Borderline hydrides

Properties

Borderline hydrides exhibit bonding characteristics between ionic and covalent bond types. A specific example of a borderline hydride is CuH, copper hydride, that appears as a spongy reddish-brown substance is a moderate reducing agent. It will catalytically oxidize hypophosphorous acid to phosphorous acid at room temperature, and it gives off hydrogen gas when subjected to heat.{{cite journal ZnH2 is also a solid at room temperature that breaks down at 90 °C, but even left alone decomposes over several days to zinc metal and hydrogen gas. Hydrogen telluride (H2Te) and hydrogen selenide (H2Se) are both borderline hydrides of high volatility that produce strong, unpleasant odors.

Examples

• (CuH)n copper hydride
• (ZnH2)n zinc(II) hydride
• HgH2 mercury(II) hydride
• TlH3 thallium hydride

Synthesis

Borderline hydrides are most commonly formed via the acidification or reduction of metal salts. For instance, copper hydride is formed by reacting copper sulfate and hypophosphorous acid at about 70 °C, forming a yellow precipitate that soon turns red-brown. Zinc hydride, ZnH2, can be formed by the reduction of either a zinc halide or dimethylzinc.

Alternative definition

A more recent definition of borderline hydrides refers to hydrides that exist between classic and non-classic dihydrides. The classic form is the dihydride M(H)2 configuration, where the metal is bound to two free hydrogen atoms. The non-classic form contains two hydrogen atoms bound to a central metal atom with an η2-H2 hapticity, indicating that a single coordination point on the metal atom bonds to two contiguous atoms from another molecule, in this case H2.{{cite journal |display-authors=etal}} A well-known example of this is from the first such molecule to be synthesized with a coordinated hydrogen ligand (dihydrogen complex): W(CO)3(PPri3)2(η2-H2).{{cite journal Classic dihydrides containing the dihydride M-(H)2 ligands are typically found as a tautomer with the non-classical dihydrogen complexes containing a M-(η2-H2) group.

Borderline hydrides exist with a bond character somewhere between the classical and non-classical hydrides.{{cite journal |display-authors=etal}} Those that are thermally unstable exhibit stretching frequencies νHH greater than 2150 cm−1 as a result of poor electron donation from the metal center. An electron dense metal center will yield hydride with a νHH less than 2060 cm−1, while anything between is considered to be in the borderline region. Kubas, et al. state that a stretching frequency of 2090 cm−1 is within the bounds of stable H2 complexes while 2060 cm−1 is right on the borderline between dihydrogen and dihydrides.

References

References

1. (1947). "Lithium Aluminum Hydride, Aluminum Hydride and Lithium Gallium Hydride, and Some of their Applications in Organic and Inorganic Chemistry". [[Journal of the American Chemical Society]].
2. Fownes, George. (1885). "Fownes' Manual of Chemistry, Theoretical and Practical". Lea Brothers & Co..