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Important Terms pertaining to Coordination Compounds

Last Updated : 16 Feb, 2022
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A coordination compound is made up of a central metal atom or ion that is surrounded by a number of oppositely charged ions or neutral molecules. All the ions or molecules in coordination compounds are connected with coordinate bonds to the central metal atom. When coordination compounds are dissolved into water, they do not dissociate into simple ions which means in water it does not lose its identity. A coordination compound is also called a complex compound. Coordination compound is very important in our day to day life. It is important to recognize that chlorophyll (Mg – complex) in plants, hemoglobin (Fe- complex) in human blood and Vitamin B12 (Co -complex) are also complex compounds.  

Definitions of Related Terms in Coordination Compounds :

Coordination entity:

A coordination entity is made up of a central metal atom or ion that is bonded to a set number of ions or molecules. Here [CoCl3 (NH3)3] is a coordination entity that has three chloride ions and three ammonia molecules that are connected to the cobalt ion.

Some important examples are:

  • [Ni(CO)4],
  • [PtCl2 (NH3)2],
  • [Fe(CN)6]4–, and
  • [Co(NH3)6]3+


Central atom/ion:

The central atom is surrounded and coordinately bonded to one or more neutral molecules or ions in a specific geometrical arrangement and are connected with a coordinate bond. The central atoms/ions in the coordination entities [NiCl2 (H2O)4], [CoCl(NH3)5]2+, and [Fe(CN)6]3– are Ni2+, Co3+, and Fe3+, respectively.

Some important examples are:

  • K4[Fe(CN)6] – central atom/ion is : Fe+2
  • [Cu(NH3)4]SO4 – central atom/ion is : Cu+2


Ligands are simple ions or molecules which has a tendency to donate an electron and can form a coordinate bond with the central atom /ion. Ligands can be simple ions like Cl– and Br,  small molecules like H2O or NH3, larger molecules like H2NCH2CH2NH2 or N(CH2CH2NH2)3.

When a ligand is bound to a metal ion through a single donor atom then the ligand is said to be unidentate. Cl , H2O or NH3 are unidentate ligands. When a ligand is connected through two donor atoms, the ligand is said to be bidentate. The H2NCH2CH2NH2 (ethane-1,2-diamine) or C2O42– (oxalate) are bidentate ligands. When there are multiple donor atoms in a single ligand, such as N(CH2CH2NH2)3, the ligand is said to be polydentate. Ethylenediaminetetraacetate ion (EDTA4–) is a hexadentate ligand it is very important. It can connect through two nitrogen and four oxygen atoms to a central metal ion. 

Ambidentate ligand is a ligand that has two different donor atoms and can be either of the two ligands in the complex. The NO2– and SCN ions are two examples of such ligands. NO2– ion can coordinate to a central metal atom/ion via either nitrogen or oxygen. Similarly, the SCN ion can coordinate via the Sulphur or nitrogen atom.

Some important examples are:

  • Unidentate ligands:
    • Cl
    • NH3
  • Bidentate ligands:
    • H2NCH2CH2NH2
    • C2O42–

Coordination number:

A metal ion’s coordination number (CN) in a complex is defined as the number of ligand donor atoms to which the metal is directly bonded. For example, the coordination numbers of Pt and Ni in the complex ions [PtCl6]2– and [Ni(NH3)4]2+ are 6 and 4, respectively. Similarly, the coordination number of both Fe and Co in the complex ions [Fe(C2O4)3]3– and [Co(en)3]3+ is 6 because C2O42– and en (ethane-1,2-diamine) are bidentate ligands.

It should be noted that the coordination number of the central atom/ion is determined solely by the number of sigma bonds formed by the ligand with the central atom/ion. This calculation excludes pi bonds formed between the ligand and the central atom/ion..

Some important examples are:

  • K4[Fe(CN)6] – coordination number = 6
  • [Ni(CO)4]   – coordination number = 4

Coordination sphere:

The coordination sphere is made up of the central atom/ion and the ligands that are attached to it. The ionisable groups are denoted by a bracket and are referred to as counter ions. 

Some important examples are:

In the complex K4[Fe(CN)6]: the coordination sphere is [Fe(CN)6]4– and the counterion is K+.

Coordination polyhedron:

A coordination polyhedron is defined by the spatial arrangement of the ligand atoms that are directly attached to the central atom/ion. Octahedral, square planar, and tetrahedral polyhedra are the most common coordination polyhedra.

Some important examples are:

  • [Co(NH3)6]3+ is octahedral,
  • [Ni(CO)4] is tetrahedral, and
  • [PtCl4]2– is square planar.

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 of the ligands and electron pairs shared with the central atom were removed. The oxidation number is represented by a Roman numeral in parenthesis after the coordination entity’s name.

Some important examples are:

  • The oxidation number of copper in [Cu(CN)4]3– is +1 and is written as Cu (I).
  • The oxidation number of ferrous in K4[Fe(CN)6] is +2 and it is written as Fe(II).

Homoleptic and heteroleptic complexes:

Homoleptic complexes are those in which a metal is bound to only one type of donor group, such as [Co(NH3)6] 3+. Heteroleptic complexes are those in which a metal is bound to more than one type of donor group, such as [Co(NH3)4Cl2] +.

Some important examples are:

  • Homoleptic complexes:
    • K2[Zn(OH)4]
    • [Ni(CO)4 ]
  • Heteroleptic complexes:
    • [Co(NH3 )4 (H2O)Cl]Cl
    • [Pt(NH3 )2Cl(NO2 )]

Sample Questions

Question 1: Write the central atom/ion in the following compound-

  1. K2[Zn(OH)4]
  2. [Co(NH3 )4(H2O)Cl]Cl
  3. K3 [Al(C2O4 )3 ]
  4. [CoCl2 (en)2 ]+
  5. [Ni(CO)4 ]


  1. Central atom of K2[Zn(OH)4] is                      –          Zn
  2. Central atom of [Co(NH3 )4 (H2O)Cl]Cl2 is     –          Co
  3. Central atom of K3 [Al(C2O4 )3 ] is                 –          Al
  4. Central atom of [Pt(NH3 )2Cl(NO2 )]+ is        –           Pt
  5. Central atom of [Ni(CO)4 ] is                         –           Ni

Question 2:  Write the coordination number of the following coordinate compounds-

  1. K4[Fe(CN)6
  2. [Ni(CO)4]
  3. K3[Al(C2O4 )3]


  1. K4[Fe(CN)6]            –   coordination number =  6
  2. [Ni(CO)4 ]               –   coordination number = 4
  3. K3 [Al(C2O4 )3 ]    –   coordination number =  6

Question 3: What are ambidentate ligands give examples?


A molecule or an ion that has two donor atoms but only one of them forms a coordinate bond with the central metal atom is referred to as a ambidentate ligands.

Some important examples are:

  • CN
  • NC:
  • NO2-
  • ONO:

Question 4: Write all the ligands present in the following coordination compounds-

  1. [Pt(NH3)6]4+
  2. [Co(NH3)2Cl2(en)]+
  3. [CO(NH3)5NO2]2+
  4. [Cu(Br)4]2-


  1. Ligands in [Pt(NH3)6]4+ are : NH3
  2. Ligands in [Co(NH3)2Cl2(en)]+ are: NH3,  Cl,  en
  3. Ligands in [CO(NH3)5NO2]2+ are: NH3,  NO2-
  4. Ligands in [Cu(Br)4]2- are: Br

Question 5: Find the oxidation number of the metals in the following complex compounds-

  1. Co(H2O)(CN)(en)2]2+ 
  2. [CoBr2(en)2]+ 
  3. [PtCl4]2- 
  4. K3[Fe(CN)6
  5. [Cr(NH3)3CI3]


1. Co(H2O)(CN)(en)2]2+ 

         Oxidation number:   x + 0 + (-1) + 0 = +2

                                         x = +3

2. [CoBr2(en)2]+

         Oxidation number:    x + 2*(-1) + 0 = +1

                                          x = +3

3. [PtCl4]2-

         Oxidation number:  x + (-4) = -2

                                        x = +2

4. K3[Fe(CN)6]

         Oxidation number:   3*(+1) + x + 6*(-1) = 0

                                         x = +3

5. [Cr(NH3)3CI3]

         Oxidation number:   x + 0 + 3*(-1) = 0

                                         x = +3

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