Synthesis and Crystal Structures of Cs 5 Nb 2 S 4 X 9 -Double Salts Containing New Cluster Anions [Nb 2 (µ-S 2 ) 2 X 8 ] 4− (X: Cl, Br).-The isotypic compounds (IV) crystallize in the tetragonal space group Immm with Z = 2. The compounds contain the new [Nb 2 S 4 X 8 ] 4− anion with the Nb atoms in a distorted square antiprismatic coordination with four S and four X atoms. These anions can be viewed as excised building blocks of polymeric Nb 2 S 2 X 2 solids. The extra halogen atom resides in the center of an octahedron formed by six Cs + or H 3 O + ions. -(SOKOLOV, MAXIM