Zinc Hydride - Molecular Form

Molecular Form

Molecular zinc(II) hydride, ZnH₂, was recently identified as a volatile product of the acidified reduction of zinc ions with sodium borohydride. This reaction is similar to the acidified reduction with lithium aluminium hydride, however a greater fraction of the generated zinc(II) hydride is in the molecular form. This can be attributed to a slower reaction rate, which prevents a polymerising concentration of building over the progression of the reaction. This follows earlier experiments in direct synthesis from the elements. The reaction of excited zinc atoms with molecular hydrogen in the gas phase was studied by Breckenridge et al using laserpump-probe techniques. Owing to its relative thermal stability, molecular zinc(II) hydride is included in the short list of molecular metal hydrides, which have been successfully identified in the gas phase (that is, not limited to matrix isolation). The only spectroscopic studies on the zinc dihydride molecule are the infrared spectra of ZnH₂ trapped in argon and krypton matrices at 10-12 K.

The average Zn-H bond energy was recently calculated to be 51.24 kcal mol-1, while the H-H bond energy is 103.3 kcal mol-1. Therefore, the overall reaction is nearly ergoneutral.

Zn_(g) + H_2(g) → ZnH_2(g)

Molecular zinc hydride was found to be linear with a Zn-H bond length of 153.5 pm.

The molecule can be found a singlet ground state of 1Σ_g+.

Quantum chemical calculations predict the molecular form to exist in a doubly hydrogen-bridged, dimeric groundstate, with little or no formational energy barrier. The dimer can be described as di-μ-hydrido-bis(hydridozinc), according to IUPAC additive nomenclature.