Linear anions [CuAs 2] 5−, [AuP 2] 5− and [AuAs 2] 5− in potassium compounds

Abstract

K 5[AuP 2], K 5[AuAs 2] and K 5[CuAs 2] were prepared from molten mixtures of the elements under an argon atmosphere in sealed niobium ampoules. The compounds form shiny black plates which are sensitive to moisture. The crystal structures of the three isotypic compounds were refined from X-ray single-crystal data. The characteristic units are linear anions [As-Cu-As] 5-, [P-Au-P] 5- and [As-Au-As] 5- with bond lengths CuAs = 230.7(2) pm, AuP = 236.8(9) pm and AuAs = 246.0(3) pm respectively, enclosed in a distorted hexagonal close packing of the potassium cations. The structural relationship to the K 5[CuSb 2] and Na 4[HgP 2] types is discussed concerning the cation polyhedra. The FIR spectra of the compounds are measured, the assignment of the observed frequencies being based on the D ∞h symmetry of the isolated [X-T-X] 5− units. The crystallographic data are as follows: space group P6 3/ mmc (no. 194), Z = 2; K 5[CuAs 2] − a = 537.7(2) pm, c = 1877.1(4) pm, c/ a = 3.49; K 5[AuP 2] − a = 563.6(2) pm, c = 1871.6(4) pm, c a = 3.32 ; K 5[AuAs 2] − a = 573.0(2) pm, c = 1921.6(4) pm, c a = 3.35 .