La8Ru4O21: a mixed-valence ternary ruthenium oxide of a new hexagonal structure type

Crystals of La8Ru4O21 were obtained by reaction of La2O3, RuO2, and NaClO3 in a KCl melt at 9500C under vacuum. La8Ru4O21 forms hexagonal crystals, space group P63cm; a = 9.885(2), c = 10.805(2) A; V = 914.3(5) A3; Z = 2. Single crystal x-ray intensity data were collected on an automated diffractometer using graphite monochromated MoKa radiation. Using 486 unique reflections (60 equal to or less than 2theta equal to or less than 600), the structure has been refined to a conventional R = 0.027 (R/sub w/ = 0.031). The significant motif in this new structure is the Ru4O21/sup 24-/ with 3m symmetry, composed of a central RuO6 octahedron that shares three mutually cis oxygen atoms with adjacent RuO6 octahedra. The mean ruthenium oxidation number is 4.5, but the averaged Ru-O distances for the two types of ruthenium atom, Ru(1), 1.96 A and Ru(2), 1.98 A, provide no support for the assignment of integral oxidation states to individual Ru ions. It is proposed that there is extensive delocalization of dπ electrons across the bridging oxygen atoms