Coordination Compound


A coordination compound contains a central metal atom or ion surrounded by number of oppositely charged ions or neutral molecules. These ions or molecules re bonded to the metal atom or ion by a coordinate bond.


They do not dissociate into simple ions when dissolved in water.

 Werner’s Theory of Coordination Compounds

Werner was able to explain the nature of bonding in complexes.

The postulates of Werner’s theory are:

a). Metal shows two different kinds of valencies: primary valence and secondary valence.

b). The ions/ groups bound by secondary linkages to the metal have characteristic spatial arrangements corresponding to different coordination numbers.

c). The most common geometrical shapes in coordination compounds are octahedral, square planar and tetrahedral.

Primary valence

·        This valence is normally ionisable.

·        It is equal to positive charge on central metal atom.

·        These valencies are satisfied by negatively charged ions.

·        Example: In , the primary valency is three. It is equal to the oxidation state of the central metal ion.

secondary valence

·        This valence is non – ionisable.

·        The secondary valency equals the number of ligand atoms coordinated to the metal. It is also called coordination number of the metal.

·        It is commonly satisfied by neutral and negatively charged, sometimes by positively charged ligands.

Important Terms of Coordination Compounds

Coordination entity

A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules.

Example: In – represents coordination entity.


A molecule, ion or group that is bonded to the metal atom or ion in a complex or coordination compound by a coordinate bond is called ligand.

It may be neutral, positively or negatively charged.

Examples: etc.

Coordination number

The coordination number (CN) of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded.

Example: In the complex  , the coordination number of Fe is 6.

Coordination sphere

The central atom/ion and the ligands attached to it are enclosed in square bracket and are collectively termed as the coordination sphere.

Example: In the complex is the coordination sphere.

Oxidation number of central atom

The oxidation number of the central atom in a complex is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom.

Homoleptic and heteroleptic complexes

Homoleptic complexes: Those complexes in which metal or ion is coordinate bonded to only one kind of donor atoms. For example:

Heteroleptic complexes: Those complexes in which metal or ion is coordinate bonded to more than one kind of donor atoms. For example:


The number of ligating (linking) atoms present in a ligand is called denticity.

Unidentate ligands:

The ligands whose only one donor atom is bonded to a metal atom are called unidentate ligands.


Didentate ligands:

The ligands which contain two donor atoms or ions through which they are bonded to the metal ion.

Examples: Ethylene diamine () has two nitrogen atoms, oxalate ion  has two oxygen atoms which can bind with the metal atom.

Polydentate ligand:

When several donor atoms are present in a single ligand, the ligand is called polydentate ligand.

Examples: In , the ligand is said to be polydentate and Ethylenediaminetetraacetate ion is an important hexadentate ligand. It can bind through two nitrogen and four oxygen atoms to a central metal ion.

Ambidentate ligand:

Ligands that can ligate (link) through two different atoms present in it are called ambidentate ligands.

Example: and SCN-. Here, NO2-can link through N as well as O while SCN- can link through S as well as N atom.

Nomenclature of Coordination Compounds

Nomenclature is important in Coordination Chemistry because of the need to have an unambiguous method of describing formulas and writing systematic names, particularly when dealing with isomers. The formulas and names adopted for coordination entities are based on the recommendations of the International Union of Pure and Applied Chemistry (IUPAC).

Formulas of Mononuclear Coordination Entities

The formula of a compound is a shorthand tool used to provide basic information about the constitution of the compound in a concise and convenient manner. Mononuclear coordination entities contain a single central metal atom.

The following rules are applied while writing the formulas:

·         Central atom is listed first.

·         Ligands are then listed in alphabetical order. The placement of a ligand in the list does not depend on its charge.

·         Polydentate ligands are also listed alphabetically. In case of abbreviated ligand, the first letter of the abbreviation is used to determine the position of the ligand in the alphabetical order.

·         The formula for the entire coordination entity, whether charged or not, is enclosed in square brackets. When ligands are polyatomic, their formulas are enclosed in parentheses. Ligand abbreviations are also enclosed in parentheses.

·         There should be no space between the ligands and the metal within a coordination sphere.

·         When the formula of a charged coordination entity is to be written without that of the counterion, the charge is indicated outside the square brackets as a right superscript with the number before the sign. For example, [Co(CN)6]3-, [Cr(H2O)6]3+, etc.

·         The charge of the cation(s) is balanced by the charge of the anion(s).

Naming of Mononuclear Coordination Compounds

The step to name a complex compound is:

·         Cations are always named before the anions.

·         The ligands are then listed in alphabetical order.

·         In case of polydentate ligandsligands are named alphabetically using a prefix di, tri, tetra, penta etc, to indicate the number of ligands of that type present.
‘ide’ are changed to ‘o’
‘ite’ are changed to ‘ito’
‘ate’ are changed to ‘ato’

·         Then the name the metal atom is written followed by its oxidation state in Roman numerals.






















·         Finally, the anion is named.

For example: [Cu(NH3)4]SO4 Tetra ammine copper (II) sulphate

If the ligands itself include di, tri etc. then we use bis – (for two), tris (for three) as prefix. For example:

If the complex ion is an anion, the name of the metal ends with the suffix – ate.

For example: [Cr(C2O4)3]3‒ Trioxalatochromate (III) ion.

·         If anion is a complex, then metal ends with ‘ate’

[Ni(CN)4]2- : tetracyanonickelate (II) ion






















  • ·         When writing (not naming) the formula of the complex:
  • ·         Complex-ion should be enclosed by square brackets.
  • ·         Ligands are placed after metal in alphabetical order but first negative ligands, then neutral, then positive.