A crystal of 2 was of a blue–purple colour at room temperature, but turned to a red–purple colour when it was cooled with liquid N2 (Fig. A.3). The colour change was reversible; hence, complex 2 was classed as being thermochromic. To confirm that 2 is thermochromic in the solid state, its absorption spectra were measured at various temperatures in solid state (Figs. 3, A.4 and A.5). Complex 2, dissolved in CH3CN, showed three weak absorption bands (λmax = 549, 758 and 803 nm) in the visible region under an ambient temperature (Fig. 3(a)), and approximately the same spectrum was found for complex 2 in a KBr pellet at 300 K (Fig. 3(b)). The bandwidth of each absorption band was narrower at 70 K (Fig. 3(c)) than at 300 K. The bands around 550 and 800 nm exhibited an extremely slight blue shift at 70 K relative to that at 300 K, and the other band exhibited an extremely slight red shift. Each band was more intense at 70 K than at 300 K. In general, the blue shift of 550 and 800 nm indicates the colour change from blue to red and from green to blue, respectively. And the red shift of 740 nm indicates the colour change from blue to green.