Titanium(IV) hydride (systematically named titanium tetrahydride) is an inorganic compound with the empirical

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 symbol ...

. It has not yet been obtained in bulk, hence its bulk properties remain unknown. However, molecular titanium(IV) hydride has been isolated in solid gas matrices. The molecular form is a colourless gas, and very unstable toward thermal decomposition. As such the compound is not well characterised, although many of its properties have been calculated via computational chemistry.

Synthesis and stability

Titanium(IV) hydride was first produced in 1963 by the photodissociation of mixtures of and , followed by immediate mass spectrometry. Rapid analysis was required as titanium(IV) hydride is extremely unstable. Computational analysis of has given a theoretical

bond dissociation energy The bond-dissociation energy (BDE, ''D''0, or ''DH°'') is one measure of the strength of a chemical bond . It can be defined as the standard enthalpy change when is cleaved by homolysis to give fragments A and B, which are usually radical ...

(relative to M+4H) of 132 kcal/mole. As the dissociation energy of is 104 kcal/mole the instability of can be expected to be

thermodynamic Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of the ...

; with it dissociating to metallic

titanium Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...

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 ...

: : (76 kcal/mole) , along with other unstable molecular titanium hydrides, (TiH, , and polymeric species) has been isolated at low temperature following laser ablation of titanium.

Structure

It is suspected that within solid titanium(IV) hydride, the molecules form aggregations (

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 a ...

s), being connected by covalent bonds. Calculations suggest that is prone to

dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ...

isation. This largely attributed to the electron deficiency of the monomer and the small size of the hydride ligands; which allows dimerisation to take place with a very low

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 joules per mole (J/mol), kilojoules pe ...

as there is a negligible increase in inter-ligand repulsion. The dimer is a calculated to be a

fluxional molecule In chemistry and molecular physics, fluxional (or non-rigid) molecules are molecules that undergo dynamics such that some or all of their atoms interchange between symmetry-equivalent positions. Because virtually all molecules are fluxional in so ...

rapidly inter-converting between a number of forms, all of which display bridging hydrogens. This is an example of

three-center two-electron bond A three-center two-electron (3c–2e) bond is an electron-deficient chemical bond where three atoms share two electrons. The combination of three atomic orbitals form three molecular orbitals: one bonding, one ''non''-bonding, and one ''anti''-b ...

ing. Monomeric titanium(IV) hydride is the simplest transition metal molecule that displays sd³ orbital hybridisation.

References

Titanium(IV) compounds Metal hydrides {{Inorganic-compound-stub