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Ferrous cyanide, Fe(NC)2
Ferrous cyanide, Fe(NC)2, is obtained as a yellow powder on heating hydrogen ferroeyanide, H4Fe(CN)6, in the absence of air to 300° C. Hydrogen cyanide is evolved. It is also obtained on heating ammonium ferroeyanide in the absence of air. Ferrous cyanide is stable, in the absence of oxygen, but even in cold air it rapidly becomes warm, and when gently heated it glows, yielding ferric oxide.
The constitution of ferrous cyanide is discussed by Browning, who suggests that it is really an isocyanide, for, when warmed with potassium ethyl sulphate in a current of hydrogen, it yields ethyl isocyanide. The formula for ferrous cyanide is thus Fe(NC)2, rather than Fe(CN)2.
The constitution of ferrous cyanide is discussed by Browning, who suggests that it is really an isocyanide, for, when warmed with potassium ethyl sulphate in a current of hydrogen, it yields ethyl isocyanide. The formula for ferrous cyanide is thus Fe(NC)2, rather than Fe(CN)2.
Ferric cyanide
Ferric cyanide is not known, but large numbers of complex derivatives of both ferrous and ferric cyanides have been prepared in which the iron enters into the negative radicle. These are known as ferro- and ferri-cyanides respectively.
Constitution of Ferro- and Ferri-cyanides
The constitution of hydrogen and alkali ferrocyanides has for many years been a matter of dispute. Both Graham and Erlenmeyer believed that the cyanogen radicles were present in groups of three, as in cyanuric acid, C3N3(OH)3, so that the formula for hydrogen ferro- cyanide becomes: -
Etard and Bemont and Friedel suggested the cyclic formula: -
whilst Browning writes hydrogen ferrocyanide as: -
The results obtained for the osmotic pressures and electric conductivities of aqueous solutions of calcium and strontium ferrocyanides indicate that these molecules possess the double formula, M4[Fe(CN)6]2. On the other hand, tetra-ethyl ferrocyanide is known to have the single formula, (C2H5)4Fe(CN)6.
Each of the two latter graphical formulae as written above assumes that the iron and hydrogen are attached directly to the nitrogen and not to the carbon atoms. This is supported, in so far as the hydrogen atoms are concerned, by Freund's preparation of ethyl ferrocyanide from silver ferrocyanide and ethyl iodide: -
4C2H6I + Ag4[Fe(CN)6] = 4AgI + (C2H5)4[Fe(CN)6].
The ester, on heating, decomposed, yielding ethyl isocyanide, C2H5NC.
As has already been mentioned, hydrogen ferrocyanide, when heated in the absence of air, yields ferrous cyanide, and if the isocyanide constitution be accepted for this salt, it may be inferred that the same grouping, namely Fe(NC)2, occurs in hydrogen ferrocyanide.
To this extent, therefore, the two foregoing formulae agree, but Browning suggests that his formula is probably more nearly correct as it offers a more ready explanation for the formation of nitroprussides, etc., no rupture of a hexatomic ring being necessary. Thus, potassium nitroprusside becomes: -
Neither formula, however, explains the fact that the potassium atoms are labile, whilst the iron atom is not. Deniges overcomes this difficulty by the formula: -
In common with Browning's formula, this scheme admits of the ready formation of nitroprussides without disrupting a ring. Furthermore, since the iron is not attached in the same way as the potassium atoms, being linked to carbon instead of nitrogen, a difference in stability may reasonably be expected.
The weakness of this formula lies in the fact that the only available evidence on the subject points to the conclusion that the iron is attached directly to nitrogen, and not to the carbon as here represented, ferrous cyanide having the isocyanic structure.
According to Werner's theory, the formulae of hydrogen ferro- and ferri-cyanide should be written [Fe(CN)6]H4 and [Fe(CN)6]H3 respectively, the six cyanogen groups being co-ordinated with the iron atom, and constituting the nucleus around which hover the replaceable hydrogen atoms.
In 1911 Briggs believed that he had succeeded in isolating two isomerides of potassium ferrocyanide, and suggested their representation by the following stereo-isomeric schemes: -
although it was not possible to determine which isomeride corresponded to each particular scheme.
Bennett, however, dissented from this view. Having prepared the two "isomerides" according to the directions given by Briggs and measured their angles, he concluded that there can be no doubt of their crystallographic identity. Briggs therefore reinvestigated the problem, and showed that the so-called ferrocyanides are in reality mixed crystals of ferrocyanide and aquopentacyanoferrite, K3[Fe(CN)5H2O].
There still remains, however, the possibility of isomerism of the inorganic ferrocyanides being ultimately discovered, comparable with the undoubted isomerism manifested by the tetramethyl salt, (CH3)4Fe(CN)6.
Now, Werner's formula does not admit of the possibility of isomerism of ferrocyanides. This is not the case, however, with the formulse of Deniges, Browning, and of Etard, several rearrangements being possible. According to Friend's shell theory three isomerides, namely, ortho, meta, and para, are possible. Thus: -
In an analogous manner three isomerides are, theoretically, possible for potassium ferricyanide, K3Fe(CN)6, in which the central iron atom is trivalent. All of these cyclic schemes are in harmony with the isocyanic structure of ferrous cyanide. They also serve to explain why the potassium ions are ionisable, whereas the iron atom, bound within the centre of the shell, constitutes part of the negative radicle.
When a solution of potassium ferrocyanide reacts with rather less than one equivalent of a ferric salt, a blue hydrated precipitate of a-soluble Prussian blue, or ferric potassium ferrocyanide Fe•••K[Fe••(CN)6], is obtained. Now, Hofmann and his co-workers have shown that this precipitate is identical with that prepared under precisely similar conditions by the addition of a ferrous salt to potassium ferricyanide, although in this case ferrous potassium ferricyanide, Fe••К [Fe•••(CN)6], might be expected. It is therefore assumed that the latter salt is unstable, and, at the moment of formation, undergoes intramolecular rearrangement to the former complex.
By adopting the shell formulae, however, the difficulty at once disappears, for the two salts are seen to be identical without rearrangement. For example, 1, 4 ferrocyanide reacts with a ferric salt yielding 2, 3, 5 ferric 6 potassium 1, 4 ferrocyanide, which is clearly the same as 1, 4 ferrous 6 potassium 2,3, 5 ferricyanide obtained by the action of a ferrous salt on potassium 1, 2, 4 ferricyanide. Thus: -
Etard and Bemont and Friedel suggested the cyclic formula: -
whilst Browning writes hydrogen ferrocyanide as: -
The results obtained for the osmotic pressures and electric conductivities of aqueous solutions of calcium and strontium ferrocyanides indicate that these molecules possess the double formula, M4[Fe(CN)6]2. On the other hand, tetra-ethyl ferrocyanide is known to have the single formula, (C2H5)4Fe(CN)6.
Each of the two latter graphical formulae as written above assumes that the iron and hydrogen are attached directly to the nitrogen and not to the carbon atoms. This is supported, in so far as the hydrogen atoms are concerned, by Freund's preparation of ethyl ferrocyanide from silver ferrocyanide and ethyl iodide: -
4C2H6I + Ag4[Fe(CN)6] = 4AgI + (C2H5)4[Fe(CN)6].
The ester, on heating, decomposed, yielding ethyl isocyanide, C2H5NC.
As has already been mentioned, hydrogen ferrocyanide, when heated in the absence of air, yields ferrous cyanide, and if the isocyanide constitution be accepted for this salt, it may be inferred that the same grouping, namely Fe(NC)2, occurs in hydrogen ferrocyanide.
To this extent, therefore, the two foregoing formulae agree, but Browning suggests that his formula is probably more nearly correct as it offers a more ready explanation for the formation of nitroprussides, etc., no rupture of a hexatomic ring being necessary. Thus, potassium nitroprusside becomes: -
Neither formula, however, explains the fact that the potassium atoms are labile, whilst the iron atom is not. Deniges overcomes this difficulty by the formula: -
In common with Browning's formula, this scheme admits of the ready formation of nitroprussides without disrupting a ring. Furthermore, since the iron is not attached in the same way as the potassium atoms, being linked to carbon instead of nitrogen, a difference in stability may reasonably be expected.
The weakness of this formula lies in the fact that the only available evidence on the subject points to the conclusion that the iron is attached directly to nitrogen, and not to the carbon as here represented, ferrous cyanide having the isocyanic structure.
According to Werner's theory, the formulae of hydrogen ferro- and ferri-cyanide should be written [Fe(CN)6]H4 and [Fe(CN)6]H3 respectively, the six cyanogen groups being co-ordinated with the iron atom, and constituting the nucleus around which hover the replaceable hydrogen atoms.
In 1911 Briggs believed that he had succeeded in isolating two isomerides of potassium ferrocyanide, and suggested their representation by the following stereo-isomeric schemes: -
although it was not possible to determine which isomeride corresponded to each particular scheme.
Bennett, however, dissented from this view. Having prepared the two "isomerides" according to the directions given by Briggs and measured their angles, he concluded that there can be no doubt of their crystallographic identity. Briggs therefore reinvestigated the problem, and showed that the so-called ferrocyanides are in reality mixed crystals of ferrocyanide and aquopentacyanoferrite, K3[Fe(CN)5H2O].
There still remains, however, the possibility of isomerism of the inorganic ferrocyanides being ultimately discovered, comparable with the undoubted isomerism manifested by the tetramethyl salt, (CH3)4Fe(CN)6.
Now, Werner's formula does not admit of the possibility of isomerism of ferrocyanides. This is not the case, however, with the formulse of Deniges, Browning, and of Etard, several rearrangements being possible. According to Friend's shell theory three isomerides, namely, ortho, meta, and para, are possible. Thus: -
 |  |  |
| 1, 2, or ortho, salt. | 1, 3, or meta, salt. | 1, 4, or para, salt. |
In an analogous manner three isomerides are, theoretically, possible for potassium ferricyanide, K3Fe(CN)6, in which the central iron atom is trivalent. All of these cyclic schemes are in harmony with the isocyanic structure of ferrous cyanide. They also serve to explain why the potassium ions are ionisable, whereas the iron atom, bound within the centre of the shell, constitutes part of the negative radicle.
When a solution of potassium ferrocyanide reacts with rather less than one equivalent of a ferric salt, a blue hydrated precipitate of a-soluble Prussian blue, or ferric potassium ferrocyanide Fe•••K[Fe••(CN)6], is obtained. Now, Hofmann and his co-workers have shown that this precipitate is identical with that prepared under precisely similar conditions by the addition of a ferrous salt to potassium ferricyanide, although in this case ferrous potassium ferricyanide, Fe••К [Fe•••(CN)6], might be expected. It is therefore assumed that the latter salt is unstable, and, at the moment of formation, undergoes intramolecular rearrangement to the former complex.
By adopting the shell formulae, however, the difficulty at once disappears, for the two salts are seen to be identical without rearrangement. For example, 1, 4 ferrocyanide reacts with a ferric salt yielding 2, 3, 5 ferric 6 potassium 1, 4 ferrocyanide, which is clearly the same as 1, 4 ferrous 6 potassium 2,3, 5 ferricyanide obtained by the action of a ferrous salt on potassium 1, 2, 4 ferricyanide. Thus: -
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