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Chemistry > Coordination Compounds > Definition of Some Important Terms Pertaining to Coordination Compounds
Coordination Compounds

Definition of Some Important Terms Pertaining to Coordination Compounds

To understand the concept of coordination compounds better, it is important to know certain terms and their definitions. Let’s take a look at these terms and what they mean in coordination chemistry.

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Coordination Chemistry

An assembly where a central atom or ion is bound to a fixed number of molecules or ions is a coordination entity. For example, in the coordination entity [CoCl3(NH3)3], the central cobalt ion is bound to three ammonia molecules and three chloride ions surrounding it. Other examples in coordination chemistry are [Ni(CO)4], [PtCl2(NH3)2], [Fe(CN)6]4–, [Co(NH3)6]3+.

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Central Atom/Ion

The atom/ion in a coordination entity that is bound to a fixed number of ions/groups in a definite geometrical arrangement is the central atom/ion. For example: In the coordination entities [NiCl2(H2O)4], [CoCl(NH3)5]2+ and [Fe(CN)6]3–, the central atoms/ions are Ni2+, Co3+ and Fe3+, respectively. These central atoms/ ions are also Lewis acids.


Ligands are the ions or molecules bound to the central atom/ion in a coordination entity. These ions may be simple ions like Clor small molecules such as NH3 or H2O. They may also be larger molecules like H2NCH2CH2NH2 or N(CH2CH2NH2)or macromolecules such as proteins.

Unidentate Ligands

In a lot of cases, only one atom in the ligand is bound to the metal ion. In these cases, the ligand is unidentate. Examples of unidentate ligands are H2O, NH3 or Cl.

Bidentate Ligands

Ligands that bind to the metal ion through two donor atoms are bidentate or didentate ligands. Examples are H2NCH2CH2NH2 (ethane-1,2-diamine) and C2O42– (oxalate).

Polydentate Ligands

Polydentate ligands are those that can bind to the metal ion through several donor atoms. Examples are  N(CH2CH2NH2)3 and Ethylenediaminetetraacetate ion (EDTA4–). EDTA is an important hexadentate ligand that can bind to a central metal ion through two nitrogen and four oxygen atoms.

Coordination Chemistry

Ethylenediaminetetraacetic acid                  [Source: Wikipedia]

A di or polydentate ligand that uses two or more of its donor atoms to bind to a single metal ion is a chelate ligand. The number of such donor groups is the denticity of the ligand. These complexes are chelate complexes and are more stable than other complexes containing unidentate ligands.

Ambidentate Ligands

Ambidentate ligands can ligate through two different atoms. Examples are  NO2 and SCN ions where the NO2– ion can ligate with a central metal atom/ion through nitrogen or oxygen and SCN– can ligate through sulfur or nitrogen.

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Bonding in Metal Carbonyls

More Important Terms in Coordination Chemistry

Coordination Number

The number of ligand donor atoms to which the metal is directly bound is the coordination number (CN) of the metal ion in the complex. For example, the CN of Pt and Ni in the complex ions [PtCl6]2– and [Ni(NH3)4]2+ are 6 and 4, respectively.

Likewise, the CN of Fe and Co in the complex ions [Fe(C2O4)3]3– and [Co(en)3]3+ is 6 because both C2O42– and en (ethane-1,2-diamine) are bidentate ligands.

Note: The CN of the central atom/ion is determined only by the number of sigma bonds between the central atom/ion and the ligand, not the Pi bonds.

Coordination Sphere

The coordination sphere consists of the central atom/ion and the ligands attached to it. The ionizable groups, called counter ions are written outside the bracket. For example, in the complex K4[Fe(CN)6], [Fe(CN)6]4– is the coordination sphere and K+ is the counter ion.

Coordination Polyhedron

It refers to the geometric pattern or spatial arrangement of the ligands directly attached to the central atom/ion. The most common polyhedra are tetrahedral, octahedral and square planar. Examples:  [Co(NH3)6]3+ is octahedral, [Ni(CO)4] is tetrahedral and [PtCl4]2– is square planar.

Coordination Chemistry

Oxidation Number of Central Atom

It is the charge the central atom would carry if all the ligands are removed along with the electron pairs that are shared with it. It is represented as the name of the coordination entity followed by the oxidation number written in Roman numerals in parenthesis. For example, in [Cu(CN)4]3–, the oxidation number of copper is +1 and is Cu(I).

Homoleptic and Heteroleptic Complexes

Homoleptic complexes are complexes where a metal is bound to only one kind of donor group whereas in heteroleptic complexes the metal is bound to more than one kind of donor group. Examples –  [Co(NH3)6]3+ is homoleptic and  [Co(NH3)4Cl2]+ is heteroleptic.

Solved Example on Coordination Chemistry

Question: Match the columns.

Ligand Example
(a) Unidentate (1) EDTA4–
(b) Bidentate (2) SCN
(c) Polydentate (3) C2O42–
(d) Ambidentate  (4) NH3

Solution: a-4, b-3, c-1, d-2.

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