New types of non-classical interligand interactions involving silicon based ligands

This review summarises recent findings and ideas about new non-classical interligand interactions in which at least one of the ligands has a silicon atom as an interacting centre. For many years the field of non-classical interactions was dominated by agostic and σ-complexes, bonding in which is often described in terms of electron-deficient 3c–2e interactions. In the case of silyl hydride compounds these early works resulted in the observation of η2-HSiR3 co-ordination mode. However, several recently reported systems can be rationalised as having more extended SiH bonding, and ligands such as (η3-H2SiR3) and even (η4-H3SiR3) can be identified. Co-ordination of these complex ligands to the metal is still electron deficient. Another series of new discovered interligand interactions include donations from basic MH (or MSi) bond orbital onto suitable antibonding orbital of a neighbouring ligand ((SiX)*, (CO)*, etc). Different types of these hypervalent interactions are discussed and their relevance to migratory insertion reactions is traced.