Titanium(III) chloride

Titanium(III) chloride is the chemical compound with the formula TiCl3. This deceptively simple name describes several distinct species as well as a hydrated salt. TiCl3 is one of the most common halides of titanium and is an important catalyst for the manufacture of polyolefins.

Electronic properties
In TiCl3, each Ti atom has one d electron, rendering most of its forms paramagnetic, i.e. the substance is attracted into a magnetic field. The paramagnetism of TiCl3's contrasts with the diamagnetism (the property of being repelled from a magnetic field) of the trihalides of hafnium and zirconium. These heavier metals engage in metal-metal bonding, but the Ti(III) ion does not typically.

Solutions of titanium(III) chloride have violet coloration which arises from excitations of its d-electron. The colour is not very intense since the transition is forbidden.

Structure
Solid TiCl3 can be obtained in one of four forms or polymorphs. These forms can be distinguished by crystallography as well as studies on their magnetic properties which probes exchange interactions.

Beta form
β-TiCl3 crystallizes as brown needle. The structure of this material features chains of TiCl6 octahedra that share opposite faces such that the closest Ti-Ti is 2.91 Å. This short distance allows strong metal-metal bonding.

"Violet layered" forms
The three violet layered" forms, named for their color and its tendency to flake, are called alpha, gamma, and delta. 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 delta (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.

Ternary alkali halides
Caesium chloride treated with titanium(II) chloride and hexachlorobenzene produces crystalline CsTi2Cl7. The product contains 1:2 molar mixtures of CsCl and TiCl3. CsCl3 and Cl4 stack alternatively in an ABAC sequence with Ti3+ in one quarter of the octahedral holes.

Synthesis and selected handling properties
Titanium(IV) chloride can be reduced to TiCl3; this conversion is generally accomplished electrochemically. It is sold as a mixture with aluminium trichloride. This mixture can be separated to afford TiCl3(THF)3.

Ti3+ is prepared in situ as part of the method of analyzing titanium-containing samples such as ores. Thus, acidic solutions of Ti4+ are reduced using Jones reductor with amalgamated zinc. The resulting Ti3+ containing product is analyzed by redox titration.

TiCl3 and most of its complexes are handled under an inert atmosphere to prevent reactions with oxygen. Slow deterioration occurs in air-exposed titanium trichloride, often results in erratic results, e.g. in reductive coupling reactions.

Applications in Ziegler-Natta catalyst
TiCl3 is a useful Ziegler-Natta catalyst, although the catalytic activities vary with the method of preparation.

Other reactions of Ti(III) chloride
TiCl3 forms a variety of coordination complexes, most of which are octahedral. The light-blue crystalline adduct TiCl3(THF)3 forms when TiCl3 is refluxed with tetrahydrofuran.
 * TiCl3 + 3C4H8O → TiCl3(OC4H8)3

A dark green charge-neutral complex arises from complexation with dimethylamine:
 * TiCl3 + 3Me2NH → TiCl3(NHMe2)3 + CH3Cl

TiCl3 reacts with acetylacetone to form a tris acetylacetonate complex.
 * TiCl3 + 3NH4(acac) → Ti(acac)3 + 3NH4Cl

The product is used as cross-linking agent for cellulose films used in polyethylene catalysts. Ti(acac)3 air-oxidizes to give orange TiO(acac)2, which is ineffective in cross-linking.

Above 200 °C, TiCl3 undergoes ammonolysis.

The ternary halides, such as A3TiCl6, have structures that depend on the cation (A) added.

Suppliers

 * Fisher Scientific
 * Sigma-Aldrich

塩化チタン(III)