Carbonyl Penta-Ferrocyanides, M₃[Fe(CN)₅CO]

Addition of ferric chloride to certain liquors produced in the manufacture of Prussian blue in a French factory by the methylamine method resulted in the precipitation of a violet compound. Several hundred grams of this were isolated by Muller, warmed with potassium carbonate and hydroxide successively, and the filtered solution allowed to crystallise. The product thus isolated crystallised in thin scales and rectangular prisms, and proved to be the potassium salt of an entirely new acid, namely hydrogen carbonyl ferrocyanide, H₃Fe(CN)₅.CO. Following up this discovery, Muller succeeded in preparing a series of well-defined salts.

The constitution to be assigned to these derivatives depends very largely upon the views accepted for the constitution of ferrocyanides in general. The carbonyl group clearly replaces а С:NH group in the ferrocyanide molecule, or, regarded from another standpoint, divalent oxygen replaces the divalent NH group without effecting any alteration in the carbon nucleus of the molecule. Adopting his cyclic formula for ferrocyanic acid, Friedel therefore suggests the following graphical representation of the change: -

	→
(ferrocyanide)		(carbonyl ferrocyanide).