The coordination geometry of an atom is the geometrical pattern defined by the atoms around the central atom. The term is commonly applied in the field of inorganic chemistry, where diverse structures are observed. The coordination geometry depends on the number, not the type, of ligands bonded to the metal centre as well as their locations. The number of atoms bonded is the coordination number.
The geometrical pattern can be described as a polyhedron where the vertices of the polyhedron are the centres of the coordinating atoms in the ligands.
The coordination preference of a metal often varies with its oxidation state. The number of coordination bonds (coordination number) can vary from two in Potassium dicyanoargentate| as high as 20 in Organothorium chemistry|.
One of the most common coordination geometries is octahedral, where six ligands are coordinated to the metal in a symmetrical distribution, leading to the formation of an octahedron if lines were drawn between the ligands. Other common coordination geometries are tetrahedral and square planar.
Crystal field theory may be used to explain the relative stabilities of transition metal compounds of different coordination geometry, as well as the presence or absence of paramagnetism, whereas VSEPR may be used for complexes of main group element to predict geometry.
Crystallography usage
In a crystal structure the coordination geometry of an atom is the geometrical pattern of coordinating atoms where the definition of coordinating atoms depends on the bonding model used.
||Ag in silver cyanide,<br />Au in AuI
||As in NiAs, Mo in molybdenum disulfide|
||La in A-
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|style="text-align: center;"|capped trigonal prismatic
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||Thorium(IV) iodide
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||Caesium chloride, calcium fluoride
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|style="text-align: center;"|hexagonal bipyramidal
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||N in lithium nitride|
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|style="text-align: center;"|8
|style="text-align: center;"|trigonal prismatic, triangular face bicapped
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||Ca in
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|style="text-align: center;"|11
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||Th in ( is bidentate)<br />
IUCr have proposed a symbol which is shown as a superscript in square brackets in the chemical formula. For example, would be Ca<sup>[8cb]</sup>F<sub>2</sub><sup>[4t]</sup>, where [8cb] means cubic coordination and [4t] means tetrahedral. The equivalent symbols in IUPAC are CU−8 and T−4 respectively.