Dihydrogen can bind to metals in a nonclassical form, in which the H-H bond is not broken. This provides a new way of activating H₂ even in d° complexes. Lone pair donors, such as NH₃, readily coordinate to metal ions. Pi-bond donors, such as ethylene also form coordination compounds. This recognition led to the development of organometallic chemistry. By the 1970‘s, the first species were found in which X-H bonds (X = C, Si) interact with metal ions by donation of the X-H σ-electrons to the metal (Brookhart et al., 1983). In the C-H case, the extra stabilizing effect of chelation is required for the complex to be isolable, but Kubas (Kubas, 1988) showed that H₂ can form a complex [W(H₂)(CO)₃-(PCy₃)₂] (1) in which H₂ binding is sufficiently strong to an give isolable species. Early reports (Dong et al., 1981) that silanes could bind as σ-bond complexes have more recently led to detailed study (Schubert et al., 1987) including confirmation of the binding mode by neutron diffraction in [CpMn(CO)₂(H-SiR₃)].