Metal-hydrogen bond polarity

The application of organometallic compounds or hydrides of the more electropositive main group metals lithium, magnesium or aluminium in organic synthesis, catalysis and polymerization is due to the polarity of the metal/carbon and metal/hydrogen bonds, i.e. to the considerably unsymmetrical electron distribution M —or —respectively. While there are... 

Metal hydride reagents act as a source of H because they contain polar metal-hydrogen bonds that place a partial negative charge on hydrogen. 

Metal hydride reagent (Section 12.2) A reagent containing a polar metal-hydrogen bond that places a partial negative charge on the hydrogen and acts as a source of hydride ions (H ). 

The metal-hydrogen bonds in LiAlR are more polar than those in NaBR. As a consequence, LiAlH4 is a stronger reducing agent than NaBH4. 

The different polarity of transition metal hydrogen bonds is reflected in the hydridic-protonic preference (A// p) scheme developed by Pearson [13]. The basis for is the following gas phase redox equilibrium, in which the protonic and hydridic dissociations of transition metal hydrides are considered. 

We define then as the Markownikoff mode of insertion that expected from a consideration of the electronic structure of the reaction partners in their ground state. With this definition in mind we shall now first discuss briefly the polarity of metal-carbon and metal-hydrogen bonds, as well as the rr-electron distribution in certain relevant olefins. We shall then proceed to a description of numerous Markownikoff and anti-Markownikoff insertion reactions cited in the literature, and we shall try to discern electronic and steric effects leading to the one or to the other. 

Further chemical evidence for the polarization of the metal-hydrogen bonds as M —H comes from cleavage reactions with aqueous acids, alcohols or phenols to produce H2, or with halogen (Br2 or I2) to give hydrogen halide, although these reactions are sometimes not quantitative. Some examples are ... 

There seems to be some confusion about the polarization of the metal-hydrogen bond in certain hydrido-transition metal-carbonyl compounds, in particular HCo(CO)4. This and some related hydrido-carbonyl compounds are slightly soluble in water, and such solutions are clearly acidic In the gas phase or in non-polar solvents, however, these complexes exhibit spectroscopic and chemical properties similar to those of other non-acidic hydride complexes. A MO calculation of the charge densities in HCo(CO)4 indicated that 1.6 electrons are associated with the hydrogen atom, and the question was discussed as to how a molecule with a negative-... 

It is generally assumed that olefin insertion into metal-carbon or metal-hydrogen bonds takes place by a concerted reaction path, that means through a more or less polar, cyclic transition state, with simultaneous bond breaking and bond making, e.g.-. 

Fluorocarbon—hydrocarbon near critical mixing temperature Liquid metal-liquid metal Hydrogen-bonded organic liquid—water Non-polar saturated organic—water Aromatic hydrocarbon/water... 

Some metal carbonyl hydrides dissolved in polar solvents such as water, alcohols, nitriles, ketones, etc., are acids. The acidic character of these complexes is due not only to the polarity of the metal-hydrogen bond, but to solvation of the proton and the carbonylate anion. Acidic metal carbonyl hydrides dissolved in nonpolar solvents do not dissociate, and their chemical properties are like those of nonacidic metal hydrides. [HCo(CO)4] and [HV(CO)6] are strong acids (Table 2.28). 

Outer Coordination Sphere Catalysts. In the classical hydrogenation catalysis shown previously, the substrate must be coordinated to the metal prior to its insertion into a metal-hydrogen bond. However, in recent years, it has been found that unsaturated polar bonds can be hydrogenated without coordination of the substrate to the metal (37). Two well-known, nonclassical possibilities for the hydrogenation of unsaturated polar bonds, such as ketones, are the metal-ligand bifunctional mechanism (38) and the ionic mechanism (39). In the metal-ligand bifunctional mechanism discovered by Noyori (recipient of the Nobel Prize in 2001) for highly efficient ruthenium amine complexes, the hydridic RuH and... 

The polarity of metal-carbon and metal-hydrogen bonds allows protolysis to occur with alcohols under moderate conditions. Consequently carbanionic metal alkyls react with alcohols to give alkoxides, in excellent yields. The reaction assumes special importance as it forms the basis for the calorimetric measurements of a large number of metal-alkyl bond dissociation energies. This preparative route tends to be more convenient owing to the volatility of the hberated alkane side-products. Organometal... 

The expression formally considered is used not because there is any doubt with most ligands about the number of valence electrons involved in a given metal-ligand bond, but because there may be room for discussion about the polarity of the bond. For example the metal-hydrogen bond, M—H, may be thought of as being formed from M +H+, M+-hH or Since the same bond is formed in all cases it... 

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