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Since a transition metal methyl complex corresponds to a compound in which one hydrogen atom of methane is replaced by a transition metal (M), the M–C linkage is considered to be a σ-bond and covalent. To understand the properties and reactivity of organometallic complexes, it is important to understand the character of M–C σ-bonds in transition metal alkyl complexes. The polarization of the bond and the effect of other C substituents on the strength of M–C bonds are explained. Taking a metal–carbonyl (CO) bond and a metal–olefin bond as examples, the σ-donation of electrons from a ligand to a metal and the π-back donation from a metal to a ligand are described. In addition, the Dewar–Chatt–Duncanson model is also explained. It is necessary to understand the application of molecular orbital theory to transition metal complexes. Although crystal field theory explains in a clear and simple way how the transition metal d orbital energies split when ligands approach the metal to form a complex, and this is sufficient for understanding Werner type complexes, it is insufficient for understanding organometallic complexes. Ligand field theory therefore emerged, combining molecular orbital and crystal field theory concepts, while remaining relatively simple. Applying this theory, it is apparent why organometallic complexes follow the 18-electron rule. In this chapter, crystal field theory, and its successor, ligand field theory, are described.

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