The key difference between inert and labile complexes is that inert complexes undergo slow substitution, whereas labile complexes undergo rapid substitution.
The terms inert complex and labile complex come under the category of transition metal complexes. A transition metal complex is an inorganic compound which has a transition metal atom or ion in the center of the complex, and there are two or more ligands attached to this metal center. We can classify these complexes into two groups as inert complexes and labile complexes, depending on the substitution reactions that they undergo.
CONTENTS
1. Overview and Key Difference
2. What are Inert Complexes
3. What are Labile Complexes
4. Side by Side Comparison – Inert vs Labile Complexes in Tabular Form
5. Summary
What are Inert Complexes?
Inert complexes are transition metal complexes that can undergo substitution reactions very slowly. Sometimes, these complexes do not undergo any substitution reaction. The inert complexes are “inert” because they have large activation energy that can prevent ligands from undergoing any substitution reaction. Therefore, inert complexes are kinetically stable compounds.
Figure 01: Hexaamminecobalt(III) Chloride
For example, cobalt(III) hexaammonium complex contains a central cobalt ion (+3 charged ion) attached to six ammonium ligands. When this complex reacts with hydronium ions, it can form a cobalt(III)hexaaqua complex. The equilibrium constant for this substitution reaction is about 1064. This large equilibrium constant indicates that the ammonium complex of cobalt is unstable than the aqua complex. Therefore, this substitution reaction is thermodynamically highly favored, but the rate of the reaction is very low due to the large activation energy barrier. This indicates that the ammonium complex of cobalt ion is an inert complex.
What are Labile Complexes?
Labile complexes are transition metal complexes that can undergo substitution reactions rapidly. In other words, labile complexes readily undergo substitution reactions when there is a suitable ligand for the substitution. These complexes undergo rapid substitution because they have a very low activation energy barrier. Therefore, these labile complexes are kinetically unstable compounds.
For example, cobalt(II) hexaammonium complex contains a central cobalt ion (with +2 electrical charge) attached to six ammonium ligands. When this complex reacts with hydronium ions, a substitution reactions occur. This reaction completes in a few seconds. This is because the hexaammonium complex of cobalt (II) is thermodynamically unstable and labile.
What is the Difference Between Inert and Labile Complexes?
There are two types of transition metal complexes as inert complexes and labile complexes. The key difference between inert and labile complexes is that inert complexes undergo slow substitution, whereas labile complexes undergo rapid substitution. This is mainly because inert complexes are thermodynamically stable complexes with a large activation energy barrier. Labile complexes, on the other hand, are thermodynamically unstable, and they have a very small activation energy barrier.
For example, cobalt(III) hexaammonium complex is an inert complex that can undergo substitution reaction with hydronium ions which takes weeks to complete. The same reaction happens in a few seconds when we use cobalt (II) hexaammonium complex, so it is a labile complex.
The following infographic summarizes the differences between inert and labile complexes in tabular form.
Summary – Inert vs Labile Complexes
Transition metal complexes are inorganic compounds containing a central metal atom or ion attached to several ligands. These complexes are in two groups as inert complexes and labile complexes. The key difference between inert and labile complexes is that inert complexes undergo slow substitution, whereas labile complexes undergo rapid substitution.
Reference:
1. “24.10: Some Kinetic Considerations.” Chemistry LibreTexts, Libretexts, 14 July 2020, Available here.
2. Anne Marie Helmenstine. “What Is a Labile Complex in Chemistry?” ThoughtCo, Available here.
Image Courtesy:
1. “CoA6Cl3” By Smokefoot – Own work (CC BY-SA 4.0) via Commons Wikimedia