IntroductionThe photo- and thermo-chromisms of organometallic compounds have been intensively studied during the last 10 years. Meriwether et al.[1,2] examined the chromic behavior of metal-dithizone(phenyldiazenecarbothionic acid 2-phenylhydrazide) complexes in detail. From the kinetic and infrared studies, they showed that the central metal of a dithizonate complex determined the photochemical stability, the rate of the thermal return reaction, and the color of the dithizonate complex. As reported by Meriwether et al.[2], the isomerization of mercury bisdithizonate[Hg(HDz)2] having tetrahedral stereochemistry occurs in each ligand independently and is unaffected by the presence of the second ligand attached to the same central metal. On the other hand, the spectral evidence suggesting the possibility of ligand-ligand interaction via a central metal was found in the square-planar dithizone complexes of Ni, Pd, and Pt[1]. Though there have been many studies regarding the thermochromism of tetrahedral Hg(HDz)2, the reaction mechanism of the thermal isomerization of the square-planar bisdithizonate complex involving the ligand-ligand interaction has not been well established yet.