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.