Pr8Cl7B7: Preparation, structure, bonding, properties

Pr8Cl7B7 is prepared from stoichiometric mixtures of PrCl3, Pr and B at 122 0 K in closed Ta capsules. Pr8Cl7B7 forms golden coloured needles sensitive to moist air. It crystallizes in the space group P (1) over bar with a = 7 73.1(2) pm, b = 903.0(2) pm, c = 1419.4(3) pm, a = 81.55(3)degrees, beta = 82.18(3)degrees, and gamma = 64.76(3)degrees. In the crystal structure Pr-6 trigonal prisms are condensed to double chains which run parallel [100]. S ome, of the prisms and rectangular prism faces are centered by boron atoms which leads to B-3, B-6, B-8 rings, and B-2 dumbbells condensed into ribbon s. These Pr8B7 strands are surrounded and held together by the Cl atoms. Pr 8Cl7B7 is a metallic conductor and shows Curie Weiss behavior with mu(eff) = 3.48 mu(B) According to extended Huckel calculations, the distribution of valence electrons is best described by a formulation Pr818+Cl77-B711-, Bon ding within the boron ribbons is thus nearly optimal, while the average 4f( 2) 5d(3/4) configuration of Pr accounts for both the observed magnetic mome nt and metallic conductivity.