Syntheses and Crystal Structures of the Cluster Compounds A2[(W6Br)Br] with A = K,Rb, Cs

K₂W₆Br₁₄ ( I ), Rb₂W₆Br₁₄ ( II ), and Cs₂W₆Br₁₄ ( III ) were formed by reactions of W₆Br₁₂ with the corresponding alkali metal bromides in evacuated silica tubes with a temperature gradient of 925 K/915 K. ( I ) crystallizes in the cubic space group Pn3 (no. 201), a = 13.808 Å, Z = 4, cP88. ( II ) crystallizes in the monoclinic space group C2/c (no. 15), a = 20.301 Å, b = 15.396 Å, c = 9.720 Å, β = 115.69°, Z = 4, mC88. ( III ) crystallizes in the trigonal space group P31c (no. 163), a = 10.180 Å, c = 15.125 Å, Z = 2, hP44. The crystal structures are composed of the isolated (W₆Br)Br]^2– cluster anions and the alkali metal cations (d(W–W) = 2.635(2) Å, d(W–Brⁱ) = 2.624(4) Å, d(W–Br^a) = 2.595(4) Å). The shape of the anions is influenced by the crystal field symmetry, but the mean bond lengths are not changed by the cation size. The packing of the cluster anions corresponds to ccp pattern in ( I ) and hcp pattern in ( II ) and ( III ), respectively. The alkali metal cations in the octahedral holes are coordinated only by the Br^a ligands while those in the tetrahedral and trigonal-bipyramidal cavities are surrounded by Br^a and Brⁱ ligands. The details will be discussed and compared with other structures.