View source for Coordination geometry

The term '''coordination geometry''' is used in a number of related fields of chemistry and solid state chemistry/physics.
==Molecules==
{{main|Molecular geometry}}
The coordination geometry of an atom is the geometrical pattern formed by atoms around the central atom.

==Inorganic coordination complexes==
In the field of inorganic coordination complexes it is the geometrical pattern formed by the atoms in the [[ligand]]s that are [[coordination (bond)|bonded]] to the central atom in a [[molecule]] or a [[coordination complex]]. The geometrical arrangement will vary according to the number and type of ligands bonded to the metal centre, and to the coordination preference of the central atom, typically a metal in a [[coordination complex]]. The number of atoms bonded, (i.e. the number of [[sigma bonds|σ-bonds]] between central atom and ligands) is termed 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.<ref name = "IUCr">Report of the International Union of Crystallography Commission on Crystallographic Nomenclature Subcommittee on the Nomenclature of Inorganic Structure Types 
By J. LIMA-DE-FARIA, E. HELLNER, F. LIEBAU, E. MAKOVICKY, E. PARTHÉ (1989) [http://ww1.iucr.org/comm/cnom/inorg/node4.html]</ref>

The coordination preference of a metal often varies with its oxidation state. The number of coordination bonds ([[coordination number]]) can vary from two as high as 20 in Th(η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)<sub>4</sub>.<ref name = "Greenwood"> {{Greenwood&Earnshaw}}</ref> <br />
One of the most common coordination geometries is [[octahedral molecular geometry|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 molecular geometry|tetrahedral]] and [[square planar molecular geometry|square planar]].<br />
[[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]].<br />
[[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.<ref name = "IUCr"/> For example in the rock salt ionic structure each sodium atom has six near neighbour chloride ions in an octahedral geometry and each chloride has similarly six near neighbour sodium ions in an octahedral geometry. In [[periodic table (crystal structure)|metals]] with the body centred cubic (bcc) structure each atom has eight nearest neighbours in a cubic geometry. In [[periodic table (crystal structure)|metals]] with the face centred cubic (fcc) structure each atom has twelve nearest neighbours in a [[cuboctahedron|cuboctahedral]] geometry.

==Table of coordination geometries==
A table of the coordination geometries encountered is shown below with examples of their occurrence in  complexes found as discrete units in compounds and coordination spheres around atoms in crystals (where there is no discrete complex).

{| class="wikitable"
!style="text-align: center;"|Coordination number !! Geometry !! !! Examples of discrete (finite) complex !! Examples in crystals
|-
|style="text-align: center;"| 2 
|style="text-align: center;"|linear 
||[[Image:Linear-3D-balls.png|100px]]
||Ag(CN)<sub>2</sub><sup>−</sup> in KAg(CN)<sub>2</sub> <ref name = "Wells"> Wells A.F. (1984) ''Structural Inorganic Chemistry'' 5th edition Oxford Science Publications ISBN 0-19-855370-6 </ref>
||Ag in [[silver cyanide]],<br />Au in AuI <ref name = "Greenwood"/>
|-
|style="text-align: center;"|3 
|style="text-align: center;"|trigonal planar
||[[Image:Trigonal-3D-balls.png|100px]]
||Cu(CN)<sub>3</sub><sup>2−</sup>  in Na<sub>2</sub>Cu(CN)<sub>3</sub>.3H<sub>2</sub>O<ref name = "Greenwood"/>
||O in [[titanium dioxide|TiO<sub>2</sub>]] rutile structure<ref name = "Wells"/>
|-
|style="text-align: center;"|4
|style="text-align: center;"|tetrahedral geometry
||[[Image:Tetrahedral-3D-balls.png|100px]]
||CoCl<sub>4</sub><sup>2−</sup><ref name = "Greenwood"/>
||Zn and S in [[zinc sulfide]], Si in  [[silicon dioxide]]<ref name = "Wells"/>
|-
|style="text-align: center;"|4 
|style="text-align: center;"|square planar
||[[Image:Square-planar-3D-balls.png|100px]]
||AgF<sub>4</sub><sup>−</sup><ref name = "Greenwood"/>
||[[copper(II) oxide|CuO]]<ref name = "Wells"/>
|-
|style="text-align: center;"|5 
|style="text-align: center;"|trigonal bipyramidal
||[[Image:Trigonal-bipyramidal-3D-balls.png|100px]]
||SnCl<sub>5</sub><sup>−</sup><ref name = "Wells"/>
||
|-
|style="text-align: center;"|5 
|style="text-align: center;"|square pyramidal
||[[Image:Square-pyramidal-3D-balls.png|100px]]
||InCl<sub>5</sub><sup>2−</sup> in (NEt<sub>4</sub>)<sub>2</sub>InCl<sub>5</sub><ref name = "Greenwood"/>
||
|-
|style="text-align: center;"|6 
|style="text-align: center;"|octahedral 
||[[Image:Octahedral-3D-balls.png|100px]]
||Fe(H<sub>2</sub>O)<sub>6</sub><sup>2+</sup><ref name = "Greenwood"/>
||Na and Cl in NaCl<ref name = "Wells"/>
|-
|style="text-align: center;"|6 
|style="text-align: center;"|trigonal prismatic
||
||Mo(SCHCHS)<sub>3</sub><ref name = "Wells"/>
||As in [[nickel arsenide|NiAs]], Mo in [[molybdenum disulfide|MoS<sub>2</sub>]]<ref name = "Wells"/>
|-
|style="text-align: center;"|7
|style="text-align: center;"|pentagonal bipyramidal
||[[Image:Pentagonal-bipyramidal-3D-balls.png|100px]]
||ZrF<sub>7</sub><sup>3−</sup>  in (NH<sub>4</sub>)<sub>3</sub>ZrF<sub>7</sub><ref name = "Wells"/>
||Pa in [[Protactinium(V) chloride|PaCl<sub>5</sub>]]
|-
|style="text-align: center;"|7 
|style="text-align: center;"|face capped octahedral
||
||[Ho<sup>III</sup>(PhCOCHCOPh)<sub>3</sub>(H<sub>2</sub>O)]<ref>Crystal and molecular structure of the heptacoordinate complex tris(diphenylpropanedionato)aquoholmium, Ho(PhCOCHCOPh)<sub>3</sub>.H<sub>2</sub>O, Zalkin A., Templeton D.H., Karraker D.G,  Inorganic chemistry, 1969, 8, 12,2680 - 2684; {{doi|10.1021/ic50082a029}}</ref>
||La in [[lanthanum(III) oxide|A-La<sub>2</sub>O<sub>3</sub>]]
|-
|style="text-align: center;"|7 
|style="text-align: center;"|trigonal prismatic, square face monocapped
||
||TaF<sub>7</sub><sup>2−</sup> in K<sub>2</sub>TaF<sub>7</sub><ref name = "Wells"/>
||
|-
|style="text-align: center;"|8 
|style="text-align: center;"|cubic 
||
||
||[[Caesium chloride]], [[calcium fluoride]]
|-
|style="text-align: center;"|8 
|style="text-align: center;"|square antiprism
||[[Image:Square antiprism.png|100px]]
||TaF<sub>8</sub><sup>3−</sup> in Na<sub>3</sub>TaF<sub>8</sub><ref name = "Wells"/>
||[[Thorium(IV) iodide]]<ref name = "Wells"/>
|-
|style="text-align: center;"|8 
|style="text-align: center;"|dodecahedral <br />(note whilst this is term generally<br /> used the correct term is bisdisphenoid<ref name = "Wells"/><br /> or [[snub disphenoid]] as this polyhedron is a [[deltahedron]])
||[[Image:snub disphenoid.png|100px]]
||Mo(CN)<sub>8</sub><sup>4−</sup> in K<sub>4</sub>[Mo(CN)<sub>8</sub>].2H<sub>2</sub>O<ref name = "Wells"/>
||Zr in K<sub>2</sub>ZrF<sub>6</sub><ref name = "Wells"/>
|-
|style="text-align: center;"|8 
|style="text-align: center;"|hexagonal bipyramidal
||[[File:Hexagonale bipiramide.png|100px]]
||
||N in [[lithium nitride|Li<sub>3</sub>N]]<ref name = "Wells"/>
|-
|style="text-align: center;"|8 
|style="text-align: center;"|octahedral, trans-bicapped
||
||
||Ni in [[nickel arsenide]], NiAs; 6 As neighbours  + 2 Ni capping<ref>David G. Pettifor, ''Bonding and Structure of Molecules and Solids'', 1995, Oxford University Press,ISBN 0198517866</ref>
|-
|style="text-align: center;"|8 
|style="text-align: center;"|trigonal prismatic, triangular face bicapped
||
||
||Ca in CaFe<sub>2</sub>O<sub>4</sub><ref name = "Wells"/>
|-
|style="text-align: center;"|8
|style="text-align: center;"|trigonal prismatic, square face bicapped
||
||
||PuBr<sub>3</sub><ref name = "Wells"/>
|-
|style="text-align: center;"|9
|style="text-align: center;"|trigonal prismatic, square face tricapped
||[[Image:Nonahydridorhenate-3D-balls.png|100px]]
||[ReH<sub>9</sub>]<sup>2−</sup> in [[potassium nonahydridorhenate]]<ref name = "Greenwood"/>
||SrCl<sub>2</sub>.6H<sub>2</sub>O , Th in RbTh<sub>3</sub>F<sub>13</sub><ref name = "Wells"/>
|-
|style="text-align: center;"|9  
|style="text-align: center;"|monocapped square antiprismatic
||
||[Th(tropolonate)<sub>4</sub>(H<sub>2</sub>O)]<ref name = "Greenwood"/>
||La in LaTe<sub>2</sub><ref name = "Wells"/>
|-
|style="text-align: center;"|10 
|style="text-align: center;"|bicapped square antiprismatic
||
||Th(C<sub>2</sub>O<sub>4</sub>)<sub>4</sub><sup>2−</sup> <ref name = "Greenwood"/>
||
|-
|style="text-align: center;"|11
||
||
||Th in [Th<sup>IV</sup>(NO<sub>3</sub>)<sub>4</sub>(H<sub>2</sub>O)<sub>3</sub>] (NO<sub>3</sub><sup>−</sup> is bidentate) <ref name = "Greenwood"/>
||
|-
|style="text-align: center;"|12 
|style="text-align: center;"|icosahedron
||[[Image:icosahedron.png|100px]]
||Th in Th(NO<sub>3</sub>)<sub>6</sub><sup>2−</sup> ion in Mg[Th(NO<sub>3</sub>)<sub>6</sub>].8H<sub>2</sub>O <ref name = "Wells"/>
|-
|style="text-align: center;"|12 
|style="text-align: center;"|cuboctahedron
||[[Image:Cuboctahedron.png|100px]]
||Zr<sup>IV</sup>(η<sup>3</sup>−(BH<sub>4</sub>)<sub>4</sub>)
||atoms in [[periodic table (crystal structure)| fcc metals]] e.g. Ca<ref name = "Wells"/>
|-
|style="text-align: center;"|12 
|style="text-align: center;"|anticuboctahedron, triangular orthobicupola
||[[Image:triangular_orthobicupola.png|100px]]
||
||atoms in [[periodic table (crystal structure)| hcp metals]] e.g. Sc <ref name = "Wells"/>
|-
|style="text-align: center;"|14 
|style="text-align: center;"|bicapped hexagonal antiprismatic
||
||U(BH<sub>4</sub>)<sub>4</sub><ref name = "Greenwood"/>
|}

==Naming of inorganic compounds==
IUPAC have introduced the [[polyhedral symbol]] as part of their [[IUPAC nomenclature of inorganic chemistry 2005|IUPAC nomenclature of inorganic chemistry 2005 recommendations]] to describe the geometry around an atom in a compound.<ref name = "IUPAC inorganic"> NOMENCLATURE OF INORGANIC CHEMISTRY IUPAC Recommendations 2005 ed. N. G. Connelly et al. RSC Publishing  http://www.chem.qmul.ac.uk/iupac/bioinorg/</ref><br />
IUCr have proposed a symbol which is shown as a superscript in square brackets in the chemical formula. For example CaF<sub>2</sub> 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.<ref name = "IUCr"/> <br />
The IUPAC symbol is applicable to complexes and molecules whereas the IUCr proposal applies to crystalline solids.

==See also==

*[[Molecular geometry]]
*[[VSEPR]]
*[[Ligand field theory]]
==References==
{{reflist}}

[[Category:molecular physics]][[Category:chemical bonding]][[Category:coordination chemistry]][[Category:inorganic chemistry]]

[[pt:Geometria de coordenação]]
[[ru:Координационная геометрия]]
[[sl:Koordinacijska geometrija]]
[[zh:配合物结构]]
[[Category:2Fix]]