Prussian blue and related compounds

3-dimensional frameworks Zn(CN)2 and Cd(CN)2 with the anti-Cu20 structure (p. 106), and Prussian blue and related compounds which are discussed in the next section. The anti-cuprite structure is shown in Fig. 22.4. As in CU2O the structure consists of two identical interpenetrating nets which are not cross-connected by any Cu-C or Cu-N bonds (see Chapter 3). 

When cyanides of certain metals, for example, Fe, Co, Mn, Cr, are redissolved by the addition of excess alkali cyanide solution, complex ions M(CN)g are formed. 

The relationship between (a) Berlin green, (b) Prussian blue, and (c) potassium 

The iron compounds have been known for several centuries and valued as pigments. The name Prussian (Berlin) blue is given to the compound formed from a solution of a ferric salt and a ferrocyanide, while that obtained from a ferrous salt and a ferricyanide has been known as Turnbull s blue. It appears from Mbssbauer studies that both of these compounds are (hydrated) Fe4 [Pe (CN)6]3. with high-spin Fe and low-spin Fe in the ratio 4 3. There is also a soluble Prussian blue with the composition KFe[Fe(CN)6] which is also a ferrocyanide. Most, if not all, of the compounds we are discussing are hydrated we consider the water of hydration later. 

An early X-ray study of Prussian blue and some related compounds showed that in ferric ferricyanide (Berlin green), FeFe(CN)6, Prussian blue, KFeFe(CN)6. and the white insoluble K2FeFe(CN)6. there is the same arrangement of Fe atoms on a cubic face-centred lattice. In Fig. 22.5 ferrous atoms are distinguished as shaded and ferric as open circles. In (a) all the iron atoms are in the ferric state in (b) one-half the atoms are Fe and the others Fe, and alkali atoms maintain electrical neutrality. These are at the centres of alternate small cubes, and it was supposed that in hydrated compounds water molecules could also be accommodated in the interstices of the main framework. Lithium and caesium, forming 

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J.E Keggin and F.D. Miles, Structure and formulae of the Prussian blue and related compounds. Nature 137, 577-578 (1936). 

Fig. 24-1. Iron-Cyanide Framework in Prussian Blue and Related Compounds...

Figure 4 The structure of Prussian blue and related compounds. If none of the cube centre sites are occupied, the structure is that of ferric ferricyanide (both black and white Fe positions occupied by Fe111) if every second cube centre site (marked with a dotted circle) is occupied by K+, the structure is that of soluble Prussian blue (black = Fe11, white = Fe111) if all the centre sites are occupied by K+ (crosses as well as dotted circles) the structure is that of dipotassium ferrous ferrocyanide...

Keggin, J.R and Miles, F.D., Structure and formulae of the prussian blues and related compounds, Nature, 58, 577,1936. 

Earliest examples of structurally investigated coordination polymers dates back to 1936, where Keggin and Miles reported the structure of Prussian blue and related compounds, and Griffth in 1943, who reported the crystal structure of silver oxalate. ... 

The electrochemical processes involving Prussian blue and organic dyes studied above can be taken as examples of solid state redox processes involving transformation of a one solid compound into another one. This kind of electrochemical reactions are able to be used for discerning between closely related organic dyes. As previously described, the electrochemistry of solids that are in contact with aqueous electrolytes involves proton exchange between the solid and the electrolyte, so that the electrochemical reaction must in principle be confined to a narrow layer in the external surface of the solid particles. Eventually, however, partial oxidative or reductive dissolution processes can produce other species in solution able to react with the dye. 

The redox chemistry of the Prussian blue family (Table 7) has attracted considerable attention. The generation of thin films of Prussian blue has led to studies of its mediation in electron transfer reactions and of the electrochemical processes involved in its deposition and redox reactions. This work has been spurred by its electrochromic properties which have been used in prototype electronic display devices based, for example, on Prussian blue modified Sn02 electrodes. A recent review deals with the electrochemistry of electrodes modified by depositing thin films of PB and related compounds on them. Interestingly, true Prussian blue is somewhat difficult to process and modern iron blue pigments such as Milori blue are derived from the oxidation of rlin white Fe(NH4)2[Fe(CN)e] to give iron(III) ammonium ferrocyanides. 

According to Heaton (1928), daylight blue was similar to Brunswick blue in being Prussian blue (blue hexacyanoferrate compounds) struck onto a base such as baryte or barium sulfate qq.v.), though as a pale tint of 5-10%. Seemingly synonymous with celestial blue and related to damp blue (Davidson, 1880). 

The intense blue color of Prussian Blue is attributed to electron transfer between the [Fe(CN) ]4 and Fe(III) ions. A related pigment called Berlin Green is obtained by oxidation of Prussian Blue. It is thought that the intense color of this other compound results only if oxidation of the fFe(CN)6] units is incomplete and some remain as hexakiscyanoferrate(4—). The compound in which only iron(III) is present, Fe[Fe(CN)6] [14433-93-3], is brown and is subject to autoreduction processes. 

Prussian blue — iron(III) hexacyanoferrate(II) is the archetype of sparingly soluble mixed valence polymeric metal hexacyanometalates with the formula Me Me(N) [Me c (CN)6] with (i), (N), and (C) indicating the position in the crystal lattice, where (i) means interstitial sites, (N) means metal coordinated to the nitrogen of the cyanides, and (C) means metal ions coordinated to the carbon of the cyanides. It is one of the oldest synthetically produced coordination compounds and was widely used as pigment in paints because of the intensive blue color. The compound has been studied extensively by electrochemical and other methods. The importance of Prussian blue in electrochemistry is related to the fact that it has two redox-active metal centers and that it has an open structure that allows small cations to... 

The use of octahedral building blocks is perhaps one of the most obvious and simplest strategies for the assembly of molecule-based three-dimensional solids. In this context, an examination of suitable octahedral [MLg] precursors reveals that some of the most inert are the hexacyano-metalate anions [M (CN)6] " (M = Cr, Mn, Fe, Co), which can be combined with divalent Lewis acids M (M = Cu, Ni, Co, Fe, Mn) to give face-centered cubic compounds closely related to Prussian blue (Figure 3). 

Paris blue, along with Berlin blue and Prussian blue (qq.v.), can be found in German hterature sources of the nineteenth and early twentieth centuries. The relationship between the three terms is somewhat complex for more information, see Berlin blue. In addition to describing Paris blue as a synonym for the violet-tinted kind of Prussian blue , Terry (1893) describes three compositions based on calcined mixtures of (a) sulfur, sodium carbonate, sodium sihcate and sodium aluminate, (b) china clay, sodium sulfate, sodium carbonate, sulfur and charcoal, and (c) sodium carbonate, orpiment, gelatinous alumina hydrate , clay and sulfur these appear likely to form ultramarine-related compounds (q.v.). 

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