Classification of Ligands As L and X
Especially in the area of organometallic chemistry, ligands are classified as L and X (or combinations of the two). The classification scheme - the "CBC Method" for Covalent Bond Classification - was popularized by M.L.H. Green and "is based on the notion that there are three basic types ... represented by the symbols L, X, and Z, which correspond respectively to 2-electron, 1-electron and 0-electron neutral ligands." L ligands are derived from charge-neutral precursors and are represented by amines, phosphines, CO, N2, and alkenes. X ligands typically are derived from anionic precursors such as chloride but includes ligands where salts of anion do not really exist such as hydride and alkyl. Thus, the complex IrCl(CO)(PPh3)2 is classified as an MXL3 complex, since CO and the two PPh3 ligands are classified as L's. The oxidative addition of H2 to IrCl(CO)(PPh3)2 gives an 18e- ML3X3 product, IrClH2(CO)(PPh3)2. EDTA4- is classified as an L2X4 ligand, as it features four anions and two neutral donor sites. Cp is classified as an L2X ligand.
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