Titanium(III) chloride

In TiCl₃, each titanium atom has one d electron, rendering its derivatives paramagnetic, that is, the substance is attracted into a magnetic field. Solutions of titanium(III) chloride are violet, which arises from excitations of its d-electron. The colour is not very intense since the transition is forbidden by the Laporte selection rule.

Four solid forms or polymorphs of TiCl₃ are known. All feature titanium in an octahedral coordination sphere. These forms can be distinguished by crystallography as well as by their magnetic properties, which probes exchange interactions. β-TiCl₃ crystallizes as brown needles. Its structure consists of chains of TiCl₆ octahedra that share opposite faces such that the closest Ti–Ti contact is 2.91 Å. This short distance indicates strong metal–metal interactions (see figure in upper right). The three violet "layered" forms, named for their color and their tendency to flake, are called alpha (α), gamma (γ), and delta (δ). In α-TiCl₃, the chloride anions are hexagonal close-packed. In γ-TiCl₃, the chlorides anions are cubic close-packed. Finally, disorder in shift successions, causes an intermediate between alpha and gamma structures, called the δ form. The TiCl₆ share edges in each form, with 3.60 Å being the shortest distance between the titanium cations. This large distance between titanium cations precludes direct metal-metal bonding. In contrast, the trihalides of the heavier metals hafnium and zirconium engage in metal-metal bonding. Direct Zr–Zr bonding is indicated in zirconium(III) chloride. The difference between the Zr(III) and Ti(III) materials is attributed in part to the relative radii of these metal centers.{{Greenwood&Earnshaw2nd}}

Two hydrates of titanium(III) chloride are known, i.e. complexes containing aquo ligands. These include the pair of hydration isomers {{chem2|[Ti(H2O)6]Cl3 and [Ti(H2O)4Cl2]Cl(H2O)2}}. The former is violet and the latter, with two molecules of water of crystallization, is green.{{Greenwood&Earnshaw2nd|page = 965}}

TiCl₃ is produced usually by reduction of titanium(IV) chloride. Older reduction methods used hydrogen:{{cite book |doi=10.1002/9780470132371.ch17|chapter=Titanium(III) Chloride and Titanium(III) Bromide|year=2007|last1=Sherfey|first1=J. M.|title=Inorganic Syntheses|pages=57–61|volume=6|isbn=978-0-470-13237-1}}

:2 TiCl₄ + H₂ → 2 HCl + 2 TiCl₃

More modern techniques prefer aluminum; the product is sold as a mixture with aluminium trichloride, TiCl₃·AlCl₃.

TiCl₃ can also be produced by the reaction of titanium metal and hot, concentrated hydrochloric acid; the reaction does not proceed at room temperature, as titanium is passivated against most mineral acids by a thin surface layer of titanium dioxide.

:2 Ti + 6 HCl → 3 H₂ + 2 TiCl₃

Treating TiCl₃ with tetrahydrofuran (THF) gives the light-blue colored,{{cite book|last1=Manzer |first1=L. E.|chapter=31. Tetragtdrfuran Complexes of Selected Early Transition Metals |title= Inorganic Syntheses|series= Inorganic Syntheses|year= 1982|volume= 21|page= 137|doi=10.1002/9780470132524.ch31|isbn=978-0-471-86520-9 }} meridional complex, TiCl₃(THF)₃:{{cite journal|last1=Handlovic |first1=M. |last2=Miklos |first2=D. |last3=Zikmund |first3=M. |title=The structure of trichlorotris(tetrahydrofuran)titanium(III) |journal=Acta Crystallographica B |date=1981 |volume=37 |issue=4 |pages=811–814 |doi=10.1107/S056774088100438X|bibcode=1981AcCrB..37..811H }}

:TiCl₃ + 3 C₄H₈O → TiCl₃(OC₄H₈)₃

TiCl₃·AlCl₃ gives the same product.{{cite journal|last1=Jones |first1=N. A. |last2=Liddle |first2=S. T. |last3=Wilson |first3=C. |last4=Arnold |first4=P. L. |title= Titanium(III) Alkoxy-N-heterocyclic Carbenes and a Safe, Low-Cost Route to TiCl₃(THF)₃|journal= Organometallics |year= 2007|volume= 26|issue=3 |pages= 755–757|doi= 10.1021/om060486d}}

An analogous dark green complex arises from complexation with dimethylamine. In a reaction where all ligands are exchanged, TiCl₃ is a precursor to the blue-colored complex Ti(acac)₃.{{cite journal |doi=10.1007/s11224-016-0864-0|title=An Historic and Scientific Study of the Properties of Metal(III) Tris-acetylacetonates|year=2017|last1=Arslan|first1=Evrim|last2=Lalancette|first2=Roger A.|last3=Bernal|first3=Ivan|journal=Structural Chemistry|volume=28|pages=201–212|s2cid=99668641}}

The more reduced titanium(II) chloride is prepared by the thermal disproportionation of TiCl₃ at 500 °C. The reaction is driven by the loss of volatile TiCl₄:{{cite book|last1=Holleman |first1=A. F. |last2=Wiberg |first2=E. |title=Inorganic Chemistry |publisher=Academic Press |location=San Diego, CA |date=2001 |isbn=0-12-352651-5}}{{page needed|date=February 2021}}

:2 TiCl₃ → TiCl₂ + TiCl₄

The trichloride is a Lewis acid, forming ternary hexahalide complexes with stoichiometry M₃TiCl₆. These have structures that depend on the cation (M⁺) added.{{cite journal|last1=Hinz |first1=D. |last2=Gloger |first2=T. |last3=Meyer |first3=G. |title= Ternary halides of the type A₃MX₆. Part 9. Crystal structures of Na₃TiCl₆ and K₃TiCl₆|journal= Zeitschrift für Anorganische und Allgemeine Chemie|year= 2000 |volume= 626|pages= 822–824|doi= 10.1002/(SICI)1521-3749(200004)626:4<822::AID-ZAAC822>3.0.CO;2-6|issue= 4}} Caesium chloride treated with titanium(II) chloride and hexachlorobenzene produces crystalline CsTi₂Cl₇. In these structures Ti³⁺ exhibits octahedral coordination geometry.{{cite journal|last1=Jongen |first1=L. |last2=Meyer |first2=G. |title= Caesium heptaiododititanate(III), CsTi₂I₇|journal= Zeitschrift für Anorganische und Allgemeine Chemie|year= 2004 |volume= 630|pages= 211–212|doi= 10.1002/zaac.200300315|issue= 2}}

TiCl₃ is the main Ziegler–Natta catalyst, responsible for most industrial production of polyethylene. The catalytic activities depend strongly on the polymorph of the TiCl₃ (α vs. β vs. γ vs. δ) and the method of preparation.{{Ullmann|first1=Kenneth S. |last1=Whiteley |first2=T. Geoffrey |last2=Heggs |first3=Hartmut |last3=Koch |first4=Ralph L. |last4=Mawer |first5=Wolfgang |last5=Immel |title=Polyolefins |year=2005 |doi=10.1002/14356007.a21_487}}

TiCl₃ is also a specialized reagent in organic synthesis, useful for reductive coupling reactions, often in the presence of added reducing agents such as zinc. It reduces oximes to imines.{{cite encyclopedia|first1=Lise-Lotte |last1=Gundersen |first2=Frode |last2=Rise |first3=Kjell |last3=Undheim |first4=José |last4=Méndez Andino |title=Titanium(III) Chloride |encyclopedia=Encyclopedia of Reagents for Organic Synthesis |year=2007 |doi=10.1002/047084289X.rt120.pub2|isbn=978-0-471-93623-7 }} Titanium trichloride can reduce nitrate to ammonium ion thereby allowing for the sequential analysis of nitrate and ammonia.{{cite journal|title=Determining Ammonium & Nitrate ions using a Gas Sensing Ammonia Electrode |journal=Soil and Crop Science Society of Florida |volume=65 |date=2006 |first1=D. W. |last1=Rich |first2=B. |last2=Grigg |first3=G. H. |last3=Snyder}} Slow deterioration occurs in air-exposed titanium trichloride, often resulting in erratic results, such as in reductive coupling reactions.{{OrgSynth|year=1981|last1= Fleming|first1= Michael P.|last2= McMurry|first2=John E. |title= Reductive Coupling of Carbonyls to Alkenes: Adamantylideneadamantane|volume = 60|pages = 113|doi = 10.15227/orgsyn.060.0113}}

TiCl₃ and most of its complexes are typically handled under air-free conditions to prevent reactions with oxygen and moisture. Samples of TiCl₃ can be relatively air stable or pyrophoric.{{cite journal|last1=Ingraham|first1=T. R.|last2=Downes|first2=K. W.|last3=Marier|first3=P.|title=The Production of Titanium Trichloride by Arc-Induced Hydrogen Reduction of Titanium Tetrachloride|journal=Canadian Journal of Chemistry|volume=35|issue=8|year=1957|pages=850–872|issn=0008-4042|doi=10.1139/v57-118}}{{cite book|title=Wiley Guide to Chemical Incompatibilities|last1=Pohanish |first1=Richard P. |last2=Greene |first2=Stanley A. |publisher=John Wiley & Sons|year=2009|edition=3rd|isbn=978-0-470-52330-8|page=1010}}

{{reflist|30em}}

{{Titanium compounds}}

{{Chlorides}}

Category:Titanium(III) compounds

Category:Chlorides

Category:Titanium halides

Category:Reducing agents