Abstract The complex bis-( N, N-dimethyl-1,2-ethanediamine) nickel(II) perchlorate undergoes a first-order thermochromic phase transition at ca. 476 K, changing its color from orange to red. The room temperature X-ray crystal structure determination showed that the nickel ion possesses a square-planar geometry with two five membered chelate rings, in the δλ conformation, forming the NiN 4 chromophore. The broad-line 1H NMR indicates the onset of a dynamic disorder of diamine chelate rings at the phase transition temperature region, while T 1 measurement of 1H affords the activation energy of the puckering metal chelate rings to be 26 kJ mol −1. The electronic spectrum revealed that a weakening of ligand field around the nickel is associated with the phase transition.