Synthesis and characterization of the "metallic salts" A5Pn4 (A = K, Rb, Cs and Pn = As, Sb, Bi) with isolated zigzag tetramers of Pn44- and an extra delocalized electron

The isostructural title compounds were prepared by direct reactions of the corresponding elements, and their structures were determined from single-crystal X-ray diffraction data in the monoclinic space group C2/m, Z = 2 (K5As4, a = 11.592(2) Angstrom, b = 5.2114(5) Angstrom, c = 10.383(3) Angstrom, beta = 113.42(1)degrees; K5Sb4, a = 12.319(1) Angstrom, b = 5.4866(4) Angstrom, c = 11.258(1) Angstrom, beta = 112.27(7)degrees; Rb5Sb4, a = 12.7803(9) Angstrom, b = 5.7518(4) Angstrom, c = 11.6310(8) Angstrom, beta = 113.701(1)degrees; K5Bi4, a = 12.517(2) Angstrom, b = 5.541(1) Angstrom, c = 11.625(2) Angstrom, beta = 111.46(1)degrees; Rb5Bi4, a = 12.945(4) Angstrom, b = 5.7851(9) Angstrom, c = 12.018(5) Angstrom, beta = 112.78(3)degrees; Cs5Bi4, a = 12.887(3) Angstrom, b = 6.323(1) Angstrom, c = 12.636(1) Angstrom, beta = 122.94(2)degrees). The compounds contain isolated and flat zigzag tetramers of Pn(4)(4-) (Pnictide (Pn) = As, Sb, Bi) with a conjugated pi -electron system of delocalized electrons. All six compounds are metallic ("metallic salts") and show temperature-independent (Pauli-like) paramagnetism due to a delocalized electron from the extra alkali-metal cation in the formula. At low temperatures (around 9.5 K) and low magnetic fields the bismuthides become superconducting.