Interpreting and controlling the structures of six-coordinate copper(ii) centres – When is a compression really a compression?

Many experimental observations of 'inverse' or 'quenched' Jahn–Teller effects in copper(II) compounds in fact reflect a normal Jahn–Teller-elongated configuration that is masked by structural disorder. On the other hand, a small number of copper(II) complexes are known that genuinely exhibit these unusual structures. It requires careful interpretation of spectroscopic and structural data to distinguish these two scenarios.