in pentane educts mixed dropwise at -78°C, stirred for 7 h; ppt. filtered, washed with pentane, filtrate concd. at - 40°C, allowed to stand for 6 d at -78°C, pptn., mother liquor siphoned off, residue dissolved in pentane at -20°C and frozen out at -78°C, dried in vac., elem. anal.;