Complex (chemistry)

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This article is about the chemical complex. For other uses of this word, see complex.

A complex in chemistry is a reversible association of molecules, atoms, or ions through weak non-covalent chemical bonds. Simple salts are usually not considered complexes.

1 Metal complexes
- 1.1 Naming complexes
2 Receptor-ligand complexes
3 See also
4 References
5 External links

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Metal complexes

Image:Copper complex.jpg A metal complex, also known as coordination compound, 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. Counter ions often surround the metal complex ion, causing the compound to have no net charge.

The ions or molecules surrounding the metal are called ligands. Ligands are generally bound to a metal ion by a coordinate covalent bond, and are thus 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).

Coordination numbers, or the number of bonds formed between the metal ions and ligands, are normally between 2 and 8 but can extend higher; it becomes very difficult to measure the number of ligands after about 8, and large numbers of ligands are uncommon. The number of bonds depends on the size, charge, and electron configuration of the metal ion. Some metal ions may have more than one coordination number. Different ligand structural arrangements result from the coordination number. A coordination number of 2 corresponds with a linear geometry; a coordination number of 4 corresponds with either a tetrahedral or square planar molecular geometry; and a coordination number of 6 corresponds with an octahedral geometry.

Simple ligands like water or chlorine form only one link with the central atom and are said to be monodentate. More examples of monodentate ligands include hydroxide, nitrite, and thiocyanate. Some ligands are capable of forming multiple links to the same metal atom, and are described as bidentate, tridentate etc. Oxalate and ethylenediamine (en) are examples of bidentate ligands, while diethylenetriamine (dien) is a tridentate ligand. 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 molecular orbitals of the ligands and the d (or f) orbitals of the metal ions. Because of this, the simple octet rule 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 and assumes that the ligands can be approximated by negative point charges. 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.

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Naming complexes

The basic procedure for naming a complex:

When naming a complex ion, the ligands are named before the metal ion.
Write the names of the ligands in alphabetical order. (Numerical prefixes do not affect the order.)
- Multiply occurring monodentate ligands receive a prefix according to the number of occurrences: di-, tri-, tetra-, penta-, or hexa. Polydentate ligands (e.g., ethylenediamine, oxalate) receive bis-, tris-, tetrakis-, etc.
- Anions end in o. This replaces the final 'e' when the anion ends with '-ate', e.g. sulfate becomes sulfato. It replaces 'ide': cyanide becomes cyano.
- Neutral ligands are given their usual name, with some exceptions: NH₃ becomes ammine; H₂O becomes aqua; CO becomes carbonyl; NO becomes nitrosyl.
Write the name of the central atom/ion. If the complex is an anion, the central atom's name will end in -ate, and its Latin name will be used if available (except for mercury).
If the central atom's oxidation state needs to be specified (when it is one of several possible, or zero), write it as a Roman numeral (or 0) in parentheses.
Name cation then anion as separate words (if applicable, as in last example)

Examples:

[NiCl₄]^2- → tetrachloronickelate(II) ion

[CuNH₃Cl₅]^3- → amminepentachlorocuprate(II) ion

[Cd(en)₂(CN)₂] → dicyanobis(ethylenediamine)cadmium(II)

[Co(NH₃)₅Cl]SO₄ → pentaamminechlorocobalt(III) sulfate

While the chemistry of the transition metals is awash with coordination complexes, it should be noted that lanthanides,. actinides, s-block metals (alkali metals and alkaline earth metals) and p-block metals (such as tin, bismuth and lead) also form a wide range of complexes.

To study the activity of complexes in solution, it is possible to record pH spectra which shows the interaction between complexing agent and central ion as a function of the degree of dissociation of their functional groups.

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Receptor-ligand complexes

Receptors are proteins that bind small ligands. A typical example of a receptor-ligand complex is a neurotransmitter bound to a neurotransmitter receptor in the cell membrane of the synapse. The dissociation constant K_d is used as an indicator of the electron affinity of the ligand to the receptor.

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