Coordination geometry

{{Short description|Term used in chemistry}} {{main|Molecular geometry}} 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.<ref name = "IUCr">{{cite journal | title = Report of the International Union of Crystallography Commission on Crystallographic Nomenclature Subcommittee on the Nomenclature of Inorganic Structure Types | journal = Acta Crystallogr. A | volume = 46 | pages = 1–11 | year = 1990 | doi = 10.1107/S0108767389008834 | author1 = J. Lima-de-Faria | author2 = E. Hellner | author3 = F. Liebau | author4 = E. Makovicky | author5 = E. Parthé | doi-access = free }}</ref>

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|{{chem2|K[Ag(CN)2]}}]] as high as 20 in {{chem2|Th(''η''^{5}\-C5H5)4}}.<ref name = "Greenwood">{{Greenwood&Earnshaw}}</ref>

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.<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 metals with the body centred cubic (bcc) structure each atom has eight nearest neighbours in a cubic geometry. In metals with the face centred cubic (fcc) structure each atom has twelve nearest neighbours in a 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 (infinite solids) |- |style="text-align: center;"| 2 |style="text-align: center;"|linear ||100px ||{{chem2|[Ag(CN)2]−}} in {{chem2|K[Ag(CN)2]}} <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, Au in AuI <ref name = "Greenwood"/> |- |style="text-align: center;"|3 |style="text-align: center;"|trigonal planar ||100px ||{{chem2|[HgI3]−}}<ref name = "Greenwood"/> ||O in {{chem2|TiO2}} rutile structure<ref name = "Wells"/> |- |style="text-align: center;"|4 |style="text-align: center;"|tetrahedral ||100px ||{{chem2|[CoCl4](2−)}}<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 ||100px ||{{chem2|[AgF4]−}}<ref name = "Greenwood"/> ||CuO<ref name = "Wells"/> |- |style="text-align: center;"|5 |style="text-align: center;"|trigonal bipyramidal ||100px ||{{chem2|[SnCl5]−}}<ref name = "Wells"/> || |- |style="text-align: center;"|5 |style="text-align: center;"|square pyramidal ||100px ||{{chem2|[InCl5](2−)}} in {{chem2|[N(CH2CH3)4]2[InCl5]}}<ref name = "Greenwood"/> || |- |style="text-align: center;"|6 |style="text-align: center;"|octahedral ||100px ||{{chem2|[Fe(H2O)6](2+)}}<ref name = "Greenwood"/> ||Na and Cl in NaCl<ref name = "Wells"/> |- |style="text-align: center;"|6 |style="text-align: center;"|trigonal prismatic ||100px ||{{chem2|W(CH3)6}}<ref>{{Housecroft2nd|page=725}}</ref> ||As in NiAs, Mo in {{chem2|MoS2}}<ref name = "Wells"/> |- |style="text-align: center;"|7 |style="text-align: center;"|pentagonal bipyramidal ||100px ||{{chem2|[ZrF7](3−)}} in {{chem2|[NH4]3[ZrF7]}}<ref name = "Wells"/> ||Pa in {{chem2|PaCl5}} |- |style="text-align: center;"|7 |style="text-align: center;"|capped octahedral ||100px ||{{chem2|[MoF7]−}}<ref>{{cite journal | title = "Non-VSEPR" Structures and Bonding in d(0) Systems | first = Martin | last = Kaupp | journal = Angew Chem Int Ed Engl | year = 2001 | volume = 40 | issue = 1 | pages = 3534–3565 | doi = 10.1002/1521-3773(20011001)40:19<3534::AID-ANIE3534>3.0.CO;2-# | pmid = 11592184 }}</ref> ||La in A-{{chem2|La2O3}} |- |style="text-align: center;"|7 |style="text-align: center;"|capped trigonal prismatic ||100px ||{{chem2|[TaF7](2−)}} in {{chem2|K2[TaF7]}}<ref name = "Wells"/> || |- |style="text-align: center;"|8 |style="text-align: center;"|square antiprismatic ||100px ||{{chem2|[TaF8](3−)}} in {{chem2|Na3[TaF8]}}<ref name = "Wells"/> {{chem2|[Zr(H2O)8](4+)}} aqua complex<ref name="Persson2010">{{cite journal|last1=Persson|first1=Ingmar|title=Hydrated metal ions in aqueous solution: How regular are their structures?|journal=Pure and Applied Chemistry|volume=82|issue=10|year=2010|pages=1901–1917|issn=0033-4545|doi=10.1351/PAC-CON-09-10-22|doi-access=free}}</ref> ||Thorium(IV) iodide<ref name = "Wells"/> |- |style="text-align: center;"|8 |style="text-align: center;"|dodecahedral (note: whilst this is the term generally used, the correct term is "bisdisphenoid"<ref name = "Wells"/> or "snub disphenoid" as this polyhedron is a deltahedron) ||100px ||{{chem2|[Mo(CN)8](4−)}} in {{chem2|K4[Mo(CN)8]*2H2O}}<ref name = "Wells"/> ||Zr in {{chem2|K2[ZrF6]}}<ref name = "Wells"/> |- |style="text-align: center;"|8 |style="text-align: center;"|bicapped trigonal prismatic ||100px ||{{chem2|[ZrF8](4−)}}<ref>{{cite journal | title = Eight-Coordination | author1 = Jeremy K. Burdett | author2 = Roald Hoffmann | author3 = Robert C. Fay | journal = Inorganic Chemistry | year = 1978 | volume = 17 | issue = 9 | pages = 2553–2568 | doi = 10.1021/ic50187a041 }}</ref> ||{{chem2|PuBr3}}<ref name = "Wells"/> |- |style="text-align: center;"|8 |style="text-align: center;"|cubic || ||{{chem2|[PaF8](3-)}} in {{chem2|Na3PaF8}}<ref>{{cite journal | last1=Brown | first1=D. | last2=Easey | first2=J. F. | last3=Rickard | first3=C. E. F. | title=Cubic co-ordination: Crystal structure of sodium octafluoroprotactinate(V) | journal=Journal of the Chemical Society A: Inorganic, Physical, Theoretical | date=1969 | page=1161 | doi=10.1039/J19690001161 | url=https://doi.org/10.1039/J19690001161 }}</ref> ||Caesium chloride, calcium fluoride |- |style="text-align: center;"|8 |style="text-align: center;"|hexagonal bipyramidal ||100px || ||N in {{chem2|Li3N}}<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|0-19-851786-6}}</ref> |- |style="text-align: center;"|8 |style="text-align: center;"|trigonal prismatic, triangular face bicapped || || ||Ca in {{chem2|CaFe2O4}}<ref name = "Wells"/> |- |style="text-align: center;"|9 |style="text-align: center;"|tricapped trigonal prismatic ||100px ||{{chem2|[ReH9](2−)}} in potassium nonahydridorhenate<ref name = "Greenwood"/> {{chem2|[Th(H2O)9](4+)}} aqua complex<ref name="Persson2010"/> ||{{chem2|SrCl2*6H2O}}, Th in {{chem2|Rb[Th3F13]}}<ref name = "Wells"/> |- |style="text-align: center;"|9 |style="text-align: center;"|capped square antiprismatic ||100px ||{{chem2|[Th(tropolonate)4(H2O)]}}<ref name = "Greenwood"/> ||La in {{chem2|LaTe2}}<ref name = "Wells"/> |- |style="text-align: center;"|10 |style="text-align: center;"|bicapped square antiprismatic || ||{{chem2|[Th(C2O4)4](2−)}}<ref name = "Greenwood"/> || |- |style="text-align: center;"|10 |style="text-align: center;"|bicapped cubic || || ||Th in {{chem2|Th(C2O4)2*6H2O}}<ref name="Bicapped Cubic">{{cite journal | last1=Sockwell | first1=A. Kirstin | last2=Sweet | first2=Teagan F. M. | last3=Barth | first3=Brodie | last4=Isbill | first4=Sara B. | last5=Diblasi | first5=Nicole A. | last6=Szymanowski | first6=Jennifer E. S. | last7=Sigmon | first7=Ginger E. | last8=Oliver | first8=Allen G. | last9=Miskowiec | first9=Andrew J. | last10=Burns | first10=Peter C. | last11=Hixon | first11=Amy E. | title=Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries* | journal=Chemistry – A European Journal | date=2023 | volume=29 | issue=47 | article-number=e202301164 | doi=10.1002/chem.202301164 | pmid=37227412 | bibcode=2023ChEuJ..29E1164S | osti=1983863 | url=https://www.researchgate.net/publication/369731537 }}</ref> |- |style="text-align: center;"|11 || || ||Th in {{chem2|[Th^{IV}(NO3)4(H2O)3]}} ({{chem2|[NO3]-}} is bidentate)<ref name = "Greenwood"/> || |- |style="text-align: center;"|12 |style="text-align: center;"|icosahedron ||100px ||Th in {{chem2|[Th(NO3)6](2−)}} ion in {{chem2|Mg[Th(NO3)6]*8H2O}}<ref name = "Wells"/> |- |style="text-align: center;"|12 |style="text-align: center;"|cuboctahedron ||100px ||{{chem2|Zr^{IV}(''η''^{3}\−[BH4]4)}} ||atoms in fcc metals e.g. Ca<ref name = "Wells"/> |- |style="text-align: center;"|12 |style="text-align: center;"|anticuboctahedron (triangular orthobicupola) ||100px || ||atoms in hcp metals e.g. Sc<ref name = "Wells"/> |- |style="text-align: center;"|12 |style="text-align: center;"|bicapped hexagonal antiprismatic || ||{{chem2|U[BH4]4}}<ref name = "Greenwood"/> |}

==Naming of inorganic compounds== IUPAC have introduced the polyhedral symbol as part of their 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> IUCr have proposed a symbol which is shown as a superscript in square brackets in the chemical formula. For example, {{chem2|CaF2}} would be Ca[8cb]F2[4t], where [8cb] means cubic coordination and [4t] means tetrahedral. The equivalent symbols in IUPAC are ''CU''−8 and ''T''−4 respectively.<ref name = "IUCr"/> The IUPAC symbol is applicable to complexes and molecules whereas the IUCr proposal applies to crystalline solids.

==References== {{reflist}}

Category:Molecular physics Category:Chemical bonding Category:Coordination chemistry Category:Inorganic chemistry