Zinc hydride is an

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemi ...

with the

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

. It is a white, odourless solid which slowly decomposes into its elements at room temperature; despite this it is the most stable of the

binary Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two digits (0 and 1) * Binary function, a function that takes two arguments * Binary operation, a mathematical operation that ta ...

first row

transition metal hydride Transition metal hydrides are chemical compounds containing a transition metal bonded to hydrogen. Most transition metals form hydride complexes and some are significant in various catalytic and synthetic reactions. The term "hydride" is used loose ...

s. A variety of

coordination compound A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...

s containing Zn–H bonds are used as

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are commonly reducing agents include the Earth m ...

s, however itself has no common applications.

Discovery and synthesis

Zinc(II) hydride was first synthesised in 1947 by Hermann Schlesinger, via a reaction between

dimethylzinc Dimethylzinc, also known as Zinc methyl, DMZ, or DMZn is a colorless volatile liquid Zn(CH3)2, formed by the action of methyl iodide on zinc at elevated temperature or on zinc sodium alloy. :2Zn + 2CH3I → Zn(CH3)2 + ZnI2 The sodium assists the ...

and

lithium aluminium hydride Lithium aluminium hydride, commonly abbreviated to LAH, is an inorganic compound with the chemical formula Li Al H4. It is a white solid, discovered by Finholt, Bond and Schlesinger in 1947. This compound is used as a reducing agent in organic ...

; a process which was somewhat hazardous due to the

pyrophoric A substance is pyrophoric (from grc-gre, πυροφόρος, , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolith ...

nature of . : Later methods were predominantly

salt metathesis reaction A salt metathesis reaction, sometimes called a double displacement reaction, is a chemical process involving the exchange of bonds between two reacting chemical species which results in the creation of products with similar or identical bonding a ...

s between zinc halides and alkali metal hydrides, which are significantly safer.Egon Wiberg, Arnold Frederick Holleman (2001) ''Inorganic Chemistry'', Elsevier Examples include: : : : Small quantities of gaseous zinc(II) hydride have also been produced by

laser ablation Laser ablation or photoablation (also called laser blasting) is the process of removing material from a solid (or occasionally liquid) surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser ...

of zinc under a hydrogen atmosphere and other high energy techniques. These methods have been used to assess its gas phase properties.

Chemical properties

Structure

New evidence suggests that in zinc(II) hydride, elements form a one-dimensional network (

polymer A polymer (; Greek ''poly-'', "many" + '' -mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic and ...

), being connected by covalent bonds. Other lower metal hydrides polymerise in a similar fashion (c.f.

aluminium hydride Aluminium hydride (also known as alane and alumane) is an inorganic compound with the formula Al H3. Alane and its derivatives are common reducing (hydride addition) reagents in organic synthesis that are used in solution at both laboratory an ...

). 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 as in liquid hydrogen.

Stability

Zinc(II) hydride slowly decomposes to metallic

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic t ...

and

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

gas at room temperature, with decomposition becoming rapid if it is heated above 90°C. : It is readily

oxidised Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a de ...

and is sensitive to both air and moisture; being hydrolysed slowly by water but violently by aqueous acids, which indicates possible passivation via the formation of a surface layer of ZnO. Despite this older samples may be pyrophoric. Zinc hydride can therefore be considered

metastable In chemistry and physics, metastability denotes an intermediate energetic state within a dynamical system other than the system's state of least energy. A ball resting in a hollow on a slope is a simple example of metastability. If the ball i ...

at best, however it is still the most stable of all the

first row

s (c.f.

titanium(IV) hydride Titanium(IV) hydride (systematically named titanium tetrahydride) is an inorganic compound with the empirical chemical formula . It has not yet been obtained in bulk, hence its bulk properties remain unknown. However, molecular titanium(IV) hydri ...

Molecular form

Molecular zinc(II) hydride, , has been identified as a volatile product of the acidified reduction of zinc ions with

sodium borohydride Sodium borohydride, also known as sodium tetrahydridoborate and sodium tetrahydroborate, is an inorganic compound with the formula Na BH4. This white solid, usually encountered as an aqueous basic solution, is a reducing agent that finds applic ...

. This reaction is similar to the acidified reduction with

, 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 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. : Molecular zinc hydride in the gas phase 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 In chemistry and physics, activation energy is the minimum amount of energy that must be provided for compounds to result in a chemical reaction. The activation energy (''E''a) of a reaction is measured in joule per mole, joules per mole (J/mol), ...

. The dimer can be described as di-μ-hydrido-bis(hydridozinc), according to IUPAC additive nomenclature.

References

{{Hydrides by group

hydride In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of ...

Metal hydrides Reducing agents