Weak acids group 16 hydrides

Since the hydrogen-element bond energy decreases from sulphur to tellurium they are stronger acids than hydrogen sulphide in aqueous solution but are still classified as weak acids—similar change in acid strength is observed for Group Vll hydrides. 

Most of the alkyls of the other early transition metals demonstrate similar reactivity.101102 Hydridic metal hydrides will similarly react with the weakly acidic OH groups of alcohols and phenols, liberating hydrogen and forming alkoxides or phenoxides. Aluminum hydride will undergo stepwise substitution by alcohols (equation 29).103... 

Hydrogen bonding is a well-known phenomenon that has led to major developments during the last decade in the field of polyhydrides. The formation of the so-called dihydrogen bond (M H- H X) can occur intermolecularly between a hydride and a weak acid (see equation 4) or intramolecularly between a hydride and a pendant ligand with an NH or OH group. This type of interaction is very important as it may control reactivity and selectivity in solution. A few examples have appeared in ruthenium chemistry. 28... 

Hydrostannation of carbonyl compounds with tributyltin hydride is promoted by radical initiation and Lewis or protic acid catalysis.The activation of the carbonyl group by the acidic species allows the weakly nucleophilic tin hydride to react via a polar mechanism. Silica gel was a suitable catalyst allowing chemoselective reduction of carbonyl groups under conditions that left many functional groups unchanged. Tributyltin triflate generated in situ from the tin hydride and triflic acid was a particularly efficient catalyst for the reduction of aldehydes and ketones with tributyltin hydride in benzene or 1,2-di-chloromethane at room temperature. Esters and ketals were not affected under these conditions and certain aldehydes were reduced selectively in preference to ketones. 

Studebaker identified the use of lithium aluminium hydride as a chemical probe.Under the right conditions it cleaves poly- and disulfide bonds, leaving the monosulfide intact. Lithium aluminum hydride reacts with polysulfides in an etheral solvent at moderate temperatures and then with a weak acid, the terminal groups are liberated as thiols and interior sulfur atoms are converted to hydrogen sulfide.Lithium... 

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. 

In a number of recent studies, metal hydrides have been shown to act as the weak base (proton acceptor) component of a hydrogen bond. Classical hydrogen bonds [2a] of type A-H -B, show a weak electrostatic attraction between a weak acid, AH, and a weak base, B, requiring the presence of a lone pair on the base, B. Both A and B are electronegative atoms or groups. In the dihydrogen bond, A-H--H-M, the role of the weak base is taken by M-H. As Morris [2b] has noted, these interactions can also be considered as arising from an attractive interaction between a proton and a hydride hence the alternative term, proton-hydride interaction. 

The boron atom is both small and has a high ionization potential, so bonds to boron are largely covalent with only small degrees of polarization. In fact boron is classed as a metalloid. It has volatile hydrides and a weakly acidic oxide. As the group is descended the ionization potentials decrease and the atomic radii increase, leading to increasingly polar interactions and the formation of distinct ions. The... 

Structures that are more representative of typical DHBs (e.g., 6-7) came from the author s and Morris groups in the form of an extensive series of metal hydrides in which a DHB is formed with OH or NH protons (= A-H) as the weak acid partners, both intramolecularly and intermolecularly. The characteristics of the DHB proved to be a short H... H distance (ca. 1.8 A versus the sum of the van der Waals radii of 2.4 A) found from x-ray and neutron diffraction work in the solid state and by NMR relaxation time studies in solution. Theoretical and experimental studies led to a typical A-H... H-M interaction strength of ca. 4-8 kcal/mol. 

It is known that transition metal hydrides exhibit acidic properties and it has been reported that in some cases complexes containing a group IV metal ligand react with weak bases ... 

---