Coordination Number : The Neighbours of The Central Atom

In order to understand coordination number, let us know about coordination compounds. We know from the study of transition metals that these metals because of their small size and high charges are capable of forming complex compounds, also known as coordination compounds. The availability of d-orbital also plays an important role in rendering these metals this characteristic complex forming property. Coordination compounds have found a significant application in many industries such as metallurgical processes; they are extensively used as catalysts, textile dyeing and many other industries. It is because of their importance in our lives that these compounds continue to be an important field of research.

The entities related to a complex compound have been given specific names, let’s discuss about some of them. The ions or the neutral molecules that surround the central atom in a coordination compound are known as ligands. The property of a coordination compound is determined by the type and number of ligands bonded to the central atom. Coordination number for a metal ion i.e. the central atom in a coordination compound is defined as the number of ligand donor atoms or molecules to which the central metal atom is directly bonded. Let us understand this through an example, suppose we have a complex compound [Fe(C2O4)3]3- .

Now in this compound the central atom is Fe and the ligand is C2O42-. C2O42- (oxalate) is a bidentate that means it bonds to the central atom through two bonds. In the complex compound [Fe(C2O4)3]3- , there are three molecules of C2O42-, each bonded to Fe through two sigma bonds. Hence the coordination number of Fe in this complex is 3×2=6.  It should be taken into note while calculating coordination number of the central metal atom that only the number of sigma bonds is to be considered. That is, the number of sigma bonds formed by the ligand with the central atom only and pi bonds are not counted while determining the coordination number.


The coordination number of a complex compound significantly affects the geometry of the compound. It also plays an important role in the properties of the complex such as magnetic properties, colour, etc. The study of coordination compounds is a vast subject; get in touch with our experts to explore the world of complex compound chemistry.

Practise This Question

At equilibrium, reaction A  B is at the stage of half completion. Which one of the followings is true?