Organometallic compounds of the transition elements. Classification of ligands and theories of bonding

Abstract

While organometallic compounds of the Main Group elements are most conveniently classified from the position of the metal in the Periodic Table, it is useful to classify d-block transition metal complexes from the organic ligands which they contain. In Table 6.1 ligands are listed according to the number of electrons which they formally contribute to the metal—ligand bonding. While there is usually no doubt as to the total number of valence electrons in a complex, confusion in counting electrons can arise if attempts are made to take into account any polarization of the metal—ligand bonds, by assigning an oxidation state to the metal, for example. A Metal—hydrogen bond can be thought of as arising from M⁻ and H⁺, M^. or M⁺ and H⁻. In all cases the same essentially covalent bond results, which contains the same number of electrons (two), independent of the final polarization M⁺—H⁻ or M⁻—H⁺. It is therefore recommended that in counting electrons, all metal—ligand bonds are treated as covalent. Thus an M—H bond is considered as arising from a metal atom M^. and the radical H^., so that H^. is formally a one electron donor. Metal alkyls and less conventionally metal halides are treatedsimilarly. A metal cyclopentadienyl bond M—C₅H₅ is also considered as arising from M ^. and cyclopentadienyl radical C₅ ^.₅ rather than from M⁺ and C₅H ⁻₅ , making C₅H₅ formally a five electron ligand.