SYNTHESIS, CRYSTAL-STRUCTURE, AND PROPERTIES OF POLYMERIC RB12NB6SE35, A NOVEL TERNARY NIOBIUM SELENIDE CONSISTING OF INFINITE ANIONIC CHAINS BUILT-UP BY NB2SE11 UNITS CONTAINING AN UNCOMMON SE-3(4-)-FRAGMENT

Rb12Nb6Se35 is the first ternary niobium polyselenide composed of infi nite anionic chains. These are well separated by rubidium cations. It crystallizes in the orthorhombic space group Pbcn with unit cell dimen sions a = 8.5811(1) Angstrom, b = 13.7152(1) Angstrom, c = 56.5225(4) Angstrom, and Z = 4. The least-squares refinement against F-2 With ani sotropic displacement parameters for all atoms yields RI = 5.78 and wR 2 = 12.81. The anionic polymeric chains are arranged in layers paralle l to the (010) plane, and successive planes are rotated by 35 degrees against each other. Nb2Se11 units composed of two face-sharing NbSe7 p entagonal bipyramids are the building blocks of the structure. They ar e interconnected by Se-2(2-) and Se-3(2-) polyanions, giving rise to i nfinite (1)(infinity)[Nb6Se35](12-) chains. Within the Nb2Se11 units a n unusual nearly linear Se, fragment is found which must be formulated as Se-3(4-). (C) 1998 Academic Press.