Electron transfer in groups of iron, cobalt, and copper triazene 1-oxides: Hammett correlation, ligand redistribution, and crystal field effects

Abstract

The electron-transfer behavior of groups of pseudooctahedral ML₃ and grossly planar Ca, complexes is reported (M Fe. Co; L triazene 1-oxide). A quasi-reversible proccess of type MLp+ e-⇌ML₃(p=2,3) is identified with electrochemical techniques. The E°₂₉₈ values (vs. SCE) fall in the following ranges: Fe, -O.76 to -0.14; Co, -0.56 to -0.08; Cu, -1.07 to -0.37. The values become more negative in the order CoL₃, FeL₃, and CuL₂. For a given metal ion, E°₂₉₈ shifts appreciably with ligand substituent. and a linear Hammett relationship is obeyed. The reaction constant is shown to decrease considerably when an extra bond intervenes between the metal and the substituent. Copper(II) complexes of bidentate ligands carrying an ortho substituent as well as the of a tetradentate ligand that enforce cis planar stereochemistry display abnormally negative shifts of E°₂₉₈. When two iron(III) complexes are mixed, ligand scrambling occurs and the presence of mixed species is neatly established by cyclic voltammetry. The mixed complexes fit smoothly into the Hammett straight lines. The thermodynamic quantities controlling the trends of E°298 are considered. An attempt is made to assess the relative importance of spherical and nonspherical parts of the crystal fields. Among complexes of the same metal ion, it is possible to express the shift of E°₂₉₈ in terms of the crystal field splitting parameter with a large proportionality constant. The copper(II) complexes show a ligand field band whose energy linearly correlates with E°₂₉₈. The shift of E°₂₉₈ between iron and cobalt couples is controlled by metal ionization potential and nearly equally important spherical and nonspherical crystal field contributions.