Crystal Structure and Thermal Properties of Double-Complex Salts [M(1)(NH(3))(6)][M(2)(C(2)O(4))(3)] (M(1), M(2) = Co, Rh) and K(3)[Rh(NH(3))(6)][Rh(C(2)O(4))(3)](2)∙6H(2)O

Here, seven new double-complex salts, [M(1)(NH(3))(6)][M(2)(C(2)O(4))(3)] (M(1), M(2) = Co, Rh) and K(3)[Rh(NH(3))(6)][Rh(C(2)O(4))(3)](2)∙6H(2)O types, are synthesised. The crystal structure and composition of DCS (double-complex salts) are studied by SCXRD, XRD, CHN and IR methods. The complex salts of the [M(1)(NH(3))(6)][M(2)(C(2)O(4))(3)] (M(1), M(2) = Co, Rh) type can be crystallised both as a crystalline hydrate [M(1)(NH(3))(6)][M(2)(C(2)O(4))(3)]·3H(2)O (sp. gr. P-3) and as an anhydrous complex (sp. gr. P-1) depending on the synthesis conditions. The process of [Rh(NH(3))(6)][Rh(C(2)O(4))(3)] formation is significantly dependent on the synthesis temperature. At room temperature, a mixture is formed comprising [Rh(NH(3))(6)][Rh(C(2)O(4))(3)] and K(3)[Rh(NH(3))(6)][Rh(C(2)O(4))(3)](2)∙6H(2)O, while the [Rh(NH(3))(6)][Rh(C(2)O(4))(3)] target product crystallises at elevated temperatures. The thermal behaviour of double-complex salts is studied by the STA, EGA-MS, IR and XRD methods. The complete decomposition of complex salts in helium and hydrogen atmospheres resulting in metals or Co(x)Rh(1−x) solid solutions is achieved at temperatures of 320–450 °C.