Chemistry:Titanium(IV) hydride

Titanium(IV) hydride
Names
	Systematic IUPAC name Titanium(IV) hydride
	Other names Titanium tetrahydride; Titane; TiH4
Identifiers
CAS Number	14902-91-1 ;
3D model (JSmol)	Interactive image;
EC Number	238-972-5;
PubChem CID	187779;
UN number	1871
	InChI InChI=1S/Ti.4H Key: XOOJFLWSRHQYJN-UHFFFAOYSA-N;
	SMILES [TiH4];
Properties
Chemical formula	TiH; 4
Molar mass	51.899 g/mol
Appearance	Colourless gas
Solubility in water	Reacts
Related compounds
Other anions	Titanium(IV) fluoride; Titanium(IV) chloride; Titanium(IV) bromide; Titanium(IV) iodide
Other cations	Methane; Silane; Germane; Stannane; Plumbane
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
	Infobox references

Titanium(IV) hydride (systematically named titanium tetrahydride) is an inorganic compound with the empirical chemical formula TiH₄. 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 TiCl
4 and H
2, followed by immediate mass spectrometry.^[1] Rapid analysis was required as titanium(IV) hydride is extremely unstable. Computational analysis of TiH
4 has given a theoretical bond dissociation energy (relative to M+4H) of 132 kcal/mole.^[2] As the dissociation energy of H
2 is 104 kcal/mole the instability of TiH
4 can be expected to be thermodynamic; with it dissociating to metallic titanium and hydrogen:

TiH
4 → Ti + 2 H
2 (76 kcal/mole)

TiH
4, along with other unstable molecular titanium hydrides, (TiH, TiH
2, TiH
3 and polymeric species) has been isolated at low temperature following laser ablation of titanium.^[3]

Structure

It is suspected that within solid titanium(IV) hydride, the molecules form aggregations (polymers), being connected by covalent bonds.^[4] Calculations suggest that TiH
4 is prone to dimerisation.^[3] 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 as there is a negligible increase in inter-ligand repulsion.

The dimer is a calculated to be a fluxional molecule rapidly inter-converting between a number of forms, all of which display bridging hydrogens.^[4] This is an example of three-center two-electron bonding.

Monomeric titanium(IV) hydride is the simplest transition metal molecule that displays sd³ orbital hybridisation.^[5]

References

↑ Breisacher, Peter; Siegel, Bernard (5 June 1963). "Formation of Gaseous Titanium(IV) Hydride and Chlorohydrides of Titanium". Journal of the American Chemical Society 85 (11): 1705–1706. doi:10.1021/ja00894a049.
↑ Hood, Diane M.; Pitzer, Russell M.; Schaefer, Henry F. (1 January 1979). "Electronic structure of homoleptic transition metal hydrides: TiH4, VH4, CrH4, MnH4, FeH4, CoH4, and NiH4". The Journal of Chemical Physics 71 (2): 705. doi:10.1063/1.438357. Bibcode: 1979JChPh..71..705H.
↑ ^3.0 ^3.1 Chertihin, George V.; Andrews, Lester (September 1994). "Reactions of laser ablated Ti atoms with hydrogen during condensation in excess argon. Infrared spectra of the TiH, TiH2, TiH3, and TiH4 molecules". Journal of the American Chemical Society 116 (18): 8322–8327. doi:10.1021/ja00097a045.
↑ ^4.0 ^4.1 Webb, Simon P.; Gordon, Mark S. (July 1995). "The dimerization of TiH₄". Journal of the American Chemical Society 117 (27): 7195–7201. doi:10.1021/ja00132a020.
↑ Jonas, V.; Boehme, C.; Frenking, G. (1996). "Bent's Rule and the Structure of Transition Metal Compounds". Inorg. Chem. 35 (7): 2097–2099. doi:10.1021/ic951397o.