Ferrous Nitroso Salts

Ferrous salts readily combine in solution with nitric oxide to form nitroso derivatives, in which one molecule of nitric oxide is combined for each atom of ferrous iron. These substances, several of which are described in connection with their respective negative radicles, readily dissociate in solution in accordance with the reversible reaction: -

Fe(NO)^•• ⇔ Fe^•• + NO.

By lowering the temperature, by raising the pressure, or by increasing the concentration of the free acid, the equilibrium is shifted from right to left, the amount of the nitroso salt being proportionately increased, but never in excess of the above-mentioned ratio.1 When the increase is due to addition of acids, the colour of the solution changes, and the absorption band in the yellow region of the spectrum disappears. This is indicative of a change of constitution, complex anions being formed which are more stable than the nitroso cations.

Nitroso derivatives of a more complex and more stable character than the foregoing were discovered by Roussin in 1858. This investigator observed that a black voluminous precipitate is obtained when a mixture of ammonium sulphide and alkali nitrite is added to an aqueous solution of ferrous sulphate. On boiling, the precipitate passes into solution. The liquid is filtered, and, upon cooling, black crystals separate out, the composition of which has been the subject of considerable discussion. The reaction does not proceed in perfectly neutral solution, a green liquor only being produced, consisting of sulphides of iron and sodium, entirely free from any nitroso derivative. The presence of a small quantity of acid, however, results in the formation of the nitroso derivative, probably because it liberates nitrous acid, which acts direct upon the ferrous salt.

Roussin, as the result of his analyses, attributed to the nitroso derivative a formula which, translated into modern equivalents, becomes H₂Fe₃(NO)₄S₅.

In 1882, however, Pavel reconsidered his formula, and suggested

MFe₄(NO)₇S₃,

where M represents ammonium or one of the alkali metals. This formula has since been accepted by the majority of workers in this field.

Cryoscopic measurements of the sodium and potassium salts give values for the molecular weights equal to half those corresponding to the simple formula MFe₄(NO)₇S₃, and thus indicate that the salts are completely dissociated into the ions M^• and Fe₄(NO)₇S'₃; also electric conductivity measurements show that the salts are derived from a monobasic acid, and that they have the simple molecular formula: -

[MFe₄(NO)₇S₃]_n, where n = 1.

The ferro-heptanitroso sulphides constitute a well-defined series of crystalline salts. In so far as the salts of the alkali metals are concerned, the order of their solubilities in water is as follows: -

The potassium salt is the most soluble, whilst the caesium salt is quite insoluble in water. Both the rubidium and the caesium salts may be prepared by double decomposition of dilute solutions of their chlorides with the nitroso salts of sodium or potassium. The thallium salt may be prepared in a similar manner by means of thallium sulphate. The nitroso salts of the alkaline earth metals - calcium, barium, and magnesium - are very soluble in water.