Meeting More Metals - Bridging Ligands

We have identified ammonia (NH3) as the classical example of a monodentate ligand, with only a single lone pair. Flowever, as already mentioned, if ammonia is stripped of a proton to form the anion NH2, it now offers two lone pairs. Although accessible in aqueous solution only at high pH, this ion is a strong base and is known for its capacity to bind 

Ammonia coordinates as a monodentate ligand to one metal. When a proton is removed it exhibits the capacity to attach the resultant additional lone pair to a second metal ion in a bridging mode. 

Deprotonation of water enhances the prospect of bridging between two metal ions by increasing electron density on the O atom. Successive deprotonation can permit multiple bridging to metal ions, resulting in small metal-oxide clusters. 

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Cyclohexane formation is entropically less favorable than cyclopentane generation, and treatment of 1,6-dienes under the conditions listed in Eq. (47) leads to the production of uncyclized, disilylated products (Eq.49) [35] or silicon bridged dimers [40]. To avoid these problems, phenylmethylsilane can be employed as the chain terminator. Utilizing this more hindered silane slows the o-bond metathesis sufficiently to prevent dimerization (Eq. 50). The trapping step can be retarded even further by the use of diphenylsilane (Eq. 51). Thus, not only can the metal and the ligand array be manipulated to bring about the desired result in the catalytic process, but the properties of the silane reagent itself can also be adjusted to meet the demands of the synthesis at hand. 

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