New stannides CaTSn2 (T = Rh, Pd, Ir) and Ca2Pt3Sn5 - Synthesis, structureand chemical bonding

New stannides CaTSn2 (T = Rh, Pd, Ir) and Ca2Pt3Sn5 were prepared as single phase materials by a reaction of the elements in glassy carbon crucibles u nder flowing purified argon. The four compounds were investigated by X-ray diffraction both on powders and single crystals and their structures were r efined from single crystal data. The stannides CaTSn2 (T = Rh, Pd, Ir) adop t the MgCuAl2 structure with space group Cmcm: a = 434.1(1), b = 1081.7(3), c = 748.8(2) pm, wR2 = 0.0400 451 F-2 values for CaRhSn2, a = 442.7(2), b = 1113.8(4), c = 745.6(2) pm, 1 wR2 = 0.0318, 471 F-2 values for CaPdSn2, a nd a = 429.5(1), b = 1079.5(3), c = 758.6(2) pm, wR2 = 0.0465, 455 F-2 valu es for CaIrSn2 with 16 variables for each refinement. Chemical bonding anal ysis leads to the description of a distorted filled CnSn(2) substructure in which the tin-tin bonding is modified by the insertion of transition metal atoms into the planar calcium layers, favoring strong tin-transition metal bonding. Sn-119 Mossbauer spectra show single signals for CaTSn2 (T = Rh, Pd, Ir) which are subjected to quadrupole splitting. The electron count of the CaTSn2 compounds correlates with the Sn-119 isomer shift. Ca2Pt3Sn5 cry stallizes with the Yb2Pt3Sn5 type structure: Pnma, a = 734.8(1), b = 445.50 (7), c = 2634.8(5) pm, wR2 = 0.0636, 1406 F-2 values and 62 variables. The platinum and tin atoms in Ca2Pt3Sn5 build a complex three-dimensional [Pt3S n5] polyanion in which the calcium cations fill distorted pentagonal and he xagonal channels. According to semi-empirical band structure calculations t he strongest bonding interactions are found for the Pt-Sn contacts, followe d by Sn-Sn bonding. The Sn-119 Mossbauer spectrum of Ca2Pt3Sn5 shows two su perimposed signals at delta = 2.10(3) and delta = 2.18(6) mm/s.