Iron complex compounds, anions potassium salt

Iron (II) complex compounds, anions, carbonyl, H2Fe(CO)4, and potassium salt, 2 243, 244... 

These two simple salts are not known but with hydrocyanic acid, with potassium cyanide, with themselves or with each other, they form well known double cyanides. These double cyanides, however, are really simple compounds which dissociate, in solution, into two ions, viz., the cation, of hydrogen or the metal, and a complex anion, consisting of iron and the cyanogen group. 

---