Titanium(IV) hydride
{{For|the titanium-hydrogen alloy|Titanium hydride}}
{{Chembox
| Name =
| ImageFile = Titanium(IV)-hydride, 2D.png
| ImageFileL1 = Titanium(IV)-hydride-3D-balls.png
| ImageFileR1 =
| ImageFile_Ref = {{chemboximage|correct|??}}
| ImageSize = 100
| IUPACName =
| SystematicName = Titanium(IV) hydride
| OtherNames = Titanium tetrahydride
Titane
TiH4
| Section1 = {{Chembox Identifiers
| CASNo = 14902-91-1
| CASNo_Ref = {{cascite|correct|??}}
| PubChem = 187779
| EINECS = 238-972-5
| UNNumber = 1871
| SMILES = [TiH4]
| StdInChI = 1S/Ti.4H
| StdInChIKey = XOOJFLWSRHQYJN-UHFFFAOYSA-N
}}
| Section2 = {{Chembox Properties
| Formula = {{Chem2|TiH4}}
| MolarMass = 51.899 g/mol
| Appearance = Colourless gas
| Solubility = Reacts
}}
| Section8 = {{Chembox Related
| OtherAnions =Titanium(IV) fluoride
Titanium(IV) chloride
Titanium(IV) bromide
Titanium(IV) iodide
| OtherCations =
Methane
Silane
Germane
Stannane
Plumbane
}}
}}
Titanium(IV) hydride (systematically named titanium tetrahydride) is an inorganic compound with the empirical chemical formula {{Chem|TiH|4}}. 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 Titanium(IV) chloride and Hydrogen, followed by immediate mass spectrometry.{{cite journal|last=Breisacher|first=Peter|author2=Siegel, Bernard |title = Formation of Gaseous Titanium(IV) Hydride and Chlorohydrides of Titanium|journal=Journal of the American Chemical Society|date=5 June 1963|volume=85|issue=11|pages=1705–1706|doi=10.1021/ja00894a049}} Rapid analysis was required as titanium(IV) hydride is extremely unstable. Computational analysis of {{chem2|TiH4}} has given a theoretical bond dissociation energy (relative to M+4H) of 132 kcal/mole.{{cite journal|last=Hood|first=Diane M.|author2=Pitzer, Russell M. |author3=Schaefer, Henry F. |title=Electronic structure of homoleptic transition metal hydrides: TiH4, VH4, CrH4, MnH4, FeH4, CoH4, and NiH4|journal=The Journal of Chemical Physics|date=1 January 1979|volume=71|issue=2|pages=705|doi=10.1063/1.438357|bibcode=1979JChPh..71..705H}} As the dissociation energy of {{chem2|H2}} is 104 kcal/mole the instability of {{chem2|TiH4}} can be expected to be thermodynamic; with it dissociating to metallic titanium and hydrogen:
:{{chem2|TiH4 → Ti + 2 H2}} (76 kcal/mole)
{{chem2|TiH4}}, along with other unstable molecular titanium hydrides, (TiH, {{chem2|TiH2}}, {{chem2|TiH3}} and polymeric species) has been isolated at low temperature following laser ablation of titanium.{{Cite journal|last=Chertihin|first=George V.|author2=Andrews, Lester |title=Reactions of laser ablated Ti atoms with hydrogen during condensation in excess argon. Infrared spectra of the TiH, TiH2, TiH3, and TiH4 molecules|journal=Journal of the American Chemical Society|date=September 1994|volume=116|issue=18|pages=8322–8327|doi=10.1021/ja00097a045}}
Structure
It is suspected that within solid titanium(IV) hydride, the molecules form aggregations (polymers), being connected by covalent bonds.{{cite journal|last1=Webb|first1=Simon P.|last2=Gordon|first2=Mark S.|title=The dimerization of {{Chem|TiH|4}}|journal=Journal of the American Chemical Society|date=July 1995|volume=117|issue=27|pages=7195–7201|doi=10.1021/ja00132a020}} Calculations suggest that {{chem2|TiH4}} is prone to dimerisation.{{Cite journal|last=Chertihin|first=George V.|author2=Andrews, Lester |title=Reactions of laser ablated Ti atoms with hydrogen during condensation in excess argon. Infrared spectra of the TiH, TiH2, TiH3, and TiH4 molecules|journal=Journal of the American Chemical Society|date=September 1994|volume=116|issue=18|pages=8322–8327|doi=10.1021/ja00097a045}} 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. 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.{{cite journal | title = Bent's Rule and the Structure of Transition Metal Compounds | journal = Inorg. Chem. | year = 1996 | volume = 35 |pages = 2097–2099 | doi = 10.1021/ic951397o | author1 = Jonas, V. | author2 = Boehme, C. | author3= Frenking, G. | issue = 7}}