Crystal Field Theory.pdf - Crystal Field Theory(CFT is a...

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Crystal Field Theory ( CFT ) is a model that describes the breaking of degeneracies of electron orbital states, usually d or f orbitals, due to a static electric field produced by a surrounding charge distribution (anion neighbors). This theory has been used to describe various spectroscopies of transition metal coordination complexes, in particular optical spectra (colors). CFT successfully accounts for some magnetic properties, colours, hydration enthalpies, and spinel structures of transition metal complexes, but it does not attempt to describe bonding. CFT was developed by physicists Hans Bethe and John Hasbrouck van Vleck in the 1930s. CFT was subsequently combined with molecular orbital theory to form the more realistic and complex ligand field theory (LFT), which delivers insight into the process of chemical bonding in transition metal complexes. Overview of crystal field theory analysis According to Crystal Field Theory, the interaction between a transition metal and ligands arises from the attraction between the positively charged metal cation and negative charge on the non-bonding electrons of the ligand. The theory is developed by considering energy changes of the five degenerate d -orbitals upon being surrounded by an array of point charges consisting of the ligands. As a ligand approaches the metal ion, the electrons from the ligand will be closer to some of the d -orbitals and farther away from others, causing a loss of degeneracy. The electrons in the d -orbitals and those in the ligand repel each other due to repulsion between like charges. Thus 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 following factors: 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. The stronger the effect of the ligands then the greater the difference between the high and low energy d groups. The most common type of complex is octahedral; here six ligands form an octahedron around the metal ion. In octahedral symmetry the d -orbitals split into two sets with an energy difference, Δ oct (the crystal-field splitting parameter) where the d xy , d xz and d yz orbitals will be lower in energy than the d z 2 and d x 2 - y 2 , which will have higher energy, because the former group is farther from the ligands than the latter and therefore experience less repulsion. The three lower-energy orbitals are collectively referred to as t 2g , and the two higher-energy orbitals as e g . (These labels are based on the theory of molecular symmetry). Typical orbital energy diagrams are given below in the section High-spin and low-spin.
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Christopher Reinemann
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