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

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 d4 bis(terpyridine)manganese(III) and d4 bis(terpyridine)chromium(II), high-spin d6 bis(terpyridine)iron(II), low-spin d7 bis(terpyridine)cobalt(II) and bis(terpyridine)nickel(III), and d9 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 C2v. Compression Jahn-Teller distortion occurs along the central metal-nitrogen bonds, with the highest symmetry in these compressed bis(terpyridine)metal compounds being D2d, although C2v 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 C2v (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.