Titanium(IV) hydride

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For the titanium-hydrogen alloy, see Titanium hydride.
Titanium(IV) hydride
Ball and stick model of titanium tetrahydride
Identifiers
CAS number 14902-91-1 YesY
PubChem 187779
EC number 238-972-5
UN number 1871
Jmol-3D images Image 1
Properties
Molecular formula TiH
4
Molar mass 51.899 g mol−1
Solubility in water Reacts
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Titanium(IV) hydride is an unstable covalent hydride with the chemical formula TiH4. It is a short-lived species which has only been detected in the gas phase, following certain high energy reactions. As such the compound is not well characterised, although many of its properties have been calculated via computational chemistry.

Synthesis and stability[edit]

Titanium(IV) hydride was first produced in 1963 by the photodissociation of mixtures of TiCl4 and H2, followed by immediate mass spectrometry.[1] Rapid analysis was required as titanium(IV) hydride is extremely unstable. Computational analysis of TiH4 has given a theoretical bond dissociation energy (relative to M+4H) of 132 kcal/mole.[2] As the dissociation energy of H2 is 104 kcal/mole the instability of TiH4 can be expected to be thermodynamic; with it dissociating to metallic titanium and hydrogen:

TiH4 → Ti0 + 2 H2 (76 kcal/mole)

TiH4, along with other unstable molecular titanium hydrides, (TiH, TiH2, TiH3 and polmeric species) has been isolated at low temperature following laser ablation of titanium.[3]

Structure[edit]

Calculations suggest that TiH4 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[edit]

  1. ^ Breisacher, Peter; Siegel, Bernard (5 June 1963). Journal of the American Chemical Society 85 (11): 1705–1706. doi:10.1021/ja00894a049. 
  2. ^ 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. 
  3. ^ a b 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" (PDF). Journal of the American Chemical Society (American Chemical Society) 116 (18): 8322–8327. doi:10.1021/ja00097a045. Retrieved 24 September 2013. 
  4. ^ Webb, Simon P.; Gordon, Mark S. (1 July 1995). "The Dimerization of TiH4". Journal of the American Chemical Society 117 (27): 7195–7201. doi:10.1021/ja00132a020. 
  5. ^ 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.