Geometric distortions and Jahn-Teller effects in Bis(terpyridine)metal complexes

Abstract

• Bis(terpyridine)metal complexes display a distorted octahedral geometry. • Elongation Jahn-Teller distortion occurs along opposite terminal metal-nitrogen bonds. • Compression Jahn-Teller distortion occurs along the central metal-nitrogen bonds. • Guidelines to identify the kind of Jahn-Teller distortion in bis(terpyridine)metal. All bis(terpyridine)metal complexes exhibit a distorted octahedral geometry due to the steric strain imposed by the terpyridine ligand. In these complexes, the central metal-nitrogen bonds are consistently shorter than the terminal metal-nitrogen bonds, indicating that every bis(terpyridine)metal complex is inherently compressed along the direction of the central metal-nitrogen bonds. However, due to the Jahn-Teller effect, additional geometric distortion and symmetry breaking occur in Jahn-Teller active bis(terpyridine)metal complexes to stabilize the system. Examples include high-spin d 4 bis(terpyridine)manganese(III) and d 4 bis(terpyridine)chromium(II), high-spin d 6 bis(terpyridine)iron(II), low-spin d 7 bis(terpyridine)cobalt(II) and bis(terpyridine)nickel(III), and d 9 bis(terpyridine)copper(II). Elongation Jahn-Teller distortion in bis(terpyridine)metal complexes occurs along the opposite terminal metal-nitrogen bonds of one of the terpyridine ligands, with the highest symmetry achieved in such elongation Jahn-Teller distorted bis(terpyridine)metal compounds being C 2v . Compression Jahn-Teller distortion occurs along the central metal-nitrogen bonds, with the highest symmetry in these compressed bis(terpyridine)metal compounds being D 2d , although C 2v symmetry is also observed, with the z-axis defined along the central metal-nitrogen bonds. Distinguishing between the two possible Jahn-Teller distorted geometries for bis(terpyridine)metal complexes under C 2v (or lower) symmetry is not straightforward. This contribution discusses, with examples, experimental and theoretical results reported on Jahn-Teller distortion observed in bis(terpyridine)metal complexes, providing guidelines on how to identify the kind of Jahn-Teller distortion observed. In bis(terpyridine)metal complexes, the elongation Jahn-Teller distortion occurs along opposite terminal metal-nitrogen bonds of one of the terpyridine ligands, whereas compression Jahn-Teller distortion occurs along the central metal-nitrogen bonds of both terpyridine ligands.