Copper(II) ions form stable complexes with oxygen and nitrogen donor ligands such as beta-diketons, carboxylates and Schiff bases, ethylenediamine, bispyridine and phenantroline. Polyazamacrocyclic ligands bind copper(ii) with excellent thermodynamic stability and kinetic inertness.
Copper exhibits a variety of compounds, many of which are colored. The two principal oxidation states of copper are +1 and +2 although some +3 complexes are known. Copper(I) compounds are expected to be diamagnetic in nature and are usually colorless, except where color results from charge transfer or from the anion. The +1 ion has tetrahedral or square planar geometry. In solid compounds, copper(I) is often the more stable state at moderate temperatures. The copper(II) ion is usually the more stable state in aqueous solutions. Compounds of this ion, often called cupric compounds, are usually colored. They exhibit a wide range of stereochemistries with four, five, and six coordination compounds predominating. However, due to the Jahn Teller effect, regular geometries are rare and distorted octahedral coordination is common. Tetracoordinated square planar complexes are easily made with ammonia, pyridine and other amines, including chelating N-donor ligands such as ethylenediamine, bispyridine and phenantroline. In basic solution, biuret reacts with copper(II) to form a complex with characteristic violet color. This is the biuret probe and used to test the presence of proteins and peptides. Copper(II) also forms stable complexes with oxygen donor ligands suc as beta-diketons, carboxylates and Schiff bases. Copper-64 (beta and positron emitter) and copper-67 (beta emitter) have proven to be useful isotopes for nuclear medicine imaging and therapy. Polyazamacrocyclic ligands bind copper(ii) with excellent thermodynamic stability and kinetic inertness. Several bifunctional ligands for copper(II) have been synthesized. These include the TETA based p-bromoacetamidobenzyl TETA, and the DOTA based p-isothiocyanatobenzyl DOTA.