The structure of a pentachromium(ii) extended metal atom chain at 3 K: Cotton's conjecture proven

Abstract

We provide definitive experimental proof that the archetypal string-like compound [Cr₅(tpda)₄(NCS)₂] has alternating long and short Cr–Cr separations in the solid state, as conjectured by F. A. Cotton, rather than essentially equally spaced Cr atoms, as initially claimed (H₂tpda = N²,N⁶-di(pyridin-2-yl)pyridine-2,6-diamine). Single-crystal X-ray data collected from 292 to 3 K revealed that the misinterpretation is caused by pseudo-merohedral twinning and that bond length alternation is enhanced at low temperature.