Cyanide, formation constants with

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 ... 

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 [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). 

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. 

Any metal ion that has an EDTA formation constant higher than calcium or magnesium will interfere. Cyanide complexes strongly with copper, cobalt, nickel, zinc, and ferrous iron. Hydroxylamine or ascorbic acid is added to reduce iron to the ferrous state. If the solution is buffered to pH 10 before the indicator is added, then iron will not interfere because it precipitates as the hydroxide before it can react with the indicator or the EDTA. 

As discussed in Section l,C,l, the association constant for formation of the cyanide adduct correlates with the inductive effects of cis 2 -substituents and not with their steric bulk. That latter correlation would have been anticipated if, as suggested by Benner, the torsional angle around the glycosidic bond influences the redox potential 12). Given the 20-mV difference in redox potential between the a- and /3-anomers of NADH 94) or araW/io-NADH 40), any significant correlation of redox potentials with changes in x related to the 2 -substituent would have been readily observable. 

The thermodynamic parameters for the formation of several metal-cyanide complexes, among others those of Ni(CN)4 , have been determined using pH-metric and calorimetric methods at 10, 25 and 40°C. In case of nickel(II), the thermodynamic data were determined by titration of Ni(C104)2 solutions with NaCN solutions. The ionic strength of the solutions were 1 < 0.02 M in all cases. The Debye-Huckel equation, related to the SIT model, was used to correct the formation constants to thermodynamic constants valid at 7 = 0. Since previous experiments indicated that the dependence of A,77° in the ionic strength in dilute aqueous solutions is small compared to the experimental error, the measured heats of reaction (A,77 = - 189.1 kJ mol at 10°C A,77 ,= -183.7 kJ mol at 40 C) were taken to be valid at 7 = 0, but the uncertainties were estimated in this review as 2.0 kJ moT. From the values of A,77 , as a function of temperature, average A,C° values were calculated. 

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