ABSTRACT 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)].
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