Acidity of H2 ligand

Hz in these examples, and other properties of H2 ligands. Most of the highly acidic complexes are found for Re and especially Os, although less stable Ru analogues exist in some cases.56,57,68... 

The higher kinetic acidity of H2 complexes requires that the reverse reaction, protonation of a metal hydride, occur at H rather than at M, for which there is ample evidence. Actually protonation at the hydride is misleading because it is really the M-H bond that is protonated to form M-j/2-H2, as pointed out in a review by Kuhlman that addresses site selectivity of protonation of hydride-halide complexes, MH(X).64 Formal protonation of a hydride ligand would give M-ff -H2, which is not known to be stable. Proton transfer to halide ligands is quite rare because an add with a lower than the coordinated HX produced is necessary. One example is protonation of trens-PtHX(P Bu3)2 with triflic acid, which gives trans-[Pffl( f2-H2)(P Bu3)2][OTf] for X = H and an unstable spedes claimed to be... 

Hydrogen-bonding interactions can exist between cis chloride and H2 ligands that demonstrate the acidity of H2 and facilitate HC1 elimination, 1 12( 2XP Pr3)2 has both intramolecular and intcrmolccular hydrogen bonding to form an infinite chain structure, as shown by neutron diffraction (see Chapter 5). Such systems can lead to catalytic reactivity, as will be discussed below. 

A species consisting of several ligands (the Lewis bases) that have an independent existence bonded to a single central metal atom or ion (the Lewis acid). Examples (1) H,N—BF, (2) [Fe(H2())J,l PtCl4. composite material A synthetic material composed of a polymer and one or more other substances that have been solidified together. 

Cofacial ruthenium and osmium bisporphyrins proved to be moderate catalysts (6-9 turnover h 1) for the reduction of proton at mercury pool in THF.17,18 Two mechanisms of H2 evolution have been proposed involving a dihydride or a dihydrogen complex. A wide range of reduction potentials (from —0.63 V to —1.24 V vs. SCE) has been obtained by varying the central metal and the carbon-based axial ligand. However, those catalysts with less negative reduction potentials needed the use of strong acids to carry out the catalysis. These catalysts appeared handicapped by slow reaction kinetics. 

Capture of Active Catalyst Using Solid Acidic Support with H2 Elution. The limit on the practical life of a catalyst solution may be determined by several factors including the presence of poisons or inhibitors, the buildup of less soluble materials such as the oxidation products of organophosphorus ligands, or an increase in the concentration of heavy aldehyde condensation products in the catalyst solution. In the latter case, there may be substantial amounts of active catalyst, but it is in a solvent that is unsuitable. Alternately, active rhodium catalyst may have been carried over with product. Technology has been disclosed [39] that permits the isolation of an active metal hydride catalyst. Steps include ... 

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