Diboron tetrachloride is a tetrahalodiborane chemical compound with the formula B<sub>2</sub>Cl<sub>4</sub>. It is a colorless liquid.
The modern synthesis involves the chlorination of diboron tetrabromide by gallium(III) chloride.
It can also be formed by the electrical discharge procedure of boron trichloride at low temperatures:
The most efficient synthesis technique uses no dechlorinating metal, instead passing radio-frequency alternating current through gaseous boron trichloride.
The molecular structure of diboron tetrachloride, B<sub>2</sub>Cl<sub>4</sub>, was determined by gas electron diffraction. The molecules have D<sub>2d</sub> symmetry, i.e. the two planar BBCl<sub>2</sub> units are perpendicular to each other (torsion angle Cl-B-B-Cl 90ð). The B-B distance is 1.70(4) à, the B-Cl distance is 1.750(5) à, the Cl-B-Cl angle is 118.7(3)ð. B<sub>2</sub>Cl<sub>4</sub> thus differs significantly from B<sub>2</sub>F<sub>4</sub>, which is a planar molecule overall.
The compound is used as a reagent for the synthesis of organoboron compounds. For instance, diboron tetrachloride adds to ethylene:
Diboron tetrachloride absorbs hydrogen quickly at room temperature:
With boranes, it replaces a hydrogen to form dichloroborane(3) and a polyhedral dichloroborane. Heat induces disproportionation back to boron trichloride and a polyhedral boron(I) chloride.