Thecoordination geometry of an atom is the geometrical pattern defined by the atoms around the central atom. The term is commonly applied in the field ofinorganic 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 thecoordination number.The geometrical pattern can be described as apolyhedron where the vertices of the polyhedron are the centres of the coordinating atoms in the ligands.[1]
The coordination preference of a metal often varies with its oxidation state. The number of coordination bonds (coordination number) can vary from two inK[Ag(CN)2] as high as 20 inTh(η5-C5H5)4.[2]
One of the most common coordination geometries isoctahedral, where six ligands are coordinated to the metal in a symmetrical distribution, leading to the formation of anoctahedron if lines were drawn between the ligands. Other common coordination geometries aretetrahedral andsquare 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 ofparamagnetism, whereasVSEPR may be used for complexes ofmain group element to predict geometry.
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.[1] 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. Inmetals with the body centred cubic (bcc) structure each atom has eight nearest neighbours in a cubic geometry. Inmetals with the face centred cubic (fcc) structure each atom has twelve nearest neighbours in acuboctahedral geometry.
A table of the coordination geometries encountered is shown below with examples of their occurrence in complexes found as discrete units in compounds andcoordination spheres around atoms in crystals (where there is no discrete complex).
| Coordination number | Geometry | Examples of discrete (finite) complex | Examples in crystals (infinite solids) | |
|---|---|---|---|---|
| 2 | linear |  | [Ag(CN)2]− inK[Ag(CN)2][3] | Ag insilver cyanide, Au in AuI[2] |
| 3 | trigonal planar |  | [HgI3]−[2] | O inTiO2rutile structure[3] |
| 4 | tetrahedral |  | [CoCl4]2−[2] | Zn and S inzinc sulfide, Si insilicon dioxide[3] |
| 4 | square planar |  | [AgF4]−[2] | CuO[3] |
| 5 | trigonal bipyramidal |  | [SnCl5]−[3] | |
| 5 | square pyramidal |  | [InCl5]2− in[N(CH2CH3)4]2[InCl5][2] | |
| 6 | octahedral |  | [Fe(H2O)6]2+[2] | Na andCl inNaCl[3] |
| 6 | trigonal prismatic |  | W(CH3)6[4] | As inNiAs,Mo inMoS2[3] |
| 7 | pentagonal bipyramidal |  | [ZrF7]3− in[NH4]3[ZrF7][3] | Pa inPaCl5 |
| 7 | capped octahedral |  | [MoF7]−[5] | La inA-La2O3 |
| 7 | capped trigonal prismatic |  | [TaF7]2− inK2[TaF7][3] | |
| 8 | square antiprismatic |  | [TaF8]3− inNa3[TaF8][3] [Zr(H2O)8]4+aqua complex[6] | Thorium(IV) iodide[3] |
| 8 | dodecahedral (note: whilst this is the term generally used, the correct term is "bisdisphenoid"[3] or "snub disphenoid" as this polyhedron is adeltahedron) |  | [Mo(CN)8]4− inK4[Mo(CN)8]·2H2O[3] | Zr inK2[ZrF6][3] |
| 8 | bicapped trigonal prismatic |  | [ZrF8]4−[7] | PuBr3[3] |
| 8 | cubic | Caesium chloride,calcium fluoride | ||
| 8 | hexagonal bipyramidal |  | N inLi3N[3] | |
| 8 | octahedral, trans-bicapped | Ni innickel arsenide, NiAs; 6 As neighbours + 2 Ni capping[8] | ||
| 8 | trigonal prismatic, triangular face bicapped | Ca inCaFe2O4[3] | ||
| 9 | tricapped trigonal prismatic |  | [ReH9]2− inpotassium nonahydridorhenate[2] [Th(H2O)9]4+ aqua complex[6] | SrCl2·6H2O, Th inRb[Th3F13][3] |
| 9 | capped square antiprismatic |  | [Th(tropolonate)4(H2O)][2][clarification needed] | La inLaTe2[3] |
| 10 | bicapped square antiprismatic | [Th(C2O4)4]2−[2] | ||
| 11 | Th in[ThIV(NO3)4(H2O)3] (NO−3 isbidentate)[2] | |||
| 12 | icosahedron |  | Th in[Th(NO3)6]2− ion inMg[Th(NO3)6]·8H2O[3] | |
| 12 | cuboctahedron |  | ZrIV(η3-[BH4]4) | atoms infcc metals e.g. Ca[3] |
| 12 | anticuboctahedron (triangular orthobicupola) |  | atoms inhcp metals e.g. Sc[3] | |
| 12 | bicappedhexagonal antiprismatic | U[BH4]4[2] |
IUPAC have introduced thepolyhedral symbol as part of theirIUPAC nomenclature of inorganic chemistry 2005 recommendations to describe the geometry around an atom in a compound.[9]
IUCr have proposed a symbol which is shown as a superscript in square brackets in the chemical formula. For example,CaF2 would be Ca[8cb]F2[4t], where [8cb] means cubic coordination and [4t] means tetrahedral. The equivalent symbols in IUPAC areCU−8 andT−4 respectively.[1]
The IUPAC symbol is applicable to complexes and molecules whereas the IUCr proposal applies to crystalline solids.