Preface to the title at left, I have tried to give an introduction to that field of chemistry which deals with the spectral and magnetic features of inorganic complexes. It has been my intention not to follow the theory in all its manifestations but merely to describe the basic ideas and applications. This has been done with an eye constantly aimed at the practical and experimental features of the chemistry of the complex ions. The book is thus primarily intended for the inorganic chemist, but it is true that, in order to follow the exposition, a course in basic quantum mechanics is needed. Simple examples are nearly always used to illustrate the arguments, but the quoted experimental evidence must of necessity be limited.
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History Edit Ligand field theory resulted from combining the principles laid out in molecular orbital theory and crystal field theory , which describes the loss of degeneracy of metal d orbitals in transition metal complexes. John Stanley Griffith and Leslie Orgel  championed ligand field theory as a more accurate description of such complexes, although the theory originated in the s with the work on magnetism of John Hasbrouck Van Vleck.
Griffith and Orgel used the electrostatic principles established in crystal field theory to describe transition metal ions in solution and used molecular orbital theory to explain the differences in metal-ligand interactions, thereby explaining such observations as crystal field stabilization and visible spectra of transition metal complexes. In their paper, they proposed that the chief cause of color differences in transition metal complexes in solution is the incomplete d orbital subshells.
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 dxy, dxz and dyz orbitals remain non-bonding orbitals. In molecular symmetry terms, the six lone-pair orbitals from the ligands one from each ligand form six symmetry adapted linear combinations SALCs of orbitals, also sometimes called ligand group orbitals LGOs. The irreducible representations that these span are a1g, t1u and eg. These orbitals are close in energy to the dxy, dxz and dyz orbitals, with which they combine to form bonding orbitals i.
It is filled with electrons from the metal d-orbitals, however, becoming the HOMO highest occupied molecular orbital of the complex. The greater stabilization that results from metal-to-ligand bonding is caused by the donation of negative charge away from the metal ion, towards the ligands. The symmetry adapted linear combinations of these fall into four triply degenerate irreducible representations, one of which is of t2g symmetry.
High and low spin and the spectrochemical series Edit See also: Magnetochemistry The six bonding molecular orbitals that are formed are "filled" with the electrons from the ligands, and electrons from the d-orbitals of the metal ion occupy the non-bonding and, in some cases, anti-bonding MOs. In complexes of metals with these d-electron configurations, the non-bonding and anti-bonding molecular orbitals can be filled in two ways: one in which as many electrons as possible are put in the non-bonding orbitals before filling the anti-bonding orbitals, and one in which as many unpaired electrons as possible are put in.
The former case is called low-spin, while the latter is called high-spin.
Ligand field theory
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