Hydrogen hydride character

In Eq. (6) the hydrogen attached to the oxygen has protonic character and the hydrogen attached to the zinc has hydridic character. Such bonds, with considerable ionic character, would be expected to yield intense bands in the IR, whereas a largely covalent bond would yield relatively weak IR... 

Within the monohydridic route, apart from the already explained inner-sphere mechanisms, there is another possibility involving the concerted outer-sphere transfer of one hydride and one proton to the corresponding substrate (Scheme 4b). This mechanism is very common to the so-called bifunctional catalysts. This term was proposed by Noyori for those catalysts having one hydrogen with hydridic character directly bonded to the metal center of the catalyst, a hydride ligand, and another hydrogen with protic character bonded to one of the ligands of the metal complex (20). In Scheme 9, examples of bifunctional catalysts that are synthesized... 

Under mild conditions, hydroformylation of olefins with rhodium carbonyl complexes selectively produces aldehydes. A one-step synthesis of oxo alcohols is possible using monomeric or polymeric amines, such as dimethylbenzylamine or anion exchange resin analog to hydrogenate the aldehyde. The rate of aldehyde hydrogenation passes through a maximum as amine basicity and concentration increase. IR data of the reaction reveal that anionic rhodium carbonyl clusters, normally absent, are formed on addition of amine. Aldehyde hydrogenation is attributed to enhanced hydridic character of a Rh-H intermediate via amine coordination to rhodium. 

As shown in Figure 3, a positive p-value (+0.92) was observed in the hydrogenation of substituted benzaldehydes, giving strong support to the postulation by Heil and Marko that the rate determining step in the formation of alcohol (Mechanism 2) is the hydride addition step. It is therefore suggested that coordination of amine to rhodium increases the hydridic character of the Rh-H bond, much the same as is postulated in cobalt-tributylphosphine complexation (20). The differing effect of amine on rhodium (promoter) and on cobalt (inhibitor) is attributed to the more hydridic nature of a Rh-H bond as compared with the very protonic HCo(CO)4. Addition of amines to HCo(CO)4 results in formation of inactive species similar to I. 

For the hydride transfer the effect of the selenium substitution goes in the opposite direction compared to the H-H bond cleavage, with a 1.5 kcal/mol lower barrier for the selenium case. This effect can be explained by a larger mobility and acidity of selenium. The mobility is indicated by the bond distance to nickel which is 2.30 A for the hydride in the sulfur case and 2.40 A for the selenium case. At the transition states the Ni-S and Ni-Se distances increase to 2.44 A and 2.61 A, respectively. It is also interesting to note that the Se-H distance of 1.71 A at the transition state is shorter than the S-H distance of 1.83 A. Since selenium is more acidic this indicates a dominating hydride character of the hydrogen at the transition state. 

This introduction has outlined problems associated with a proper description of the hydrogen in motion. In many reactions, hydride motion is associated with complete transfer of an X—H bond, having hydridic character, to the ends of an unsaturated array. Isolation of the characteristics of the hydride transfer is rarely straightforward. 

As far as hydrogenation activity is concerned, the superior activity of the modincd system has been ascribed to an increase in the hydridic character of the cobalt hydride in HCo(CO)3pR3 as compared to HCo(CO)4. 

Because of the Lowry-Bronsted treatment of acids and bases, we tend to think of hydrogen chiefly in its proton character. Actually, its hydride character has considerably more reality. Solid lithium hydride, for example, has an ionic crystalline lattice made up of Li+ and H " by contrast, a naked unsolvated proton is not encountered by the organic chemist. 

Pd and Pt supported catalysts show completely different properties, due to different reaction mechanisms. The hydridic character of adsorbed H atoms and the easy cleavage of C-Cl bonds speeies lead to almost complete hydrogenation of CCI4 to hydrocarbons on Pd/MgO materials. Instead, Pt catalysts show very high activity and selectivity to partial hydrodechlorination to trichloromethane, with methane as major byproduct. 

Therefore, the isomerization depends crucially on the hydride character of the hydrogen atom. The hydride hgands of soft complexes such as [HRh(CO)(PPh3)3] preferably undergo anti-Markownikow addition with the following polarization ... 

With harder compounds such as [HCo(CO)4], in which the hydrogen atom has more protic than hydridic character, Markownikow addition is followed by isomerization (Eq. 2-88). 

---