Hydrogen Bonds to Metals and Metal Hydrides

Epstein, L. M., Shubina, E. S., New types of hydrogen bonding in organometallic chemistry . Coord. Chem. Rev. 2002, 231, 165-181. 

Ititermolecular mullicenlre hetero-acceptor (IMH) lCuL2Ly.2H20 (c) 

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Crabtree, R.H. (2004) Hydrogen Bonds to Metals and Metal Hydrides, in Encyclopedia of Supramolecular Chemistry (eds J.L. Atwood and J.W., Steed), Dekker, New York, pp. 666-672. 

Hydrogen Bonds to Metals and Metal Hydrides, p 666 Second-Sphere Coordination, p. 1209 Secondary Bonding, p. 1215 X Ra Crystallography, p 1586... 

We recentlyf came to the conclusion that the large dominance of isomer Ib over Ic is due to the stabilization elfects of intramolecular hydrogen-bonds involving metal hydrides as proton acceptors from N H bonds. In solution, the occurrence of this type of interaction has been unambiguously demonstrated by evaluating the contribution to the relaxation of Hy and He arising from the N-H moiety. 

The crystal engineering community is also at the forefront of work into new kinds of intermolecular interactions. Recent single-crystal neutron diffraction studies revealed detailed information on OH-. -tc interactions " and on unconventional hydrogen bonds to metal center- as] well as MH-. HX-type interactions, where MH is a hydridic metal hydrogen moiety, and HX contains protic hydrogen. 

In a sense the formation of t) -H2 complexes can be thought of as an intermediate stage in the oxidative addition of H2 to form two M-H bonds and, as such, the complexes might serve as a model for this process and for catalytic hydrogenation reactions by metal hydrides. Indeed, intermediate cases between and... 

The oxidative addition of silanes (with silicon-hydrogen bonds) to coordinatively unsaturated metal complexes is one of the most elegant methods for the formation of metal-silicon bonds. Under this heading normally reactions are considered which yield stable silyl metal hydrides. However, in some cases the oxidative addition is accompanied by a subsequent reductive elimination of, e.g., hydrogen, and only the products of the elimination step can be isolated. Such reactions are considered in this section as well. 

As a result of the recognized role of transition metal hydrides as l reactive intermediates or catalysts in a broad spectrum of chemical reactions such as hydroformylation, olefin isomerization, and hydrogenation, transition metal hydride chemistry has developed rapidly in the past decade (J). Despite the increased interest in this area, detailed structural information about the nature of hydrogen bonding to transition metals has been rather limited. This paucity of information primarily arises since, until recently, x-ray diffraction has been used mainly to determine hydrogen positions either indirectly from stereochemical considerations of the ligand disposition about the metals or directly from weak peaks of electron density in difference Fourier maps. The inherent limi-... 

A chiral Ru hydride 23 is formed and it is assumed that the hydrogen transfer occurs via metal-ligand bifunctional catalysis. The N-H linkage may stabilize a transition state 24 by formation of a hydrogen bond to the nitrogen atom. Stereochemistry is determined by formal discrimination of the enantiofaces at the sp2 nitrogen atom of the cyclic imine. 

All these ligands have extensive chemistry here we note only a few points that are of interest from the point of view of catalysis. The relatively easy formation of metal alkyls by two reactions—insertion of an alkene into a metal-hydrogen or an existing metal-carbon bond, and by addition of alkyl halides to unsaturated metal centers—are of special importance. The reactivity of metal alkyls, especially their kinetic instability towards conversion to metal hydrides and alkenes by the so-called /3-hydride elimination, plays a crucial role in catalytic alkene polymerization and isomerization reactions. These reactions are schematically shown in Fig. 2.5 and are discussed in greater detail in the next section. 

Ruthenium, osmium and iridium hydrides insert N,N -di-p-tolylcarbodiimide into the metal-hydrogen bonds to give N,N -p-tolylformamidinato derivatives. ... 

Germanium forms a wide range of compounds with hydrogen, silicon, the heavier group 14 elements, some main group metals, and transition metals. Catenated hydrides, Ge H2 +2, have already been discussed. The majority have carbon bonded to Ge and are included in the next article (see Germanium Organometallic Chemistry). 

There exist two broad families of complex transition metal hydrides. The first contains hydrogen bonded to transition elements only and has the general composition... 

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