Chemical elements
  Iron
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    Mineralogy
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    Physical Properties
    Chemical Properties
      Iron Hydride
      Ferrous fluoride
      Aluminium pentafluoferrite
      Ferric fluoride
      Ammonium ferrifluoride
      Barium ferrifluoride
      Potassium ferrifluoride
      Sodium ferrifluoride
      Thallous ferrifluoride
      Ferrous diferrifluoride
      Ferrous monoferrifluoride
      Ferrous chloride
      Ammonium tetrachlorferrite
      Ferric chloride
      Tetrachlorferrates
      Pentachlorferrates
      Ferroso-ferric chloride
      Ferrous perchlorate
      Ferric perchlorate
      Ferrous chlorate
      Ferric chlorate
      Ferrous Oxychlorides
      Ferrous bromide
      Ferric bromide
      Ferric chloro-bromide
      Ferrous bromate
      Ferrous iodide
      Ferric iodide
      Ferric iodate
      Ferrous oxide
      Ferrous hydroxide
      Triferric tetroxide
      Ferric oxide
      Ferrous acid
      Calcium ferrite
      Cobalt ferrite
      Cupric ferrite
      Cuprous ferrite
      Magnesium ferrite
      Nickel ferrite
      Potassium ferrite
      Sodium ferrite
      Zinc ferrite
      Barium ferrate
      Strontium ferrate
      Barium perferrate
      Calcium perferrate
      Potassium perferrate
      Sodium perferrate
      Strontium perferrate
      Iron Subsulphides
      Ferrous sulphide
      Ferric sulphide
      Potassium ferric sulphide
      Sodium ferric sulphide
      Cuprous ferric sulphide
      Iron disulphide
      Ferrous sulphite
      Ferric sulphite
      Potassium ferri-tetrasulphite
      Potassium ferri-disulphite
      Potassium ferri-sulphite
      Ammonium ferri-sulphite
      Sodium ferri-disulphite
      Sodium hydrogen ferri-tetrasulphite
      Ferrous sulphate
      Ferrous copper sulphate Fe
      Ferrous ammonium sulphate
      Ferrous potassium sulphate
      Ferrous aluminium sulphate
      Basic ferrous sulphate
      Ferric sulphate
      Ammonium ferri-disulphate
      Trisodium ferri-trisulphate
      Ferric Alums
      Ferric ammonium alum
      Ferric potassium alum
      Ferric rubidium alum
      Ferroso-ferric sulphate
      Ferrous amido-sulphonate
      Ferric amido-sulphonate
      Ferrous thiosulphate
      Ferrous pyrosulphate
      Ferrous tetrathionate
      Ferric selenide
      Iron diselenide
      Iron Selenites
      Ferrous selenate
      Ferric rubidium selenium alum
      Ferric caesium selenium alum
      Ferric tellurite
      Ferrous chromite
      Ferrous chromate
      Iron nitride
      Nitro-Iron
      Ferrous nitrate
      Ferric nitrate
      Ferrous Nitroso Salts
      Potassium ferro-heptanitroso sulphide
      Sodium ferro-heptanitroso sulphide
      Ammonium ferro-heptanitroso sulphide
      Tetramethyl ammonium ferro-heptanitroso sulphide
      Ferro-dinitroso Sulphides
      Potassium ferro-dinitroso thiosulphate
      Triferro phosphide
      Diferro phosphide
      Iron monophosphide
      Iron sesqui-phosphide
      Ferrous hypophosphite
      Ferric hypophosphite
      Ferrous phosphite
      Ferric phosphite
      Ferrous orthophosphate
      Ferrous hydrogen orthophosphate
      Ferrous dihydrogen orthophosphate
      Ferric orthophosphate
      Sodium ferri-diorthophosphate
      Ammonium ferri-diorthophosphate
      Sodium ferri-triorthophosphate
      Ferric dihydrogen orthophosphate
      Acid ferric orthophosphate
      Ferrous metaphosphate
      Ferric metaphosphate
      Ferrous pyrophosphate
      Ferric pyrophosphate
      Hydrogen ferri-pyrophosphate
      Sodium ferro-pyrophosphate
      Ferrous thio-orthophosphite
      Ferrous thio-orthophosphate
      Ferrous thio-pyrophosphite
      Ferrous thio-pyrophosphate
      Iron sub-arsenide
      Iron mon-arsenide
      Iron sesqui-arsenide
      Iron di-arsenide
      Iron thio-arsenide
      Ferrous met-arsenite
      Ferric arsenite
      Ferrous ortho-arsenate
      Ferric ortho-arsenate
      Ferro mono-antimonide
      The di-antimonide
      Ferrous thio-antimonite
      Ferric ortho-antimonate
      Triferro carbide
      Diferro carbide
      Iron dicarbide
      Iron pentacarbonyl
      Diferro nonacarbonyl
      Iron tetracarbonyl
      Ferrous carbonate
      Ferrous bicarbonate
      Ferrous potassium carbonate
      Complex Iron Carbonates
      Ferrous thiocarbonate
      Ferrous thiocarbonate hexammoniate
      Ferrous cyanide
      Ferro-cyanic acid
      Aluminium ferrocyanide
      Aluminium ammonium ferrocyanide
      Ammonium ferrocyanide
      Barium ferrocyanide
      Calcium ferrocyanide
      Calcium ammonium ferrocyanide
      Cobalt ferrocyanide
      Copper ferrocyanide
      Ammonium cuproferrocyanide
      Barium cuproferrocyanide
      Lithium cuproferrocyanide
      Magnesium cuproferrocyanide
      Potassium cuproferrocyanide
      Sodium cuproferrocyanide
      Ammonium cupriferrocyanide
      Potassium cupriferrocyanide
      Potassium ferrous cupriferrocyanide
      Sodium cupriferrocyanide
      Strontium cupriferrocyanide
      Lithium ferrocyanide
      Magnesium ferrocyanide
      Magnesium ammonium ferrocyanide
      Manganese ferrocyanide
      Nickel ferrocyanide
      Potassium ferrocyanide
      Potassium aluminium ferrocyanide
      Potassium barium ferrocyanide
      Potassium calcium ferrocyanide
      Potassium cerium ferrocyanide
      Potassium magnesium ferrocyanide
      Potassium mercuric ferrocyanide
      Silver ferrocyanide
      Sodium ferrocyanide
      Sodium cerium ferrocyanide
      Strontium ferrocyanide
      Thallium ferrocyanide
      Zinc potassium ferrocyanide
      Ferricyanic acid
      Ammonium ferricyanide
      Barium ferricyanide
      Barium potassium ferricyanide
      Calcium ferricyanide
      Calcium potassium ferricyanide
      Cobalt ferricyanide
      Copper ferricyanide
      Lead ferricyanide
      Magnesium ferricyanide
      Mercuric ferricyanide
      Mercurous ferricyanide
      Potassium ferricyanide
      Sodium ferricyanide
      Strontium ferricyanide
      Zinc ferricyanide
      Ferrous hydrogen ferrocyanide
      Ferrous potassium ferrocyanide
      Prussian Blues
      Ferrous ferrocyanide
      Ferric ammonium ferrocyanide
      Nitroprussic acid
      Sodium nitroprusside
      Ammonium nitroprusside
      Barium nitroprusside
      Cobalt nitroprusside
      Nickel nitroprusside
      Potassium nitroprusside
      Carbonyl Penta-Ferrocyanides
      Carbonyl ferrocyanic acid
      Barium carbonyl ferrocyanide
      Copper carbonyl ferrocyanide
      Ferric carbonyl ferrocyanide
      Potassium carbonyl ferrocyanide
      Silver carbonyl ferrocyanide
      Sodium carbonyl ferrocyanide
      Strontium carbonyl ferrocyanide
      Uranyl carbonyl ferrocyanide
      Sodium ammonio ferrocyanide
      Potassium aquo ferrocyanide
      Potassium aquo ferricyanide
      Sodium aquo penta-ferricyanide
      Potassium sulphito ferrocyanide
      Ferrous thiocyanate
      Ferric thiocyanate
      Sodium ferrothiocyanate
      Sodium ferrithiocyanate
      Potassium ferrithiocyanate
      Iron subsilicide
      Iron monosilicide
      Iron disilicide
      Triferro disilicide
      Ferrous orthosilicate
      Ferrous magnesium orthosilicate
      Ferrous metasilicate
      Ferric silicate
      Diferro boride
      Iron monoboride
      Iron diboride
      Ferrous chlorborate
      Ferrous bromborate
    Corrosion
    Iron Salts
    PDB 101m-1aeb
    PDB 1aed-1awd
    PDB 1awp-1beq
    PDB 1bes-1c53
    PDB 1c6o-1ci6
    PDB 1cie-1cry
    PDB 1csu-1dfx
    PDB 1dgb-1dry
    PDB 1ds1-1e08
    PDB 1e0z-1ehj
    PDB 1ehk-1f5o
    PDB 1f5p-1fnp
    PDB 1fnq-1fzi
    PDB 1g08-1gnl
    PDB 1gnt-1h43
    PDB 1h44-1hdb
    PDB 1hds-1i5u
    PDB 1i6d-1iwh
    PDB 1iwi-1jgx
    PDB 1jgy-1k2o
    PDB 1k2r-1kw6
    PDB 1kw8-1lj0
    PDB 1lj1-1m2m
    PDB 1m34-1mko
    PDB 1mkq-1mun
    PDB 1muy-1n9x
    PDB 1naz-1nx4
    PDB 1nx7-1ofe
    PDB 1off-1p3t
    PDB 1p3u-1pmb
    PDB 1po3-1qmq
    PDB 1qn0-1ra0
    PDB 1ra5-1rxg
    PDB 1ry5-1smi
    PDB 1smj-1t71
    PDB 1t85-1u8v
    PDB 1u9m-1uyu
    PDB 1uzr-1vxf
    PDB 1vxg-1wri
    PDB 1wtf-1xlq
    PDB 1xm8-1y4r
    PDB 1y4t-1ygd
    PDB 1yge-1z01
    PDB 1z02-2a9e
    PDB 2aa1-2azq
    PDB 2b0z-2boz
    PDB 2bpb-2ca3
    PDB 2ca4-2cz7
    PDB 2czs-2dyr
    PDB 2dys-2ewk
    PDB 2ewu-2fwl
    PDB 2fwt-2gl3
    PDB 2gln-2hhb
    PDB 2hhd-2ibn
    PDB 2ibz-2jb8
    PDB 2jbl-2mgh
    PDB 2mgi-2o01
    PDB 2o08-2ozy
    PDB 2p0b-2q0i
    PDB 2q0j-2r1h
    PDB 2r1k-2spm
    PDB 2spn-2vbd
    PDB 2vbp-2vzb
    PDB 2vzm-2wiv
    PDB 2wiy-2xj5
    PDB 2xj6-2ylj
    PDB 2yrs-2zon
    PDB 2zoo-3a17
    PDB 3a18-3aes
    PDB 3aet-3bnd
    PDB 3bne-3cir
    PDB 3ciu-3dax
    PDB 3dbg-3e1p
    PDB 3e1q-3eh4
    PDB 3eh5-3fll
    PDB 3fm1-3gas
    PDB 3gb4-3h57
    PDB 3h58-3hrw
    PDB 3hsn-3ir6
    PDB 3ir7-3k9y
    PDB 3k9z-3l4p
    PDB 3l61-3lxi
    PDB 3lyq-3mm8
    PDB 3mm9-3n62
    PDB 3n63-3nlo
    PDB 3nlp-3o0f
    PDB 3o0r-3p6o
    PDB 3p6p-3prq
    PDB 3prr-3sel
    PDB 3sik-3una
    PDB 3unc-4blc
    PDB 4cat-4erg
    PDB 4erm-4nse
    PDB 4pah-8cat
    PDB 8cpp-9nse

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.


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: -

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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