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In chemistry, a complex is a structure composed of a central metal atom or ion, generally a cation, surrounded by a number of negatively charged ions or neutral molecules possessing lone pairs. A complex may also be called a coordination compound or metal complex.

The ions or molecules surrounding the metal are called ligands. A ligand that is bound to a metal ion is said to be coordinated with the ion. The process of binding to the metal ion with more than one coordination site per ligand is called chelation. Compounds that bind avidly to form complexes are thus called chelating agents (for example, EDTA).

Simple ligands like water or chlorine form only one link with the central atom and are said to be monodentate. Some ligands are capable of forming multiple links to the same metal atom, and are described as bidentate, tridentate etc. EDTA is hexadentate, which accounts for the great stability of many of its complexes.

Typically, the chemistry of complexes is dominated by interactions between s and p orbitals of the ligands and the d (or f) orbitals of the metal ions. Because of this, simple octet bonding theory fails in the case of complexes and to understand the chemistry of these systems, a deeper understanding of chemical bonding rules is necessary.

One such rule is called electron countingIn the inorganic chemistry and organometallic chemistry of transition metals, electron counting is a formalism used for characterizing a compound and for understanding its electronic structure and bonding. The valence shells of a transition metal are fill, or the rule of 18. Crystal field theoryCrystal field theory is used to describe the bonding in transition metal complexes. It is successful in describing the magnetic properties, colors, hydration enthalpies and spinel structures of transition metal complexes. Crystal field theory was develope, introduced by Hans BetheHans Albrecht Bethe (born July 2, 1906), is an American physicist from Strassburg (then part of Germany, now Strasbourg, France). Bethe (pronounced Bey-ta studied physics at Frankfurt and obtained his doctorate from the University of Munich. He left Germa in 1929Centuries: 19th century 20th century 21st century Decades: 1870s 1880s 1890s 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s Years: 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 See also 1929 in aviation 1929 in film 1929 in literature 1929 in mu, is a more quantum mechanicallywavefunctions of an electron in a hydrogen atom possessing definite energy (increasing downward: n 1,2,3,. and angular momentum (increasing across: s p d . Brighter areas correspond to higher probability density for a position measurement. The angular mom based attempt at understanding complexes. But crystal field theory treats all interactions in a complex as ionic. Ligand field theory , introduced in 1935Events January January 1 Italian colonies of Tripoli and Kyrenaika are joined together as Libya January 7 World War II: Italian premier Benito Mussolini and French foreign minister Pierre Laval conclude agreement in which each power undertakes not to oppo and built from molecular orbital theory, can handle a broader range of complexes and can explain complexes in which the interactions are covalent. The chemical applications of group theory can aid in the understanding of crystal or ligand field theory, by allowing simple, symmetry based solutions to the formal equations.