Co2As2O5 was obtained as a by‐product from the single crystal growth of CoAs2. The triclinic crystal structure ($P{\bar 1}$, a = 6.223(1), b = 9.169(2), c = 9.259(2) Å, α = 66.95(3), β = 82.18(3), γ = 79.28(3)°, Z = 4, R1(F) = 0.057, wR2(F2) = 0.141) represents a new structure type. As typical for AsIII it contains trigonal‐pyramidal ψ‐tetrahedra [AsO3]3−. The four crystallographically different As‐atoms are arranged in monomeric units and trimeric chain fragments [As3O7]5−. The surroundings of the four cobalt(II) atoms are remarkably unequal and show tetrahedral, trigonal‐bipyramidal and distorted octahedral coordinations. The octahedra form double‐chains linked to layers by the trigonal bipyramids and dimers of the tetrahedra.

Single crystals of the high-temperature modification of RhSn4 were obtained from a tin flux (1:20 molar ratio; final annealing at 920 K; dissolution of the tin matrix in 2N HCl). The structure was refined from single-crystal X-ray diffractometer data: I41/acd, a=629.73(5), c=2288.36(18) pm, wR2=0.0382, 447 F2 values and 14 variables. β-RhSn4 is isotypic with β-IrSn4. The rhodium atoms have slightly distorted square-antiprismatic tin coordination with Rh–Sn distances of 4×273.4 and 4×274.1 pm. The RhSn8 units are condensed via common edges to layers that are staggered with respect to each other and stacked in ABCD sequence. A 119Sn Mössbauer spectroscopic characterization of ß-RhSn4 and the stannides RhSn3 and α-RhSn4 shows the typical isomer shifts for transition metal stannides. Only for α-RhSn4 the three crystallographically independent tin sites could be resolved, a consequence of the different s-electron density. Treatment of α-RhSn4 under high-pressure (up to 10 GPa)/high-temperature (up to T=1370 K) conditions leads to decomposition into Rh1.5Sn, RhSn2 and β-Sn.