Complex 2 was subjected to X-ray crystallographic analysis at 100 K to examine the relationship between the change in the crystal colour and the crystal structure. Selected bond lengths and angles at a low (100 K) and a high (300 K) temperature were compared and are summarised in Table 2. The crystal packing and hydrogen bonding were approximately the same at both temperatures, and the bond angles were also almost the same. However, the bond length changed anisotropically. The two Ni–N(terminal N of dien) bond lengths and the one Ni–O bond length were shortened; however, the other atom bonds were elongated at low temperature. The colour change from blue–purple at high temperatures to red–purple at low temperatures might, therefore, be caused by the anisotropic structural change. The structural change is minuscule and it could be caused by measurement deviation.