Borderline hydrides

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.^[3] Zinc hydride, ZnH₂, can be formed by the reduction of either a zinc halide or dimethylzinc.

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.^[4] 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).^[5] 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.^[6] Those that are thermally unstable exhibit stretching frequencies ν_HH 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 ν_HH 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.^[5]