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
Inorganic chemistry deals with chemical synthesis, synthesis and behavior of inorganic compound, inorganic and organometallic chemistry, organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subj ...
, 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
In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central ion ...
.
The geometrical pattern can be described as a
polyhedron
In geometry, a polyhedron (: polyhedra or polyhedrons; ) is a three-dimensional figure with flat polygonal Face (geometry), faces, straight Edge (geometry), edges and sharp corners or Vertex (geometry), vertices. The term "polyhedron" may refer ...
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
In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central ion ...
) can vary from two in
as high as 20 in
.
One of the most common coordination geometries is
octahedral
In geometry, an octahedron (: octahedra or octahedrons) is any polyhedron with eight faces. One special case is the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. Many types of i ...
, where six ligands are coordinated to the metal in a symmetrical distribution, leading to the formation of an
octahedron
In geometry, an octahedron (: octahedra or octahedrons) is any polyhedron with eight faces. One special case is the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex. Many types of i ...
if lines were drawn between the ligands. Other common coordination geometries are
tetrahedral
In geometry, a tetrahedron (: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (geometry), vertices. The tet ...
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
Paramagnetism is a form of magnetism whereby some materials are weakly attracted by an externally applied magnetic field, and form internal, induced magnetic fields in the direction of the applied magnetic field. In contrast with this behavior, ...
, whereas
VSEPR may be used for complexes of
main group element
In chemistry and atomic physics, the main group is the group (periodic table), group of chemical element, elements (sometimes called the representative elements) whose lightest members are represented by helium, lithium, beryllium, boron, carbon ...
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.
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
A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. These properties are all associated with having electrons available at the Fermi level, as against no ...
with the body centred cubic (bcc) structure each atom has eight nearest neighbours in a cubic geometry. In
metals
A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. These properties are all associated with having electrons available at the Fermi level, as against no ...
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).
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.
[NOMENCLATURE OF INORGANIC CHEMISTRY IUPAC Recommendations 2005 ed. N. G. Connelly et al. RSC Publishing http://www.chem.qmul.ac.uk/iupac/bioinorg/]
IUCr have proposed a symbol which is shown as a superscript in square brackets in the chemical formula. For example, would be Ca
cb/sup>F2 t/sup>, where cbmeans cubic coordination and tmeans tetrahedral. The equivalent symbols in IUPAC are ''CU''−8 and ''T''−4 respectively.
The IUPAC symbol is applicable to complexes and molecules whereas the IUCr proposal applies to crystalline solids.
See also
*Molecular geometry
Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometrical parameters that det ...
* VSEPR
* Ligand field theory
*Cis effect
In inorganic chemistry, the cis effect is defined as the labilization (or destabilization) of CO ligands that are ''cis'' to other ligands. CO is a well-known strong pi-accepting ligand in organometallic chemistry that will labilize in the ''cis ...
* Addition to pi ligands
References
{{reflist
Molecular physics
Chemical bonding
Coordination chemistry
Inorganic chemistry