Titanium(III) chloride
Titanium chloride is the inorganic compound with the formula TiCl3. At least four distinct species have this formula; additionally hydrated derivatives are known. TiCl3 is one of the most common halides of titanium and is an important catalyst for the manufacture of polyolefins.
Structure and bonding
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 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. 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 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 and Ti materials is attributed in part to the relative radii of these metal centers.
Two hydrates of titanium chloride are known, i.e. complexes containing aquo ligands. These include the pair of hydration isomers. The former is violet and the latter, with two molecules of water of crystallization, is green.
Synthesis and reactivity
TiCl3 is produced usually by reduction of titanium(IV) chloride. Older reduction methods used hydrogen: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.
Treating TiCl3 with tetrahydrofuran gives the light-blue colored, meridional complex, TiCl33:
TiCl3·AlCl3 gives the same product.
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.
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:
The trichloride is a Lewis acid, forming ternary hexahalide complexes with stoichiometry M3TiCl6. These have structures that depend on the cation added. Caesium chloride treated with titanium chloride and hexachlorobenzene produces crystalline CsTi2Cl7. In these structures Ti3+ exhibits octahedral coordination geometry.