# Titanium(III) chloride

Titanium(III) chloride is the inorganic compound with the formula TiCl$_3$. At least four distinct species have this formula; additionally hydrated derivatives are known. TiCl$_3$ is one of the most common halides of titanium and is an important catalyst for the manufacture of polyolefins.

## Structure and bonding

In TiCl$_3$, 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 TiCl$_3$ 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-TiCl$_3$ crystallizes as brown needles. Its structure consists of chains of TiCl$_6$ 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-TiCl$_3$, the chloride anions are hexagonal close-packed. In γ-TiCl$_3$, 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 TiCl$_6$ 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.
Two hydrates of titanium(III) chloride are known, i.e. complexes containing aquo ligands. These include the pair of hydration isomers [Ti(H$_2$O)$_6$]Cl$_3$ and [Ti(H$_2$O)$_4$Cl$_2$]Cl(H$_2$O)$_2$. The former is violet and the latter, with two molecules of water of crystallization, is green.

## Synthesis and reactivity

TiCl$_3$ is produced usually by reduction of titanium(IV) chloride. Older reduction methods used hydrogen:
    2 TiCl$_4$ + H$_2$ → 2 HCl + 2 TiCl$_3$
It 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 H$_2$ + 2 TiCl$_3$
It is conveniently reduced with aluminium and sold as a mixture with aluminium trichloride, TiCl$_3$·AlCl$_3$. This mixture can be separated to afford TiCl$_3$(THF)$_3$. The complex adopts a meridional structure. This light-blue complex TiCl$_3$(THF)$_3$ forms when TiCl$_3$ is treated with tetrahydrofuran (THF).
    TiCl$_3$ + 3 C$_4$H$_8$O → TiCl$_3$(OC$_4$H$_8$)$_3$
An analogous dark green complex arises from complexation with dimethylamine. In a reaction where all ligands are exchanged, TiCl$_3$ is a precursor to the blue-colored complex Ti(acac)$_3$.
The more reduced titanium(II) chloride is prepared by the thermal disproportionation of TiCl$_3$ at 500 °C. The reaction is driven by the loss of volatile TiCl$_4$:
    2 TiCl$_3$ → TiCl$_2$ + TiCl$_4$
The ternary halides, such as A$_3$TiCl$_6$, have structures that depend on the cation (A$^+$) added. Caesium chloride treated with titanium(II) chloride and hexachlorobenzene produces crystalline CsTi$_2$Cl$_7$. In these structures Ti$^{3+}$ exhibits octahedral coordination geometry.

## Applications

TiCl$_3$ is the main Ziegler–Natta catalyst, responsible for most industrial production of polyethylene. The catalytic activities depend strongly on the polymorph of the TiCl$_3$ (α vs. β vs. γ vs. δ) and the method of preparation.

### Laboratory use

TiCl$_3$ 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. Titanium trichloride can reduce nitrate to ammonium ion thereby allowing for the sequential analysis of nitrate and ammonia. Slow deterioration occurs in air-exposed titanium trichloride, often resulting in erratic results, such as in reductive coupling reactions.

## Safety

TiCl$_3$ and most of its complexes are typically handled under air-free conditions to prevent reactions with oxygen and moisture. Samples of TiCl$_3$ can be relatively air stable or pyrophoric.