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

The ions/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 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 counting, or the rule of 18. Crystal field theory[?], introduced by Hans Bethe in 1929, is a more quantum mechanically based attempt at understanding complexes. But crystal field theory treats all interactions in a complex as ionic. Ligand field theory[?], introduced in 1935 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.

Naming complexes

The basic procedure for naming a complex:
  1. Write the number of ligands surrounding the central metal ion/atom using: mono-, di-, tetra-, penta-, and hexa.
  2. Write the name of the ligands (in alphabetical order) appending an o for anions: fluoro, chloro, cyano. Neutral ligands keep their original name.
  3. Name the central atom/ion appending -ate if the atom is negative. Neutral or positive atoms/ions keep their original name.

eg. [NiCl4]2- = tetrachloronickelate (II) ion

Transition metals make good central ions for complexes.

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