Crystal and Ligand Field Theories
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The magnitude of the ligand field splitting parameter in the octahedral field can be determined from the frequency of maximum absorption in the optical absorption spectrum. This absorption arises from an electronic transition from the t2g level to the eg level. This is the most important form of electronic transition in the transition metal complexes, but others are also observed, and these transitions are generally observed in the visible and ultraviolet regions.
The effects of the application of a field to a metal ion, in the formation of a complex, whether examined through the Crystal Field Theory of the Ligand Field Theory, have so far been limited to octahedral complexes. This is because it is easy to visualize the orientation of ligands and orbitals in the octahedral symmetry environment, and the points at the vertices of the octahedron lie on the cartesian axes.
The enthalpies of hydration of d-metal complexes. The trend in observed hydration enthalpies of the hexa-aquo transition metal complexes of the first row transition metals can be explained in terms of the ligand field stabilization energy.
Crystal Field Theory describes the interactions between the ligands and the metal ion orbitals on an electrostatic basis, and examines the effect the ligands have on the metal center as a result.
The value of the ligand field splitting parameter, ie. the amount by which the degeneracy of the d-orbitals is disturbed by the effect of the electrostatic field generated by the ligands, depends upon the identity of the ligands.
When a metal is subjected to the perturbation of an octahedral field, the energies of the d-orbitals split into two groups, the lower energy t2g, at -0.4Δo, and the higher energy eg, at 0.6Δo, where Δo is the ligand field splitting parameter.
Ligand and Crystal Field theories are used to describe the nature of the bonding in transition metal complexes. Crystal Field Theory is based upon the effect of a perturbation of the d-orbitals consisting of electronic interaction between the metal cation nucleus and the negatively charged electrons of the ligands: the metal-ligand interactions are electrostatic only. Ligand Field Theory treats the metal-ligand interaction as a covalent bonding interaction, and depends upon considering the overlap between the d-orbitals on the metals and the ligand donor orbitals.