The preparation of Cs 3 Mo(CN) 8 ·2H 2 O and Cs 3 W(CN) 8 ·2H 2 O

[HN(n‐C 4 H 9 ) 3 ] 3 M(CN) 8 4 H 2 O, where M = Mo or W, is formed as a light yellow or white water insoluble precipitate by adding tri‐n‐butylammonium ions to M(CN)   8 3− , obtained by oxidation of K 4 Mo(CN) 8 · 2H 2 O or K 4 W(CN) 8 ·2H 2 O with concentrated nitric acid. The precipitates are soluble in methyl and ethyl alcohol, benzene and acetonitrile. Crystal data were determined for crystals of the W v compound. The tri‐n‐butylammonium compound can be nearly quantitatively converted to Cs 3 M(CN) 8 · 2H 2 O with CsCl, which is fairly soluble in methyl alcohol. Crystal data were determined for light yellow crystals obtained by recrystallization. The potassium complex of W v can be prepared by using potassium acetate instead of CsCl.