| Names | |
|---|---|
| IUPAC name Zinc(II) hydride | |
| Systematic IUPAC name Zinc dihydride | |
| Other names Zinc hydride Zincane | |
| Identifiers | |
3D model (JSmol) | |
| ChemSpider |
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| |
| |
| Properties | |
| ZnH2 | |
| Molar mass | 67.425 g/mol |
| Appearance | White crystals |
| Structure | |
| linear at Zn | |
| linear | |
| 0 D | |
| Related compounds | |
Related compounds | Mercury(II) hydride Cadmium(II) hydride |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Zinc hydride is aninorganic compound with thechemical formulaZnH2. It is a white, odourless solid which slowly decomposes into its elements at room temperature; despite this it is the most stable of thebinary first rowtransition metal hydrides. A variety ofcoordination compounds containing Zn–H bonds are used asreducing agents,[1] butZnH2 itself has no common applications.
Zinc(II) hydride was first synthesized in 1947 byHermann Schlesinger, via a reaction betweendimethylzincZn(CH3)2 andlithium aluminium hydrideLi[AlH4];[2] a process which was somewhat hazardous due to thepyrophoric nature ofZn(CH3)2.
Later methods were predominantlysalt metathesis reactions between zinc halides and alkali metal hydrides, which are significantly safer.[3][4] Examples include:
Small quantities of gaseous zinc(II) hydride have also been produced bylaser ablation of zinc under a hydrogen atmosphere[5][6] and other high energy techniques. These methods have been used to assess its gas phase properties.
New evidence suggests that in zinc(II) hydride, elements form a one-dimensional network (polymer), being connected bycovalent bonds.[7] Other lower metal hydrides polymerise in a similar fashion (cf.aluminium hydride). Solid zinc(II) hydride is the irreversible autopolymerisation product of the molecular form, and the molecular form cannot be isolated in concentration. Solubilising zinc(II) hydride in non-aqueous solvents, involve adducts with molecular zinc(II) hydride, such asZnH2·H2 in liquid hydrogen.
Zinc(II) hydride slowly decomposes to metalliczinc andhydrogen gas at room temperature, with decomposition becoming rapid if it is heated above 90°C.[8]
It is readilyoxidised and is sensitive to both air and moisture; being hydrolysed slowly by water but violently by aqueous acids,[3] which indicates possiblepassivation via the formation of a surface layer ofZnO. Despite this older samples may be pyrophoric.[3] Zinc hydride can therefore be consideredmetastable at best, however it is still the most stable of all thebinary first rowtransition metal hydrides (cf.titanium(IV) hydride).
Molecular zinc(II) hydride,ZnH2, has been identified as a volatile product of the acidified reduction of zinc ions withsodium borohydride.[citation needed] This reaction is similar to the acidified reduction withlithium 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 Breckenridgeet al using laserpump-probe techniques.[citation needed] 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 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.[citation needed] Therefore, the overall reaction is nearly ergoneutral.
Molecular zinc hydride in the gas phase was found to be linear with a Zn–H bond length of 153.5 pm.[9]
The molecule can be found a singlet ground state of1Σg+.
Quantum chemical calculations predict the molecular form to exist in a doubly hydrogen-bridged, dimeric groundstate, with little or no formationalenergy barrier.[citation needed] The dimer can be called di-μ-hydrido-bis(hydridozinc), per IUPAC additive nomenclature.
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