Stability of Complexes and CFSE | JEE Important Topic

There are two ways to view the stability of coordination complexes. Thermodynamic stability and kinetic stability are the first two. The free energy change (G) of a process leading to the development of a coordination complex is referred to as its thermodynamic stability. The ligand substitution is referred to as the kinetic stability of a coordination complex. Complexes can occasionally experience a fast ligand replacement and are known as labile complexes.

However, certain compounds only slowly (or never) undergo ligand substitution and are known as inert complexes. If a metal complex doesn't react with water, which would lower the system's free energy and indicate thermodynamic stability, it is considered to be stable. On the other hand, if the complex combines with water to create a stable product, it is considered to have kinetic stability.

Factors Affecting Stability of Complexes

Stability of complexes mainly depends on two factors:

1. Central Metal ion 

2. Nature of ligands

Central Metal Ion 

Any change in the nature of central metal ion affects stability of complex

Size of Metal Ion: As the size of the metal cations decreases, so does the stability of the coordination complexes. Hence, as we go along the 3d series, the size decreases, increasing the stability of metal complexes. Ba²⁺<Sr²⁺<Ca²⁺<Mg²⁺<Mn²⁺<Fe²⁺<Co²⁺<Ni²⁺<Cu²⁺  

Charge of Metal Ion: A smaller, more strongly charged ion enables ligands to approach more quickly and closely while also having a stronger attraction. Higher oxidation states in metal cations produce more stable complexes than lower oxidation states, when combined with ligands like NH₃, H₂O, etc.

Nature of Ligands

Basic Nature of Ligands: The basic nature is the tendency to donate electron pairs to metal. The greater is the basic nature of ligand, the more easily it can donate its lone pair of electrons to the central metal ion. As basic nature increases, the stability of complexes increases.

Thus, F^- should form more stable complexes than Cl^-, Br^- and I^-. Similarly, ammonia should form more stable complexes than H²O and HF.

Steric Hindrance of Ligands: When a bulky group is attached to the metal atom, it causes repulsions between the molecules, and the stability of the compound decreases.

For example: 2-methyl-8-hydroxy quinoline is less stable than 8-hydroxyquinoline.

Chelate Effect of Ligands

The chelation effect states that complexes formed when metal ions are coordinated with chelating ligands are thermodynamically much more stable than complexes formed when metal ions are coordinated with non-chelating ligands. Chelating ligands, also known as multidentate ligands, are compounds that can form several bonds with a single metal ion. Oxalate and ethylenediamine are two basic examples. $[Co(en)_3]^{3+}$ is more stable than $[Co(NH_3)_6]^{3+}$

Macrocyclic Effect 

A macrocyclic ligand is a cyclic compound with three or more potential donor atoms that can coordinate to the metal ion and nine or more atoms in the cyclic structure. It has been found that complexes coordinated to open-ended multidentate chelating ligands are less stable than complex compounds coordinated to macrocyclic ligands of the proper size.

Crystal Field Stabilization Energy

The five d-orbitals of a single transition metal ion have the same energy . Based on the geometric structure of the molecule, some ligands encounter more d-orbital electron repulsion when they approach the metal ion than others. Not all d-orbitals directly interact because ligands arrive from various angles. The electrostatic environment, however, causes a splitting as a result of these interactions. Take an octahedral-shaped molecule as an illustration. Along the x, y, and z axes, ligands approach the metal ion. Since these orbitals are located along these axes, the electrons in the dz² and dx²y² orbitals are more attracted to one another. These orbitals demand more energy to keep an electron in them than other orbitals. Hence, the orbitals split according to their energies in the presence of ligands. This phenomenon of splitting of orbitals is known as crystal field splitting as a consequence of which energy is released. The energy released is crystal field stabilization energy.

Crystal Field Splitting in Octahedral Complexes

Factors Affecting CFSE

Nature of Metal Ion: When the number of d-electrons in the central metal cation increases, CFSE decreases for complexes with the same geometry and ligands but varying numbers of d-electrons. When the number of d-electrons is the same, CFSE increases as the oxidation state increases. CFSE increases with increasing principal quantum 3d < 4d < 5d when the  ligand, oxidation state and d-electrons are the same.

Nature of Ligands: The magnitude of CFSE varies from stronger ligand to weaker ligand, which indicates that CFSE increases with stronger ligand and decreases with weaker ligand.

For example: Fluoride being a weak ligand has CFSE as compared to cyanide ligand which is a strong field ligand.

Geometry of Complex: Octahedral complexes have higher CFSE than tetrahedral complexes.

$\Delta \mathrm{t}=\dfrac{4}{9} \Delta \mathrm{o}$

Conclusion

There are two ways to view the stability of coordination complexes. Thermodynamic stability and kinetic stability are the first two. The free energy change (G) of a process leading to the development of a coordination complex is referred to as its thermodynamic stability. The ligand substitution is referred to as the kinetic stability of a coordination complex. We also discussed the factors affecting stability of coordination compounds. The nature of metal ion and ligand both affects the stability of complexes as well as crystal field splitting energy. Moreover, complexes can occasionally experience a fast ligand replacement. These complexes are known as labile complexes. Compounds which slowly (or never) undergo ligand substitution, these complexes are known as inert complexes.