Crystal structure and properties of chromium(II)tetrachloroaluminate

Chromium(II)tetrachloroaluminate can be prepared by reaction of chromium, chromium trichloride and aluminum trichloride in the molar ratio 1 : 1.94 : 5.79 or by reaction of stoichiometric amounts of chromium dichloride and aluminum trichloride. Sublimation of the compound yields phase-pure samples of colourless crystals. Cr(AlCl4)(2) crystallizes in the non-centrosymmetric orthorhombic space group Pca2(1) (No. 29), a = 1511.38(3), b = 604.71(l), c = 1301.76(3) pm. The aggregation of Cr(AlCl4)(2) molecules leads to a square-bipyramidal coordination of the chromium atom typical of high-spin d(4) ions due to the Jahn-Teller effect. The relationship to centrosymmetric Pd(AlCl4)(2) (P2(1)/c) which is built up of similar molecules was established by means of a group-subgroup symmetry tree. The temperature dependence of the magnetic susceptibility agrees with Curie-Weiss behaviour (mu(eff) = 4.70 mu(B), Theta = - 1.3 K)