversible 1-equiv oxidation of Tc(III) to Tc(IV). E/sup 0/' values for the Tc(III)/Tc(II) couple range from -1.11 to -0.69 V (vs. Ag/AgCl/NaCl (3 M)) while E/sup 0/' values for the Tc(IV/Tc(III) couple range from + 0.62 to + 0.79 V (vs. the same reference). The difference between E/sup 0/' /sub IV/III/ and E/sup 0/'/sub III/II/ ranges from 1.5 to 1.75 V, reflecting the large range of stability of the Tc(III) state. The formal potentials of both redox processes depend on the nature of the phosphine and Schiff base ligands. These dependencies are readily explained in terms of ..pi.. back-bonding from low-valent technetium to both the phosphine and Schiff base ligands. All the technetium complexes, including the electrogenerated Tc(II) and Tc(IV) species, exhibit characteristic metal-to-ligand charge-transfer (MTLCT) bands in the visible region of the spectrum. The energies of these MTLCT tranisitions are a function of the oxidation state of the technetium, the nature of the phosphine ligands, andmore » the nature of the Schiff base ligand. The energy of the Tc(III) MTLCT band is linearly related to the redox potential of the Tc(IV)/Tc(III) couple.« less