in hydrogen fluoride Cs2NiF6 was placed in trap, suspended at -60°C in ClF5, HF was condensed at -60°C, AsF5 was added, stirred at -60°C, 0.5 h at -60-40°C, 14 h at -60-10°C; volatiles removed in vac. at 25°C, suspended in HF at 25.degre.C,filtered under Ar, cooled to -196°C under vac., stirred with HF at 25°C, filtered at -78°C under Ar, washed with HF at -78 °C, dried in vac. for 12 h;