The key difference between coordination number and oxidation number is that coordination number is the number of ligands attached to the metal centre of a coordination compound, whereas oxidation number is the charge of the central atom if we remove all the ligands and electron pairs around it.
We encounter the two terms, coordination number and oxidation number, in the field of coordination chemistry. In coordination chemistry, we are dealing with complex compounds containing a metal centre surrounded by chemical species called ligands.
CONTENTS
1. Overview and Key Difference
2. What is Coordination Number
3. What is Oxidation Number
4. Side by Side Comparison – Coordination Number vs Oxidation Number in Tabular Form
5. Summary
What is Coordination Number?
Coordination number is the number of ligands in a coordination complex that are attached to the metal centre. Therefore, we sometimes call it as ligancy. These ligands can be atoms, ions or molecules. Usually, we can observe coordination numbers from 2 to 9 in coordination compounds, but higher coordination numbers are very rare.
Figure 01: A Coordination Compound having Coordination Number 6
However, this number does not involve the number of electron pairs around the metal centre. For, example, [Mo(CN)8]4- is a coordination complex having Molybdenum as the metal centre and the coordination number is 8 because there are eight ligands attached to the metal centre.
What is Oxidation Number?
The oxidation number is the charge the central atom would have if all ligands and electron pairs shared with the atom were removed. Moreover, this number is for transition metals because the coordination complexes form via binding ligands with transition metals. Transition metals are the chemical elements in group 3 to 12 in d block of the periodic table. Typically, they can form several oxidation states due to the low reactivity of unpaired d electrons. For example, the oxidation number of iron (Fe) in the coordination complex [Fe(CN)6]3- is +3. We can obtain this value by removing all the ligands (CN ligands) with their charge.
Overall charge is -3
Remove -1 x 6 (because there are six ligands and a Cn ligand has -1 charge)
Then, oxidation number of Fe = -3 – (-6) = +3
What is the Difference Between Coordination Number and Oxidation Number?
The key difference between coordination number and oxidation number is that coordination number is the number of ligands attached to the metal centre of a coordination compound whereas oxidation number is the charge of the central atom if we remove all the ligands and electron pairs around it. Further, we give the coordination number for a whole coordination compound, but the oxidation number is for the metal centre. For example, the coordination number of the [Mo(CN)8]4- complex is 8 while the oxidation number of iron (Fe) in the coordination complex [Fe(CN)6]3- is +3.
The below infographic summarizes the difference between coordination number and oxidation number.
Summary – Coordination Number vs Oxidation Number
Coordination number and oxidation number are two different terms in coordination chemistry. Coordination number is the number of ligands the metal centre of a coordination compound holds. In contrast, the oxidation number is the charge of the central atom if we remove all the ligands and electron pairs around it. So, this is the key difference between coordination number and oxidation number.
Reference:
1. Helmenstine, Todd. “The Difference Between Oxidation State and Oxidation Number.” ThoughtCo, Jul. 3, 2019, Available here.
Image Courtesy:
1. “Trans-dichlorotetraamminecobalt(III)” By Benjah-bmm27 – Own work (Public Domain) via Commons Wikimedia