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Definition of Important Terms Pertaining to Co-Ordination Compounds

Coordination Entity:

Coordination entity is an ion or a neutral molecule, composed of a central atom, usually a metal and the array of other atoms or groups of atoms (ligands) that are attached to it. In the formula, the coordination entity is enclosed in square brackets. For example, in potassium ferrocyanide, K4[Fe(CN)6], the coordination entity is [Fe(CN)6]4-. In nickel tetracarbonyl, the coordination entity is [Ni(CO)4].

Central Atom/Ion:

The central atom/ion is the one that occupies the central position in a coordination entity and binds other atoms or groups of atoms (ligands) to itself, through a coordinate covalent bond. For example, in K4[Fe(CN)6], the central metal ion is Fe2+. In the coordination entity [Fe(CN)6]4-, the Fe2+ accepts an electron pair from each ligand, CNand thereby forming six coordinate covalent bonds with them. since, the central metal ion has an ability to accept electron pairs, it is referred to as a Lewis acid.


The ligands are the atoms or groups of atoms bound to the central atom/ion. The atom in a ligand that is bound directly to the central metal atom is known as a donor atom. For example, in K4[Fe(CN)6]4- the ligand is CN ion, but the donor atom is carbon and in [Co(NH3)6]Cl3 the ligand is NH3 molecule and the donor atom is nitrogen.

Coordination Sphere:

The complex ion of the coordination compound containing the central metal atom/ion and the ligands attached to it, is collectively called coordination sphere and are usually enclosed in square brackets with the net charge. The other ionisable ions, are written outside the bracket are called counter ions. For example, the coordination compound K4[Fe(CN)6] contains the complex ion [Fe(CN)6]4- and is referred as the coordination sphere. The other associated ion K+ is called the counter ion.

Coordination Polyhedron:

The three dimensional spacial arrangement of ligand atoms/ions that are directly attached to the central atom is known as the coordination polyhedron (or polygon). For example, in K4[Fe(CN)6], the coordination polyhedra is octrahedral. The coordination polyhedra of [Ni(CO)4] is tetrahedral.

Coordination Number:

The number of ligand donor atoms bonded to a central metal ion in a complex is called the coordination number of the metal. In other words, the coordination number is equal to the number of σ-bonds between ligands and the central atom.

For example,

  • In K4[Fe(CN)6], the coordination number of Fe2+ is 6.
  • In [Ni(en)3]Cl2, the coordination number of Ni2+ is also 6. Here the ligand ‘en’ represents ethane-1,2-diamine (NH2-CH2-CH2-NH2) and it contains two donor atoms (Nitrogen).
  • Each ligand forms two  coordination bonds with nickel. So,totally there are six coordination bonds between them.

Oxidation State (Number):

The oxidation state of a central atom in a coordination entity is defined as the charge it would bear if all the ligands were removed along with the electron pairs that were shared with the central atom. In naming a complex, it is represented by a Roman numeral.

For example, in the coordination entity [Fe(CN)6]4-, the oxidation state of iron is represented as (II). The net charge on the complex ion is equal to the sum the oxidation state of the central metal and the charge the on the ligands attached to it. Using this relation the oxidation number can be calculated as follows Net charge = (oxidation state of the central metal) + [(No. of ligands) × (charge on the ligand)]

Example 1:

In [Fe(CN)6]4-, let the oxidation number of iron is x:
The net charge: – 4 = x + 6 (-1) ⇒ x = +2

Example 2:

In [Co(NH3)5Cl]2+, let the oxidation number of cobalt is x:
The net charge: +2 = x + 5 (0) + 1 (-1) ⇒ x = +3

Types of Complexes:

The coordination compounds can be classified into the following types based on

  • The net charge of the complex ion
  • Kinds of ligands present in the coordination entity.

Classification based on the net charge on the complex:

A coordination compound in which the complex ion

(i) Carries a net positive charge is called a cationic complex. Examples: [Ag(NH3)2]+, [Co(NH3)6]3+,
[Fe(H2)O6]2+, etc

(ii) Carries a net negative charge is called an anionic complex. Examples: [Ag(CN)2], [Co(CN)6]3-,
[Fe(CN)6]4-, etc

(iii) Bears no net charge, is called a neutral complex. Examples: [Ni(CO)4], [Fe(CO)5], [Co(NH3)3(Cl3)].

Classification Based on Kind of Ligands:

A coordination compound in which

(i) The central metal ion/atom is coordinated to only one kind of ligands is called a homoleptic complex.
Examples: [Co(NH3)6]3+, [Fe(H2O)6]2+.

(ii) The central metal ion/atom is coordinated to more than one kind of ligands is called a heteroleptic complex. Example, [Co(NH3)5Cl]2+, [Pt(NH3)2Cl2)].