The coordination chemistry of divalent zinc triad metal ions is complicated by coordination number and geometry plasticity. A series of symmetric and asymmetric tridentate N and N,S donor ligands have been used to explore the intramolecular isomerization of bis-tridentate chelates under conditions of slow intermolecular exchange. There are nine regular polyhedral forms of bis-tridentate chelates, seven of which have been observed crystallographically for divalent zinc triad metal ions. Unfortunately, crystallization can selectively remove minor solution species with lower solubility. Variable temperature solution-state proton NMR has been used to evaluate the contribution of crystallized forms to solution equilibria. Rapid intramolecular exchange is commonly considered de rigueur for complexes of d¹⁰ metal ions. Surprisingly, bis-tridentate chelates of N-(2-pyridylmethyl)-N-(2-(methylthio)ethyl)amine exhibited slow exchange on the chemical shift time scale between several regular polyhedral forms. The relevance of these observations to the development of new catalysts and to the understanding of physiological differences between these metal ions will be discussed.