Titanium(III) chloride

In TiCl₃, each Ti atom has one d electron, rendering its derivatives paramagnetic, i.e. 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, 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. 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.[1]

TiCl₃ is produced usually by reduction of titanium(IV) chloride. Older reduction methods used hydrogen:[2]

2 TiCl₄ + H₂ → 2 HCl + 2 TiCl₃

It is conveniently reduced with aluminium and sold as a mixture with aluminium trichloride, TiCl₃·AlCl₃. This mixture can be separated to afford TiCl₃(THF)₃.[3] The complex adopts a meridional structure.[4]

Its hydrate can be synthesised by dissolving titanium in aqueous hydrochloric acid.

2 Ti + 6 HCl + 6 H₂O → 2 TiCl₃(H₂O)₃ + 3 H₂

TiCl₃ forms a variety of coordination complexes, most of which are octahedral. The light-blue crystalline adduct TiCl₃(THF)₃ forms when TiCl₃ is treated with tetrahydrofuran.[5]

TiCl₃ + 3 C₄H₈O → TiCl₃(OC₄H₈)₃

An analogous dark green complex arises from complexation with dimethylamine. In a reaction where all ligands are exchanged, TiCl₃ is a precursor to the tris acetylacetonate complex.

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:[6]

2 TiCl₃ → TiCl₂ + TiCl₄

The ternary halides, such as A₃TiCl₆, have structures that depend on the cation (A⁺) added.[7] Caesium chloride treated with titanium(II) chloride and hexachlorobenzene produces crystalline CsTi₂Cl₇. In these structures Ti³⁺ exhibits octahedral coordination geometry.[8]

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.[9]

TiCl₃ and most of its complexes are typically handled under air-free conditions to prevent reactions with oxygen and moisture. Depending on the method for its preparation, samples of TiCl₃ can be relatively air stable or pyrophoric.[13][14]