Ternary Polar Intermetallics within the Pt/Sn/R Systems (R = La-Sm): Stannides or Platinides?

Starting generally with a 4:6:3 molar ratio of Pt, Sn, and R (where R = La-Sm), with or without the application of a NaCl flux, seven ternary compounds were obtained as single crystals. The platinides Pt₄Sn₆R₃ (R = La-Nd) crystallize with the Pt₄Ge₆Pr₃ type of structure (oP52, Pnma, a = 27.6-27.8 Å, b = 4.59-4.64 Å, c = 9.33-9.40 Å). With R = Pr, Pt₄Sn₆Pr_3- (oP52, Pnma, a = 7.2863(3) Å, b = 4.4909(2) Å, c = 35.114(1) Å) is also obtained, which might be considered a high-temperature polymorph with disorder on the Sn- and Pr-sites. For R = Nd and Sm, a structurally related isostructural series with a slightly different composition Pt₃Sn₅R_2- (oP52, Cmc2₁, a = 4.50-4.51 Å, b = 26.14-26.30 Å, c ≈ 7.29 Å) has been observed, together with Pt₇Sn₉Sm₅ (oS42, Amm2, a = 4.3289(5) Å, b = 28.798(4) Å, c = 7.2534(9) Å) under the same conditions. The latter exhibits the rare Zr₅Pd₉P₇-type structure, linking polar intermetallics to metal phosphides, in accord with P₇Pd₉Zr₅≡Pt₇Sn₉Sm₅. All structures may be described in terms of either negative Pt/Sn networks encapsulating positive R atoms, or {PtSn} clusters (x = 5, 6, or rarely 7) sharing vertices and edges with R in the second coordination sphere and with considerable heterometallic Pt-R bonding contributions.