THE COLORLESS CHAMELEON OR THE PECULIAR PROPERTIES OF ZN2+ IN COMPLEXES IN SOLUTION - QUANTIFICATION OF EQUILIBRIA INVOLVING A CHANGE OF THE COORDINATION-NUMBER OF THE METAL-ION

The adaptability of the coordination sphere of Zn2+ in biological systems is well known. Less appreciated is the fact that equilibria between Zn2+ species with coordination numbers 6 and (5 or) 4 are indeed quite common in solution in the presence of low-molecular-weight ligands. In this essay ways are described that allow a quantification of these equilibria in binary and in ternary (mixed ligand) complexes. For example. the 1:1 and 1:2 complexes formed by Zn2+ and ethylenediaminc exists in aqueous solution in about 50 and 90% respectively, as tetrahedral species; the remaining parts being octahedral.