Sigma bonds as ligand donor groups in transition metal complexes

Abstract

Covalent X-H bonds, particularly where X is H, C, B, and Si, can act as Lewis base ligands in forming metal complexes of general type LnM(H-X), where the M-H-X angle is strongly bent so that the coordination can best be considered as side-on. The binding is greatly enhanced by back donation from the metal into the X-H σ* orbital. This elongates and eventually breaks the X-H bond, leading to characteristic structural, physicochemical characteristics and alteration of the reactivity of the ligand. The requirement for back donation means that the appropriate LnM fragment must usually have appreciable π donor character. Since the binding of the X-H bond to the metal center is relatively weak, and the binding of the deprotonated X- group left behind is much stronger, the binding also facilitates proton loss from the X-H bond. This electron redistribution results in reactivity differences which may be exploited. The case of H2 is treated in most detail in this review, because of its central place in the field.