New Stannides CaTSn2 (T = Rh, Pd, Ir) and Ca2Pt3Sn5 - Synthesis, Structure and Chemical Bonding

Abstract

New stannides CaTSn2 (T = Rh, Pd, Ir) and Ca2PhSn5 were prepared as single phase materials by a reaction o f the elements in glassy carbon crucibles under flowing purified argon. The four compounds were investigated by X-ray diffraction both on powders and single crystals and their structures were refined from single crystal data. The stannides CaTSn2 (T = Rh, Pd, Ir) adopt the MgCuAl, structure with space group Cmcm: a = 434.1(1), b = 1081.7(3), c = 748.8(2) pm, wR2 = 0.040Ö, 451 F2 values for CaRhSn2, a = 442.7(2), b = 1113.8(4), c = 745.6(2) pm, wR2 = 0.0318, 471 F ; values for CaPdSn2, and a = 429.5(1), b = 1079.5(3), c = 758.6(2) pm, wR2 = 0.0465, 455 F2 values for CaIrSn2 with 16 variables for each refinement. Chemical bonding analysis leads to the description o f a distorted filled CaSni substructure in which the tin-tin bonding is modified by the insertion o f transition metal atoms into the planar calcium layers, favoring strong tin-transition metal bonding. 119Sn Mössbauer spectra show single signals for CaTSn2 (T = Rh, Pd, Ir) which are subjected to quadrupole splitting. The electron count o f the CaTSn2 compounds correlates with the ll9Sn isomer shift. Ca2Pt3Sn3 crystallizes with the Yb2Pt3Sn5 type structure: Pnma, a = 734.8(1), b = 445.50(7), c = 2634.8(5) pm, wR2 = 0.0636, 1406 F2 values and 62 variables. The platinum and tin atoms in Ca2Pt3 Sns build a complex three-dimensional [Pt3Sn5] polyanion in which the calcium cations fill distorted pentagonal and hexagonal channels. According to semi-empirical band structure calculations the strongest bonding interactions are found for the Pt-Sn contacts, follow ed by Sn-Sn bonding. The 119Sn Mössbauer spectrum of Ca2Pt3Sn5 shows two superimposed signals at δ = 2.10(3) and δ= 2.18(6) mm/s