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

We provide definitive experimental proof that the archetypal string-like compound [Cr 5 (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 (H 2 tpda = N 2 , N 6 -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.