Alkyl ligands form metal carbon σ bonds. Usually they occur in conjunction with other organic ligands or CO, but may be achieved on their own, as in tungsten hexamethyl (1), and in [Ti(CH2SiMe3)4] where the bulky groups are helpful in stabilizing the compound. Elements with H attached to β carbons (the nomenclature being M-Cα-Cβ-Cγ) tend to be unstable to β-hydride elimination of an alkene fragment, defined below. The surprising structure of (1), trigonal prismatic (D3h) rather than octahedral as collect in WCl6, has been attributed to the orientation of d orbitals accept for σ bonding. In an octahedral complex only two d orbitals may be involved, but four in the trigonal prismatic structure.
Alkylidyne and alkylidene ligands need metal-carbon π bonding in addition to σ.
Among the many reactions of organometallic elements, ones having elimination and insertion of ligands are important in applications to synthesis and catalysis. As like carbonyl insertion is:
in which a Mn-CH3 bond is changed by Mn-C(O)-CH3. The terminology is misleading as it is published by isotopic labeling that the incoming CO is not the one inserted. The first step is a opposite alkyl migration leading to a 16- electron intermediate which then picks up another CO molecule as show in 8.
Fig. Reaction steps involved in the catalytic Monsanto acetic acid process.