Coordination properties of didentate N,O heterocyclic alcohols and aldehydes towards Cu(II), Co(II), Zn(II) and Cd(II) ions in the solid state and aqueous solution

A critical discussion of the coordination properties of some heterocyclic alcohols and aldehydes derived from imidazole, pyrazole or pyridine are the subject of this review. The versatility of these ligands, which may act as monodentate or didentate, either neutral or deprotonated, and which undergo tautomeric equilibria upon coordination to Cu(II), Zn(II), Co(II) and Cd(II) is discussed and illustrated using relevant examples. Special attention is paid to a comparison of results obtained by the X-ray diffraction method in the solid state with those acquired by the potentiometric, NMR and EPR methods in aqueous solution. A discussion of the crystal structures, as well as a correlation of the stability constants of reported complexes is also provided. Additionally, differences in the geometries of the central ions of these complexes, which extend from tetrahedral (most common for zinc) to the octahedral form for cobalt and dodecahedral for cadmium (and copper) complexes respectively, are extensively discussed.