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Coordination geometry

    The term '''coordination geometry''' is used in a number of related fields of chemistry and solid state chemistry/physics

    Molecules

    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 ligands that are 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 (ie the number of σ-bonds between central atom and ligands) is termed the coordination numberThe geometrical pattern can be described as a polyhedron where the vertices of the polyhedron are the centres of the coordinating atoms in the ligandsReport of the International Union of Crystallography Commission on Crystallographic Nomenclature Subcommittee on the Nomenclature of Inorganic Structure TypesBy J. LIMA-DE-FARIA E. HELLNER F. LIEBAU E. MAKOVICKY E. PARTHÉ (1989) [1]
    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(η5-C5H5)4 Other common coordination geometries are tetrahedral and square planarCrystal 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 paramagnetismmain 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 usedsodium 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)
    Coordination number Geometry Examples of discrete (finite) complex Examples in crystals
    2linear
    100px
    Ag(CN)2 in KAg(CN)2 Wells AF (1984) Structural Inorganic Chemistry 5th edition Oxford Science Publications ISBN 0-19-855370-6 Ag in silver cyanide
    3trigonal planar
    100px
    Cu(CN)32− in Na2Cu(CN)33H2OO in TiO2 rutile structure
    4tetrahedral geometry
    100px
    CoCl42−Zn and S in zinc sulfide Si in silicon dioxide
    4square planar
    100px
    AgF4CuO
    5trigonal bipyramidal
    100px
    SnCl5
    5square pyramidal
    100px
    InCl52− in (NEt4)2InCl5
    6octahedral
    100px
    Fe(H2O)62+Na and Cl in NaCl
    6trigonal prismaticMo(SCHCHS)3As in NiAs Mo in MoS2
    7pentagonal bipyramidal
    100px
    ZrF73− in (NH4)3ZrF7Pa in PaCl5
    7face capped octahedral[2]Crystal and molecular structure of the heptacoordinate complex tris(diphenylpropanedionato)aquoholmium Ho(PhCOCHCOPh)3H2O Zalkin A., Templeton DH Karraker DG Inorganic chemistry 1969 8, 122680 - 2684; La in A-La2O3
    7trigonal prismatic square face monocappedTaF72− in K2TaF7
    8cubicCaesium chloride calcium fluoride
    8square antiprism
    100px
    TaF83− in Na3TaF8Thorium iodide
    8dodecahedral
    polyhedron is a deltahedron)
    Mo(CN)84− in K4[3]2H2OZr in K2ZrF6
    8hexagonal bipyramidal100pxN in Li3N
    8octahedral trans-bicappedNi in nickel arsenide NiAs; 6 As neighbours + 2 Ni cappingDavid G. Pettifor Bonding and Structure of Molecules and Solids 1995 Oxford University PressISBN 0198517866
    8trigonal prismatic triangular face bicappedCa in CaFe2O4
    8trigonal prismatic square face bicappedPuBr3
    9trigonal prismatic square face tricapped
    100px
    [4]2− in potassium nonahydridorhenateSrCl26H2O , Th in RbTh3F13
    9monocapped square antiprismatic[5]La in LaTe2
    10bicapped square antiprismaticTh(C2O4)42−
    11Th in [6] (NO3 is bidentate)
    12icosahedronTh in Th(NO3)62− ion in Mg[7]8H2O
    12cuboctahedron
    100px
    ZrIV3−(BH4)4)atoms in fcc metals eg Ca
    12anticuboctahedron triangular orthobicupolaatoms in hcp metals eg Sc
    14bicapped hexagonal antiprismaticU(BH4)4

    Naming of inorganic compounds

    IUPAC have introduced the polyhedral symbol as part of their IUPAC nomenclature of chemistry 2005|IUPAC nomenclature of inorganic chemistry 2005 recommendations] to describe the geometry around an atom in a compound NOMENCLATURE OF INORGANIC CHEMISTRY IUPAC Recommendations 2005 ed. N. G. Connelly et al. RSC Publishing http://wwwchemqmulacuk/iupac/bioinorg/chemical formula For example CaF2 would be Ca[8]F2[9] where [10] means cubic coordination and [11] means tetrahedral The equivalent symbols in IUPAC are CU−8 and T−4 respectively

    See also

    References