Coordination chemistry studies complexes or coordination compounds. Metal complexes consist of molecules or ions that have a central metal ion, (coordination center), and neutral or charged groups, ligands, that are arranged around the central atom.
Coordination chemistry studies complexes or coordination compounds. Metal complexes consist of molecules or ions that have a central metal ion, often called the coordination center, and neutral or charged groups, ligands, that are arranged around the central atom. The set of ligands makes up the coordination sphere. A ligand may contain one or more donor atoms that are available to bond the central atom. The number of available donor atoms is called the denticity of the ligand. Examples of monodentate ligands include halogenides (fluoride, chloride, bromide, iodide), ligands with O-donor atom (water, hydroxide, alcohols, ethers, carboxylates, oxoanions), ligands with S-donor atom (sulfides, thiocyanates, thiols, thioethers, thioacids), ligands with N-donor atom (nitrogen, azide, nitride, imides, amides, amines), and several other ligands with phosphorus, arsenic, carbon, silicon and mixed donor atoms. Ligands that can bind with more than one donor atom to the same center act as chelate ligands or chelators. They form chelate complexes with chelate rings that can be 3 to 6-membered. Five-membered chelate rings are especially stable and are formed by polyamino-polycarboxylate type chelators such as EDTA, DTPA, DOTA, TETA. Important features of complex centers include their electron configuration (high spin / low spin complexes), their oxidation states and their coordination numbers. The configuration or stereochemistry of complexes describes the spatial arrangement of the ligands around the metal center. For example, four ligands can be arranged around a metal center either in tetrahedral or in square planar geometry. The thermodynamic stability of complexes is characterized by the formation or dissociation constant, which is expressed as the equilibrium constant for the reaction in which water molecules attached to the metal ion are replaced by the ligands. The larger the value of the formation constant, the higher the thermodynamic stability of the complex.