Hydride donor strength

Hydride donor strengths can be summarized as follows 24 > 19 > 12 > 22. This type of reaction provides a useful alternative to formyl synthesis by borohydride reductants neutral metal carbonyl by-products are produced instead of boron-containing species. 

These benzhydrilium carbocations were also used to develop a comprehensive scale of hydride donor strength. Both Si-H and C-H donors were evaluated by their second-order rate constants of hydride transfers to the carbocation electrophiles. The respective nucleophilicity parameter, N and %, were then determined. The benzhydryl cation series was also used to evaluate the nucleophilic reactivities of azolium enolates (1). ... 

Finally the reversal in the most favorable relative a donor strength of the two apical ligands on going from the pathway B to the pathway C may be related to a different nature of the transferred hydrogen The pathway B may be viewed as involving a transfer of a hydride, II , to a coordinatively unsaturated trigonal pyramid moiety acting as a Lewis acid. The pathway C, on the other hand may be viewed formally as an oxidative addition of on the coordinatively saturated square pyramidal d center. 

MR donor bonds, is o-bond metathesis. Alternatively, as the Lewis-acid strength of M increases, the tendency toward agostic or bridging interactions can finally result in H—H bond scission and formal migration of hydride to the metal atom,... 

On the basis of relative electronegativities (Section 3.2.5), the order of donor (nA) strengths of group 5-7 AH hydrides is expected to be... 

The mechanism of proton transfer to hydridic hydrogens depends strongly on the strengths of proton-donor and proton-acceptor sites, the nature of the element, bonding to a hydridic hydrogen, and also on the aggregate states (the solid state, the gas phase, or solution) and medium polarity. 

This analogy is plausible on energetic grounds, since the decreased base strength of the proton acceptor should be approximately compensated by the increased acid strength of the proton donor. In view of the different species involved, however, it is reasonable to expect appreciable differences in the configurations of the transition states and hence in the activation barriers for the two paths. Therefore, the failure to observe an acid-catalyzed exchange reaction cannot be taken as conclusive evidence in favor of the alternative (hydride ion) mechanism. 

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