2004/09/21 | 晶体场理论[Crystal Field Theory]
类别(℃〖化学〗) | 评论(0) | 阅读(449) | 发表于 14:13
An ionic theory which is an offshoot of electrostatic theory. It ignores all covalent bonding effects. It was developed by Hans Bethe in 1929 by applying group theory and quantum mechanics to electrostatic theory. It was further developed by physicists during the 1930s and 1940s. It can be used to predict chemical properties, kinetic properties, reaction mechanisms, magnetic and spectral properties, and thermodynamic data. It cannot, however, be applied to sulfides, since sulfide forms mainly covalent bonds.
A splitting of energy levels ("crystal field splitting") occurs because the orientation of the d orbital wavefunctions will increase an electron's energy when the orbital is located in a region of high electron density, and lower it when the reverse is true. In crystals, the ,,, and orbitals split up as depicted below, depending on their cation's coordination. The total energy splitting is termed the crystal field stabilization energy. may be estimated from

where r is the radius of the d orbital and R is the metal-ligand internuclear distance. A large crystal field splitting energy is provided by ligands with high negative charge and small radius, and by metal cations with a large oxidation number.
Let denote the cubic splitting, the tetrahedral splitting, and the octahedral splitting. Then


Cubic splitting is illustrated above.

For octahedral splitting (Cotton 1990, p. 266), in order of increasing , some common species are



In the presence of octahedral crystal field splitting (with a configuration), there are two possible states for compounds: high spin () and low spin (). Depending on the system, high spin or low spin may be favored.

Tetrahedral splitting (Cotton 1990, p. 266) is illustrated above.

Hexagonal planar splitting is illustrated above.

Linear splitting is illustrated above.

Octahedral splitting with 2 short bonds is illustrated above.

Square planar splitting is illustrated above.

Tetragonal splitting is illustrated above.

The crystal field states corresponding to the Russell-Saunders states have been calculated by Bethe (Cotton 1990, p. 264). For other symmetry environments of , the crystal field states can be found in Appendix IIB of Cotton (1990, p. 437).

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