Black platelets of Bi_39.67(7)Ru₂Br_35.0(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 [(Bi₈²⁺)Ru²⁺(Bi ₈²⁺)] 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 η⁴-coordination of the two Bi₈²⁺ square antiprisms to the Ru^II atom in the sandwich complex resembles the bonding of Bi₅⁺ and Bi₈²⁺ in the clusters [(Bi₅⁺)Ru⁺(Bi₄Br ₄)Ru⁺(Bi₅⁺)] and [(Bi ₈²⁺)Ru⁺(Bi₄Br₄)Ru ⁺(Bi₅⁺)] as well as of Bi₄ ^2- in1∞[(Bi₄^2-)Ru⁺(Bi ₄Br₄)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 η⁴-coordinating bismuth polycations and polyanions act as six electron donors, comparable to nido-carborane ligands, Zintl anions E₉^4- 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.