Difference Between Crystal Field Theory and Ligand Field Theory     

Key Difference – Crystal Field Theory vs Ligand Field Theory
 

Crystal field theory and ligand field theory are two theories in inorganic chemistry that are used to describe the bonding patterns in transition metal complexes. Crystal field theory (CFT) considers the effect of a perturbation of electron containing d-orbitals and their interaction with the metal cation and, in CFT, the metal-ligand interaction is considered as electrostatic only. Ligand Field Theory (LFT) considers the metal-ligand interaction as a covalent bonding interaction and depends on the orientation and the overlap between the d-orbitals on the metals and the ligand. This is the key difference between crystal field theory and Ligand field theory.

What is Crystal Field Theory?

Crystal Field Theory (CFT) was proposed by the physicist Hans Bethe in 1929, and then some alterations were proposed by J. H. Van Vleck in 1935. This theory describes some important properties of transition metal complexes such as magnetism, absorption spectra, oxidation states, and coordination. CFT basically considers the interaction of d-orbitals of a central atom with ligands and these ligands are considered as point charges. In addition, the attraction between the central metal and ligands in a transition metal complex is considered as purely electrostatic.

What is Ligand Field Theory?

Ligand field theory provides a more detailed description of bonding in coordination compounds. This considers the bonding between the metal and the ligand according to the concepts in the coordination chemistry. This bond is considered as a coordinated covalent bond or a dative covalent bond to show that both electrons in the bond have come from the ligand. The basic principles of the crystal field theory are closely similar to those in the molecular orbital theory.

What is the difference between Crystal Field Theory and Ligand Field Theory?

Basic Concepts:

Crystal Field Theory: According to this theory, the interaction between a transition metal and ligands is due to the attraction between the negative charge on the non-bonding electrons of the ligand and the positively charged metal cation. In other words, the interaction between the metal and the ligands are purely electrostatic.

Ligand Field Theory:

• One or more orbitals on the ligand overlap with one or more atomic orbitals on the metal.
• If the orbitals of the metal and ligand have similar energies and compatible symmetries, a net interaction exists.
• The net interaction results in a new set of orbitals, one bonding and the other anti-bonding in nature. (An * indicates an orbital is anti-bonding.)
• When there is no net interaction; the original atomic and molecular orbitals are not affected, and they are nonbonding in nature as regards the metal-ligand interaction.
• Bonding and anti-bonding orbitals have sigma (σ) or pi (π) character, depending on the orientation of the metal and the ligand.

Limitations:

Crystal Field Theory: Crystal field theory has several limitations. It takes into account only the d-orbitals of the central atom; the s and p orbitals are not considered.  In addition, this theory fails to explain the reasons for large splitting and the small splitting of some ligands.

Ligand Field Theory: Ligand field theory does not have such limitations as in the crystal field theory. It can be considered as the extended version of the crystal field theory.

Applications:

Crystal Field Theory: Crystal Field Theory provides valuable insights into the electronic structure of transition metals in crystal lattices,

Crystal field theory explains the breaking of orbital degeneracy in transition metal complexes due to the presence of ligands. It also describes the strength of the metal-ligand bonds. The energy of the system is altered based on the strength of the metal-ligand bonds, which may lead to a change in magnetic properties as well as color.

Ligand Field Theory: This theory is concerned with the origins and consequences of metal– ligand interactions to elucidate the magnetic, optical, and chemical properties of these compounds.