Cyanides complexing action

Uses. The extraction or cyanidation of precious metal ores was the first, and is stiU the largest, use for black cyanide (71). The leaching action of the cyanide results from the formation of soluble cyanide complexes. 

Further investigations proved an assumption stated before, concerning destruction of cyanide complexes and cyanide ions under the action of nonequilibrium plasma [5], A number of experiments on destructing cyanide ions were conducted both in model solutions and technological solutions containing complex cyanides of various metals (composition of the solutions is given in Table 1.). 

Scheme 6.165 Enantioselective Strecker reactions catalyzed by biflinctional hydrogen-bonding guanidine organocatalyst 178. Catalytic action of 178 HCN hydrogen bonds to 178 and generates a guanidinium cyanide complex after protonation, which activates the aldimine through single hydrogen bonding and facilitates stereoselective cyanide attack and product formation.

Discovery. These catalysts were discovered during a study of the use of transition metal cyanides in combination with metal alkyl and hydride reducing agents in polymerizations. The combination of nickel cyanide and lithium aluminum hydride complexed very strongly with tetrahydrofuran. A similar complexing action occurred with propylene oxide and nickel hexacyanoferrate(II)-lithium aluminum hydride. This led to speculation as to the role of the double-metal cyanide itself. 

A number of valence states of metals, too unstable to exist in solution as ordinary ions, are greatly stabilized by the strong coordinating action of the cyanide ion. Thus, cyanide complexes of Mn(I), Mn(III), Ni(I), Cu(I), Mo(IV), Au(I), and W(XV) are known although simple salts of the metals in these oxidation states do not survive in solution. These complexes will be described in the chapters on their respective metals. 

The complexing action of cyanide is also important in the metallurgy of silver and gold. Both gold and silver in the elemental state will dissolve in a solution of cyanide if air is present to effect an oxidation both metals form complexes of the type M(CN)7, from which the metals themselves may be recovered by reduction with metallic zinc. 

A number of useful replacement reactions in the 2,1,3-benzothiadiazole series involve the production and reactions of their nitriles. 4,7-Dicyano-2,l,3-benzothiadiazole is of particular interest because of its potent herbicidal and defoliating activity. It is produced by the action of a pyridine-copper(n) cyanide complex on the readily available 4,7-dibromo-compound. Other nitriles of this series are similarly obtained. A second method, of more limited applicability, is the Sandmeyer reaction, which, in the present instance, gives improved results when the amino-2,1,3-benzothiadiazoles are diazotized with nitrosyl-sulphuric acid. The nitriles are convertible by conventional methods into other functional derivatives. Successive coupling with diazotized aniline, and... 

Probably the most extensively applied masking agent is cyanide ion. In alkaline solution, cyanide forms strong cyano complexes with the following ions and masks their action toward EDTA Ag, Cd, Co(ll), Cu(ll), Fe(ll), Hg(ll), Ni, Pd(ll), Pt(ll), Tl(lll), and Zn. The alkaline earths, Mn(ll), Pb, and the rare earths are virtually unaffected hence, these latter ions may be titrated with EDTA with the former ions masked by cyanide. Iron(lll) is also masked by cyanide. However, as the hexacy-anoferrate(lll) ion oxidizes many indicators, ascorbic acid is added to form hexacyanoferrate(ll) ion. Moreover, since the addition of cyanide to an acidic solution results in the formation of deadly... 

Another type of demasking involves formation of new complexes or other compounds that are more stable than the masked species. For example, boric acid is used to demask fluoride complexes of tin(IV) and molybdenum(VI). Formaldehyde is often used to remove the masking action of cyanide ions by converting the masking agent to a nonreacting species through the reaction ... 

This is by far the most stable and best-known oxidation state for chromium and is characterized by thousands of compounds, most of them prepared from aqueous solutions. By contrast, unless stabilized by M-M bonding, molybdenum(III) compounds are sparse and hardly any are known for tungsten(III). Thus Mo, but not W, has an aquo ion [Mo(H20)g] +, which gives rise to complexes [MoXg] " (X = F, Cl, Br, NCS). Direct action of acetylacetone on the hexachloromolybdate(III) ion produces the sublimable (Mo(acac)3] which, however, unlike its chromium analogue, is oxidized by air to Mo products. A black cyanide,... 

The mechanism of action of the cyanation reaction is considered to progress as follows an oxidative addition reaction occurs between the aryl halide and a palladium(O) species to form an arylpalladium halide complex which then undergoes a ligand exchange reaction with CuCN thus transforming to an arylpalladium cyanide. Reductive elimination of the arylpalladium cyanide then gives the aryl cyanide. 

These are thermodynamically relatively weak oxidants (Table 18) and their action is relatively restricted, for example, to inorganic ions of moderate reducing power such as iodide, to polyfunctional organic compounds such as hydroxy-acids, and, in the cases of Ag(I) and Cu(II), to CO and H2. Fe(III) is particularly affected by hydrolysis and all these oxidants form complexes with suitable ligands. Cyanide ion and 1,10-phenanthroline form strong complexes with Fe(III) which greatly affect its behaviour. Tris-l,10-phenanthrolineiron(III) (ferriin) displays... 

Fig. 2(b) represents similar dependencies for technological solutions. Solutions were obtained by means of cyanidation of specified quantities of one metal (Au, Ag, Cu, Zn, curves l -6 ), or the ore concentrate containing all the above stated metals (curve 7 ). Figures prove that process of cyanide destruction is determined by the time of plasma action on the solution. For technological solutions, time of treatment required for complete destruction of cyanide ions depends on composition of the solution. The more complex is the composition, the longer time is required for complete degradation of cyanides. Character of the curves is changed as well. 

The therapeutic effects of sodium nitroprusside depend on release of nitric oxide which relaxes vascular muscle. Sodium nitroprusside is best formulated as a nitrosonium (NO+) complex. Its in vivo activation is probably achieved by reduction to [Fe(CN)5NO]3, which then releases cyanide to give [Fe(CN)4NO]2, which in turn releases nitric oxide and additional CN to yield aquated Fe(II) species and [Fe(CN)6]4 (502). There are problems associated with its use, namely reduced activity due to photolysis (501) and its oxidative breakdown due to the action of an activated immune system (503), both of which release cyanide from the low-spin d6 iron complex. 

The A-trimethylsilylimines 68 (R = t-Bu, Ph, 2-MeCgH4 or 2-BrC6H4), which are prepared by the reaction of non-enolizable aldehydes with lithium bis(trimethylsilyl)amide, followed by trimethylsilyl chloride, undergo pinacolic coupling induced by NbCLt 2THF to yield the vicinal diamines 69 as mixtures of dl- and meso-isomers, in which the former predominate. Another method for the preparation of 1,2-diamines is by the combined action of the niobium tetrachloride/tetrahydrofuran complex and tributyltin hydride on cyanides RCN (R = /-Hu. Ph, cyclopentyl or pcnt-4-en-l-yl) (equation 32)82. 

The electron transport chain is vital to aerobic organisms. Interference with its action may be life threatening. Thus, cyanide and carbon monoxide bind to haem groups and inhibit the action of the enzyme cytochrome c oxidase, a protein complex that is effectively responsible for the terminal part of the electron transport sequence and the reduction of oxygen to water. 

Figure 7.68 The site of action of cyanide in the electron transport chain. I, II, III, and IV complexes in the electron transport chain. Cyanide blocks the action of a3 and stops the reduction of water and the movement of electrons and protons. Therefore, ATP production stops (j). Abbreviations Q, coenzyme Q cyt0, cytochrome c a3 cytochrome a3.

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