Hydrides, Hydrogen Bonding and Dihydrogen Activation

After completing our work on C-H activation in 1985, we participated in the development of the chemistry of dihydrogen complexes by showing their generality and that they can be synthesised by protonation of compounds with a terminal M-H bond (Equation 1).  

Such compounds can be hard to characterise by classical methods, so we suggested use of the excess relaxation of the hydride signal in the NMR 

From 1995, we and Morris group found a long series of metal hydrides in which a transition metal M-H bond acts as the weak base (proton acceptor) in a hydrogen bond with OH or NH protons ( = A-H) as the weak acid partners. This was shown by the short H-H distance ca. 1.8 A) and by studies that identified the A-H-H-M interaction strength as ca. 4-8 kcal moP Classical hydrogen bonds, A-H-B, require a lone pair on the base, B, and both A and B are electronegative. In A-H-H-M, the lone pair of the weak base, B, is apparently replaced by the M-H (x-bond, and both M and H are much more electropositive than the N, O or F atoms common in the classical type, so we have a new class of hydrogen bond. Morris has used an alternative descriptive term, proton-hydride interaction. 

The conformational preference usually seen in compounds having a dihydro- 

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As shown in Figure 1, the next step in the catalytic cycle of carbon dioxide hydrogenation is either reductive elimination of formic acid from the transition-metal formate hydride complex or CT-bond metathesis between the transition-metal formate complex and dihydrogen molecule. In this section, we will discuss the reductive elimination process. Activation barriers and reaction energies for different reactions of this type are collected in Table 3. 

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