Developed in the first half of the 20th century to describe the electron systems of transition-metal complexes (a phenomenon within the branch of inorganic chemistry), ligand field theory is an extension of crystal field theory. Thus, it is also known as "adjusted crystal field theory." Ligand field theory and its predecessors represented huge advances in scientists' understanding of the structure, bonding, and behavior of transition metal complexes. Chemists use ligand field theory to interpret the spectra (colors) of transition metals and to understand how they react with other substances.
Somewhat similar to Linus Pauling's valence-bond model, ligand field theory is a subset of molecular orbit theory. It is more accurate and more powerful than its predecessor (crystal field theory) for explaining the chemical properties of transition metal complexes. To understand ligand field theory, it is first necessary to know the basic precept of crystal field theory, since they are so closely related.
Physicist Hans Bethe introduced the idea in 1929 to explain the magnetic and spectral properties of inorganic materials. A way to analyze inorganic complexes' electron structures, crystal field theory assumes that an inorganic complex consists of a central metal atom surrounded by ionic ligands. It can be used to predict kinetic and chemical properties, spectral and magnetic characteristics, thermodynamic properties, and reaction mechanisms. It explains the properties of coordination complexes in terms of the electrostatic pull between the ligands and the metal ion and how this interaction changes the orbitals' energies (in this case the d orbitals).
The ligand field theory is different from crystal field theory because it takes into account the overlap of electrons as they orbit the central metal atom. In addition, ligand field theory is applied to transition-metal complexes because these employ covalent (electron sharing) bonds, rather than ionic (electron transferring) bonds to stick together. Crystal field theory cannot explain the covalent bonding that occurs in coordination complexes.
The ligand field theory is especially useful for explaining the spectroscopic, magnetic, and optical characteristics of ions in the transition- metal and rare-earth groups. Many transition metals have an incomplete d orbital, meaning there is a gap in the area where their electron is usually found. Examples of transition metals include vanadium, chromium, iron, cobalt, manganese, nickel, scandium, yttrium, zirconium, silver, and gold.
The ligand field theory's main concern is the effects of ligands on the energy levels of the central atom or ion. Ligands are the ions, molecules, or atoms that surround a central ion in a complex. They produce an electrical field that influences the electron system of that central ion, and are usually the source of donated electrons within a coordination complex. The ligand field's strength and symmetry determine the optical, stability, and magnetic characteristics of a transition-metal complex.
Ligand field theory, like crystal field theory, concentrates on what happens when ligands split the central metal atom's inner orbitals. For instance, the size of the split in a complex's crystal field indicates what frequency of light it will absorb, and therefore, what color it will be.
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