Complex (coordination complex)

complex (or coordination complex) in chemistry and biochemistry is the product of the formation, often reversible, of a bond between a central atom or ion (or coordinating ion) and the atoms, ions, or molecules (called ligands or coordinated ions) that surround the central atom (called the coordination center).

A more rigorous definition of a chemical complex can be:

A complex is a chemical compound in which an atom binds a number of other chemical species greater than its oxidation number.

In other words a coordination complex is any compound that contains a coordination entity. A coordination entity is a neutral ion or molecule consisting of a central atom, usually that of a metal, to which is attached a surrounding matrix of other atoms or groups of atoms, each of which is called a ligand. This definition allows for a wider range of coordination compounds that would otherwise not be properly classified.

Parameters that affect the coordination number are:

  • the size and charge of the central ion;
  • the type of ligands (very large and bulky ligands reduce the coordination number);
  • the interactions within the complex.

There are many types of complexes, ranging from simple metal in aqueous solution (therefore coordinated by water molecules) to metal-enzyme complexes, which take part in various biochemical processes, to organometallic complexes, which play an important role as catalysts.

The transition metals, which in the elementary state possess partially occupied d or f levels, form a large class of compounds, called complexes or coordination complexes in which the central metal M (in the neutral or ionized state) forms dative (or coordination) covalent bonds with a series of atoms or chemical groups, called ligands, neutral or of opposite charge with respect to the central atom. The central metal acts as a Lewis acid (electron acceptor, electrophile) towards the ligands that behave as Lewis bases (donors of electronic doublets, nucleophiles).

Coordination complexes are much more common in nature than might be thought; many proteins and enzymes that regulate metabolic functions in our own and other organisms turn out to be metal-proteins, very large organic molecules composed of long chains of amino acids that also contain a metal atom via coordination bonds.

Coordination number and geometry of the complex

Coordination numberShapeGeometryExamples
2LinearLinear[CuCl2], [Ag(NH3)2]+
3Triangular planarTrigonal(very rare)
[BeF3], [HgI3]
4TetrahedralTetrahedral(quite common)
[ReO4]2–, Ni(CO)4
4Square planarSquare planarXeF4, [AuCl4], [PtCl4]2–
5Trigonal bipyramidalTetrahedral[CdCl5]3–, Fe(CO)5
5Square-based pyramidalSquare pyramidal[NbCl4(O)], [V(acac)2(O)]
6OctahedralOctahedral(very common)
[Cr(H2O)6]3+, [Fe(CN)6]3–
7Pentagonal bi-pyramidalPentagonal bipyramidal(rare)
8Square anti-prismaticSquare antiprismatic[Mo(CN)8]4–, [ReF8]2–
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