Niobium−Arsenic Zintl Phases: A₆NbAs₅ (A = K, Rb, Cs), K₆NbTlAs₄, and K₈NbPbAs₅ with Edge-Bridged Niobium-Centered Tetrahedra of Arsenic, [NbAs₄M]<i>ⁿ</i>^- Where M = As, Tl, Pb

The five title compounds were prepared by direct reactions of the corresponding elements at high temperature. Their structures contain isolated anions of tetrahedral NbAs4 where one of the edges of the tetrahedron is bridged by a third atom. The bridging atom is arsenic in A6NbAs5 (monoclinic, P21/c, Z = 8; with a = 25.774(3) Å, b = 9.335(1) Å, c = 13.012(1) Å, β = 101.05(1)° for A = K; a = 27.629(1) Å, b = 9.925(1) Å, c = 14.111(1) Å, β = 101.63(1)° for A = Rb; and a = 27.405(1) Å, b = 9.9447(6) Å, c = 13.9964(8) Å, β = 101.210(1)° for A = Cs), thallium in K6NbTlAs4 (orthorhombic, Pnma, Z = 4, a = 18.786(1) Å, b = 10.4442(4) Å, c = 7.715(1) Å), and lead in K8NbPbAs5 (monoclinic, C2/c, Z = 8, a = 31.597(9) Å, b = 9.353(1) Å, c = 13.427(2) Å, β = 95.25(1)°). The lead atom in the latter is bonded to a third arsenic atom as well. Magnetic measurements showed diamagnetic behavior, and therefore, the compounds are electronically balanced, closed-shell type compounds and can be described as transition-metal Zintl phases. The bonding in the anion NbAs56- is discussed in detail.