Electronic structure of the glyoxalbis(2-hydroxyanil) (gha) ligand in [CoIII(gha)(PPh3)2]+: radical vs. non-radical states

Abstract

The synthesis, structure and spectroscopic properties of a complex salt [Co^III(gha)(PPh₃)₂][Co^IICl₃(PPh₃)]·C₂H₅OH (1) are reported; gha = glyoxalbis(2-hydroxyanil). This is the first single crystal X-ray structure of a (gha)²⁻ complex with a transition element. Though the determined bond parameters and UV-Vis spectroscopic data correlate well with a diradical description for the cation in 1, detailed electronic structure calculations using density functional theory confirm that [Co(gha)(PPh₃)₂]⁺ can be described as a closed shell singlet species which nevertheless displays an interesting electronic structure with significant electron transfer to the formally unoccupied LUMO of the square planar [Co(gha)]⁺ fragment. It was found that without the phosphine coligands, the [Co(gha)]⁺ unit has a triplet ground state with the lowest energy singlet diradical state lying only 1 kcal mol⁻¹ higher in energy. The chemistry of the gha ligand is of interest as a spin diverse redox active system.