Hexacyanoferrate ions

The fact that Prussian blue is indeed ferric ferrocyanide (Fe4in[Fen(CN)6]3) with iron(III) atom coordinated to nitrogen and iron(II) atom coordinated to carbon has been established by spectroscopic investigations [4], Prussian blue can be synthesized chemically by the mixing of ferric (ferrous) and hexacyanoferrate ions with different oxidation state of iron atoms either Fe3+ + [Fen(CN)6]4 or Fe2+ + [Fem(CN)6]3. After mixing, an immediate formation of the dark blue colloid is observed. However, the mixed solutions of ferric (ferrous) and hexacyanoferrate ions with the same oxidation state of iron atoms are apparently stable. 

Under traditional conditions with the hexacyanoferrate ion, the major product from (1) is the quininoid ether (3), whereas it is the minor product under phase-transfer catalytic conditions. Similarly, the carbinol (5) is oxidized to the ketone (6) by the quaternary ammonium salt, whereas the quinone (7) is obtained in the absence of the quaternary ammonium ion [5]. 

Cr . Obviously, the incorporation of the hexacyanoferrate ions is preferred. The content of hexacyanochromate in the precipitated solid compound is less than in the solution used for precipitation [35]. 

For MCD measurements, the instrument is calibrated using the natural CD of D-10-camphorsulfonic acid as a standard. The magnetic field is determined with freshly prepared hexacyanoferrate ion, Ae422//7 = 3.0 (cm M T)"1, where IT = 10000 G. When the direction of the magnetic field is parallel to the Poynting vector,... 

It has already been noted that hexacyanoferrate ions may be exchanged on to protonated and quatemized PVP (Section 57.3.2.2(ii)), and indeed other polymeric ion exchange materials may be used, e.g. poly(vinyl sulfate) and polystyrene sulfonate).72 Also, similar redox species may be anchored using chlorosilanes, e.g. (24).64... 

The low association constants show that the binding of hexacyano-ferrate to azurin prior to electron transfer is relatively weak. Still, the formation of presumably electrostatic complexes between the negatively charged hexacyanoferrate ion and azurin is somewhat surprising, as the overall net charge of azurin at pH 7.0 is also negative (p7 = 4.9). This probably implies the existence of a patch of positively charged residues on the protein surface where hexacyanoferrate ions may bind and the electron transfer takes place. A similar situation is proposed to prevail... 

With the cyanide ion, salts of the hexacyanoferrate ion and the free acid, H3[Fe(CN)6], are well known. In contrast to [Fe(CN)6]4 the [Fe(CN)6]3 ion is quite poisonous for kinetic reasons the latter dissociates and reacts rapidly, whereas the former is not labile. There are a variety of substituted ions [Fe(CN)sX], where X = H20, N02, etc., of which the best known is the nitroprusside ion20 [Fe(CN)5NO]2- this is attacked by OH- to give [Fe(CN)5N02]2-. 

K3Fe(CN), potassium ferricyanide) is an orange crystalline compound. Its solution gives a deep blue precipitate with iron(II) ions, and is used as a test for iron(II) (ferrous) compounds. Prassian blue is a blue pigment containing hexacyanoferrate ions. 

Compounds based around the hexacyanoferrate ion (principally the iron(ll) ion, [Fe(ll)(CN)6] ) form a significant and historically important group, not least because so-called Prussian blue q.v.) is a member, a term which might reasonably be considered applicable to any of the blue hexacyanoferrate(ll) pigments. 

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