Cyanides formation constants

Hexa.cya.no Complexes. Ferrocyanide [13408-63 ] (hexakiscyanoferrate-(4—)), (Fe(CN) ) , is formed by reaction of iron(II) salts with excess aqueous cyanide. The reaction results in the release of 360 kJ/mol (86 kcal/mol) of heat. The thermodynamic stabiUty of the anion accounts for the success of the original method of synthesis, fusing nitrogenous animal residues (blood, horn, hides, etc) with iron and potassium carbonate. Chemical or electrolytic oxidation of the complex ion affords ferricyanide [13408-62-3] (hexakiscyanoferrate(3—)), [Fe(CN)g] , which has a formation constant that is larger by a factor of 10. However, hexakiscyanoferrate(3—) caimot be prepared by direct reaction of iron(III) and cyanide because significant amounts of iron(III) hydroxide also form. Hexacyanoferrate(4—) is quite inert and is nontoxic. In contrast, hexacyanoferrate(3—) is toxic because it is more labile and cyanide dissociates readily. Both complexes Hberate HCN upon addition of acids. 

The basis for the toxicological activity of this substance is the reaction of cobalt ion with cyanide ion to form a relatively nontoxic and stable ion complex. The hexacyanocobaltate ion contains a Co2+ central metal ion with six cyanide ions as ligands. This coordination complex involves six coordinate covalent bonds whereby each cyanide ion supplies a pair of electrons to form each covalent bond with the central cobalt ion. The formation constant for the hexacyanocobaltate ion is even larger than for dicobalt EDTA,3 and thus the cobalt ion preferentially exchanges an EDTA ligand for six cyano ligands ... 

Chelating agents, 5 708-739 12 61, 122 applications, 5 731-732 bifunctional, 5 7236 classes of, 5 712-713t concentration formation constants of metal chelates, 5 717t cyanide applications, 8 183 dispersants contrasted, 8 686 economic aspects, 5 729-730 environmental, health, and safety factors, 5 731... 

It should be noted that the larger the formation constant the more stable the complex. Consider the following example of Ni+ complexes with cyanide and ammonia ... 

The effect of halide, cyanate, cyanide, and thiocyanate ions on the partitioning of Hg in [BMIM][PF6]/aqueous systems (Figure 3.3-2) has been studied [8]. The results indicate that the metal ion transfer to the IL phase depends on the relative hydrophobicity of the metal complex. Hg-I complexes have the highest formation constants, decreasing to those of Hg-F [42]. Results from pseudohalides, however, suggest a more complex partitioning mechanism, since Hg-CN complexes have even higher formation constants [42], but display the lowest distribution ratios. 

The [Ni(CN)4]2 anion is one of the most stable nickel(II) complexes and an overall formation constant as high as about 1030 has been determined.627,62 The structure of the complex is square planar with the nickel(II) bound to carbon atoms of cyanides and with linear Ni—C—N linkages (Table 37).629 630 The planar [Ni(CN)4]2 units are stacked in columns in the crystal lattice with Ni—Ni interlayer distances as short as 330 pm. C-bonded CN- is a strong field donor and the electronic spectrum of [Ni(CN)4]2 shows two weak d-d bands at 444 and 328 nm. 

The results summarized in Table II illustrate the increase of the photo induced formation of cyanide achieved by IT excitation of Cu( 11 )/tMo(CN)g]4 as compared with K [Mo(CN)g]. However, despite the increase of cyanide formation the efficiency of the spectral sensitization is rather low. The low efficiency is due to the circumstance that the rate (k ) o cyanide aquation in the valence isomeric form Cu( I)/TMo(CN)gl is low compared with the very fast back electron transfer (k ).°ln order to make the proper choice of a scavenging reaction (k ) which may compete successfully with back electron transfer, we have attempted a rough estimate of the rate constant k of the back electron transfer following the theoretical treatment proposed by Hush (20). 

Equations can also be written for the addition of the third and fourth cyanide, with constants K3 and K4. In addition to the stepwise formation equilibria, we can write a single overall equation for the formation of a complex containing several ligands from the free cation and ligands (actually a summation of the other chemical equilibria). Further, Ks = K1K2K3K4. 

The results suggest that the complex cyanides can be grouped into three categories depending on the cumulative formation constant and stability of the complex. 

The existence of Tl cyanide complexes has been mentioned previously however, on the basis of the analogy with other Tl -pseudohalide redox reactions, and by analysis of the redox potentials, the existence of the T1(CN) "" complexes was not widely accepted for a time. However, a detailed investigation of this system using ° T1 and NMR spectroscopy has indicated that Tl indeed forms very stable cyanide complexes (the overall formation constants,... 

Numerical values for solubility-product constants, dissociation constants, and formation constants are conveniently evaluated through the measurement of cell potentials. One important virtue of this technique is that the measurement can be made without appreciably affecting any equilibria that may be present in the solution. For example, the potential of a silver electrode in a solution containing silver ion, cyanide ion, and the complex formed between them depends on the activities of the thiee species. It is possible to measure this potential with negligible current. 

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