Difference Between Crystal Field Theory And Ligand Field Theory

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Crystal Field Theory

The Crystal Field Theory (CFT) is a model for the bonding interaction between transition metals and ligands. It describes the effect of the attraction between the positive charge of the metal cation and negative charge on the non-bonding electrons of the ligand. When the ligands approach the central metal ion, the degeneracy of electronic orbital states, usually d or f orbitals, are broken due to the static electric field produced by a surrounding charge distribution. CFT successfully accounts for some magnetic properties, colors, and hydration energies of transition metal complexes, but it does not attempt to describe bonding.

The electrons in the d orbitals of the central metal ion and those in the ligand repel each other due to repulsion between like charges. Therefore, the d electrons closer to the ligands will have a higher energy than those further away, which results in the d orbitals splitting in energy. This splitting is affected by:

  • the nature of the metal ion
  • the metal’s oxidation state (a higher oxidation state leads to a larger splitting)
  • the arrangement of the ligands around the metal ion
  • the nature of the ligands surrounding the metal ion

What You Need To Know About Crystal Field Theory

  • The crystal field theory is a model for the bonding interaction between transition metals and ligands. It describes the effect of the attraction between the positive charge of the metal cation and negative charge on the non-bonding electrons of the ligand.
  • The crystal field theory was developed by the U.S physicist Hans Albrecht Bethe and is widely accepted theory than the valence bond theory.
  • When the ligands approach the central metal ion, the degeneracy of electronic orbital states, usually d or f orbitals, are broken due to the static electric field produced by a surrounding charge distribution.
  • Crystal field theory, successfully accounts for some magnetic properties, color, hydration enthalpies and spinel structures of the transition metal complexes, but it does not attempt to describe bonding.
  • Crystal field theory only describes electrostatic interactions between metal ions and ligands.
  • Crystal field theory is comparatively unrealistic. This theory fails to explain the reasons for large splitting and the small splitting of some ligands. It also takes into account only the d-orbitals of the central atom; the s and p orbitals are not considered.

Ligand Field Theory

Ligand field theory, in chemistry, one of several theories that describe the electronic structure of coordination or complex compounds, notably transition metal complexes, which consist of a central metal atom surrounded by a group of electron-rich atoms or molecules called ligands. The ligand field theory deals with the origins and consequences of metal– ligand interactions as a means of elucidating the magnetic, optical, and chemical properties of these compounds.

Attributed mainly to the works of the U.S. physicist J.H. Van Vleck, the ligand field theory evolved from the earlier crystal field theory, developed for crystalline solids by the U.S. physicist Hans Albrecht Bethe. Bethe’s theory considers the metal–ligand linkage as a purely ionic bond; i.e., the bond between two particles of opposite electrical charges. It further assumes that the electronic structure of the metal atom is altered by the electrical field generated by the surrounding negative charges (the ligand field). In particular, the effects of the ligand field on the five d orbitals of an inner electron shell of the central atom are considered. (The d orbitals are regions within an electron shell with certain preferred orientations in space; in transition metals these orbitals are only partly occupied by electrons.) In the isolated metal atom, the d orbitals are of the same energy state and have equal probabilities of being occupied by electrons. In the presence of the ligand field these orbitals may be split into two or more groups that differ slightly in energy; the manner and the extent of orbital splitting depend on the geometric arrangement of the ligands with respect to the orbitals and on the strength of the ligand field.

What You Need To Know About Ligand Field Theory

  • Ligand field theory is a modification or an extension of crystal field theory and molecular obital theory. It describes bonding in transition metal complexes and how the orbitals arrange to a more energetically favored geometry based on a ligands electron donating.
  • Ligand field theory represents an application of molecular orbital theory to transition metal complexes. A transition metal ion has nine valence atomic orbitals-consisting of five nd, one (n+1)s, and three (n+1)p orbitals.
  • In Ligand field theory, the various d orbitals are affected differently when surrounded by a field of neighboring ligands and are raised or lowered in energy based on the strength of their interaction with the ligands.
  • The Ligand Field Theory analysis is highly dependent on the geometry of the complex, but most explanations begin by describing octahedral complexes, where six ligands coordinate to the metal.
  • The ligand field theory was developed by John Hasbrouck Van Vleck and Hans Albrecht Bethe.
  • Ligand field theory describes both electrostatic interactions and covalent bonding between metal ions and ligands.
  • Ligand field theory is more realistic than crystal field theory. It delivers insights into the process of chemical bonding in transition metal complexes.

Difference Between Crystal Field Theory And Ligand Field Theory In Tabular Form

Basis Of Description Crystal Field Theory Ligand Field Theory
Definition The crystal field theory is a model for the bonding interaction between transition metals and ligands.   Ligand field theory is a modification or an extension of crystal field theory and molecular obital theory.
Function It describes the effect of the attraction between the positive charge of the metal cation and negative charge on the non-bonding electrons of the ligand.  It describes bonding in transition metal complexes and how the orbitals arrange to a more energetically favored geometry based on a ligands electron donating.
Development The crystal field theory was developed by the U.S physicist Hans Albrecht Bethe and is widely accepted theory than the valence bond theory.   The ligand field theory was developed by John Hasbrouck Van Vleck and Hans Albrecht Bethe.  
Nature Crystal field theory is comparatively unrealistic. Ligand field theory is more realistic than crystal field theory.
Description Crystal field theory only describes electrostatic interactions between metal ions and ligands.   Ligand field theory describes both electrostatic interactions and covalent bonding between metal ions and ligands.