[Ru(Bi8)2]6+ - A Cluster in a Highly Disordered Crystal Structure is the Key to the Understanding of the Coordination Chemistry of Bismuth Polycations

Black platelets of Bi39.67(7)Ru2Br35.0(2) were crystallized from a melt of Bi, BiBr3, and Ru. In the tetragonal crystal structure [P4/mbm, a = 1311.92(6) pm, c = 3018.2(4) pm at 155(5) K], cluster cations [(Bi-8(2+))Ru2+(Bi-8(2+))] are embedded in a matrix of disordered bromido-bismuthate(III) groups. A thorough analysis of the disorder in the anionic part enabled the unambiguous assignment of the cluster charge. The eta(4)-coordination of the two Bi-8(2+) square antiprisms to the Ru-II atom in the sandwich complex resembles the bonding of Bi-5(+) and Bi-8(2+) in the clusters [(Bi-5(+))Ru+(Bi4Br4)Ru+(Bi-5(+))] and [(Bi-8(2+))Ru+(Bi4Br4)Ru+(Bi-5(+))] as well as of Bi-4(2-) in (1)(infinity)[(Bi-4(2-))Ru+(Bi4Br4)Ru+]. By combining crystal chemical considerations for all mentioned compounds and using quantum chemical calculations, a common bonding scheme for the coordination compounds of bismuth polycations and polyanions was established. The eta(4)-coordinating bismuth polycations and polyanions act as six electron donors, comparable to nido-carborane ligands, Zintl anions E-9(4-) and E-5(6-) (E = Si to Pb), cyclopentadienyl ligands, or the cyclobutadiene dianion. The electron count for the transition metal is 18 in all finite clusters.