Site selectivity in reactions of metal hydride halide complexes with acids

Protonolysis, protonation and hydrogen bond-formation reactions with metal hydride halide (LnMHX) complexes are reviewed. The site selectivity (hydride vs halide) for these reactions is found to be highly variable, with many examples of either case. In fact, several systems are in equilibrium between protonated (or hydrogen-bended) hydride and halide ligands. The more reactive site depends primarily on the accessibility of the hydride ligands and the polarity of the metal hydride bonds. Protonation at hydride is a common synthetic method for metal halide dihydrogen complexes, M(H-2)X, which are rather well represented. In contrast, halide protonation gives metal hydride (hydrogen halide) complexes, MH(XH), for which only a few examples are established. However, H...X bonding in M(H-2)X complexes, "invisible" to NMR techniques, has generally not been ruled out. The lighter halides form stronger hydrogen bonds, and are more prone to protonation than the heavier halides. Thus, fluoride is the best halide ligand for electrophilic removal, while iodide ligands are best suited to withstand acidic conditions. (C) 1997 Elsevier Science S.A.