Complexes semi-coordinated ligands

Figure 4.45 A metal-ligand m,—orbital splitting diagram depicting interaction of the metal-atom d NAO and ligand nL NBO to form semi-localized NLMOs of the coordination complex, with splitting energy Aed. = < d/NLMO — fd> (NAO).

A similar H2 activation mechanism was proposed for the [Pd(NN S)Cl] complexes (5 in Scheme 4.5) in the semi-hydrogenation of phenylacetylene [45] after formation of the hydride 14 (Scheme 4.9), coordination of the alkyne occurs by displacement of the chloride ligand from Pd (15). The observed chemos-electivity (up to 96% to styrene) was indeed ascribed to the chloride anion, which can be removed from the coordination sphere by phenylacetylene, but not by the poorer coordinating styrene. This was substantiated by the lower che-moselectivities observed in the presence of halogen scavengers, or in the hydrogenations catalyzed by acetate complexes of formula [Pd(NN S)(OAc)]. Here, the acetate anion can be easily removed by either phenylacetylene or styrene. 

---