[Chemistry Class Notes] on Coordination Number Pdf for Exam

In chemistry, crystallography, and material sciences, the transitional elements/metals (primarily the d-block elements) give rise to a large number of complex compounds in which the metal atoms are bound to several anions (negatively charged ions)  or neutral molecules. Such compounds, by the modern definition, are called coordination compounds. One of the basic terms used in Coordination Compound Chemistry is the coordination number.

 

The coordination number (CN) of a metal ion/atom in a given complex may be defined as the number of ligand donor atoms to which the metal is bonded directly or a central atom or ion holds as its nearest neighbours in a complex or coordination compound or in a crystal form. Ligands are the ions (charged) or molecules (neutral) bound to the central atom/ion in the coordination compound.

Coordination Number Examples

As discussed earlier, the coordination number of a crystalline solid is the number of atoms, ions, or molecules that a central atom/ion holds in the crystalline solid or the coordination compound as its closest neighbours.  Our findings are that magnesium has a firm coordination number of 6 with an overwhelming preference for water or other oxygen-containing ligands. 

 

For example, in the complex ions, [PtCl6]2– and [Fe(H2O)6]2+, the coordination number of Pt and Fe are 6 and 6, respectively. Here Pt and Fe have connected to six monodentate ligands Cl– and H2O, respectively. 

 

Taking another example, [Cr(NH3)2Cl2Br2]–. Here the central atom Cr has coordination number 6 again because the total number of atoms/ions/molecules bonded to Cr is found to be 6. In the complex ion [Co(en)3 ]3+, the coordination number Co is 6 because en (ethylenediamine) are bidentate ligands.

In some compounds, the metal-ligand bonds may not all be at the same distance. In some other cases, a different definition of coordination number is used that includes atoms at a greater distance than the nearest neighbours. Some metals have irregular structures. Many chemical compounds have distorted structures. Unlike sodium chloride, where the chloride ions are cubic close-packed, the arsenic anions are hexagonal close-packed. 

 

Coordination Number of A Central Atom

In a coordination compound, the ligands are attached to the central metal atom/ion through coordinate bonds. Therefore to calculate the coordination of the central metal atom/ion, we need to calculate the total number of coordinate bonds made by all the ligands with the metal atom/ion. 

 

Let us take an example:

 

The compound [Pt(NH3)2Cl2]

 

 

Here, it is very clear that Pt carries two coordinate bonds with NH3 and two with Cl, respectively. The total number of coordinate bonds made by all the ligands stands as 4. Hence, the coordination number (CN) of the compound [Pt(NH3)2Cl2] is 4. 

 

Geometry of Molecules Based on Coordination Number

The coordination number of the central metal atom/ion may be used to deduce the molecular geometry of the coordination compound. The geometrical pattern can be described as a polyhedron where the vertices of the polyhedron are the centers of the coordinating atoms in the ligands.

 

Based on the coordination number (CN), the molecular geometry of the coordination compounds are given as follows:

 

 

Some examples include:

• [Ag(NH3)2]+ where Ag has coordination number 2 and the molecular geometry of the compound is linear.

• [NiCl4]2− where Ni has coordination number 4 and the molecular geometry of the compound is square planar.

• [CoCl6]3− where Co has coordination number 6 and the molecular geometry of the compound is octahedral.

• [ZrF7]3− where Zr has coordination number 7 and the molecular geometry of the compound is pentagonal bipyramid.

• [CoCl5]2− where Co has coordination number 5 and the molecular geometry of the compound is trigonal bipyramidal.