Cyanide, co-ordinated

Hughes, D.L., Mohammed, M.Y. and Pickett, C.J. Electroreduction of co-ordinated cyanide to the aminocarbyne ligand (CNH2) and a pathway for isomerisation of ligating methyleneamide (NCH2) Reactions at molybdenum of relevance to cyanide reduction by nitrogenase J.Chem.Soc., Chem. Commun., (1989), 1399-1400... 

Complex ions used for electroplating are anions. The cathode tends to repel them, and their transport is entirely by diffusion. Conversely, the field near the cathode assists cation transport. Complex cyanides deserve some elaboration in view of their commercial importance. It is improbable that those used are covalent co-ordination compounds, and the covalent bond breaks too slowly to accommodate the speed of electrode reactions. The electronic structure of the cyanide ion is ... 

The precipitate redissolves in excess soluble cyanide, and the complex ion is probably an ion-dipole co-ordination compound, i.e. 

The solubility of Ag(CN)2"in water stems from the overall negative charge encouraging solvation with water dipoles, which uncharged AgCN does not. It is likely that the other cyanide complex ions of low co-ordination number have a similar structure. 

The complex cyanides of transition metals, especially the iron group, are very stable in aqueous solution. Their high co-ordination numbers mean the metal core of the complex is effectively shielded, and the metal-cyanide bonds, which share electrons with unfilled inner orbitals of the metal, may have a much more covalent character. Single electron transfer to the ferri-cyanide ion as a whole is easy (reducing it to ferrocyanide, with no alteration of co-ordination), but further reduction does not occur. 

Most low-valence metal complexes are generally deactivated by air and sometimes also by water. Carbon monoxide, hydrogen cyanide, and PH3 frequently act as poisons for these catalysts. Poisoning by strongly co-ordinating molecules occurs by formation of catalytically inert complexes. An example is the poisoning of Wilkinson s catalyst for alkene hydrogenation ... 

Double cyanides of trivalent vanadium have been prepared which show the properties of co-ordinated compounds, e.g. Ks[V(CN)g] compare with K3[Cr(CN)6] and K3[Fe(CN)6]. 

In this book we are not concerned with organometallic reactivity of compounds containing M-C bonds, and so to all intents and purposes this section is about the reactions of nitriles or cyanides with nucleophiles. Reactions of co-ordinated alkynes are adequately discussed elsewhere. 

Figure 6-14. The reduction of co-ordinated macrocyclic imines provides a method for the preparation of macrocyclic amines. The reaction above illustrates one of the standard methods for the preparation of cyclam. The metal ion may be removed from the nickel(n) complex by prolonged reaction with cyanide.

In addition to their thermodynamic stability, complexes of macrocyclic ligands are also kinetically stable with respect to the loss of metal ion. It is often very difficult (if not impossible) to remove a metal from a macrocyclic complex. Conversely, the principle of microscopic reversibility means that it is equally difficult to form the macrocyclic complexes from a metal ion and the free macrocycle. We saw earlier that it was possible to reduce co-ordinated imine macrocycles to amine macrocyclic complexes in order to remove the nickel from the cyclam complex that is formed, prolonged reaction with hot potassium cyanide solution is needed (Fig. 6-24). 

The commonest reactions involve the displacement of halide by hydroxide or cyanide ion to yield co-ordinated phenols or nitriles. Once again, the metal may play a variety of different functions. The polarisation of the C-Cl bond is the most obvious, but stabilisation of the product may be of equal importance, as could the involvement of a metal coordinated nucleophile. The availability of a one-electron redox inter-conversion between copper(n) and copper(i) also opens up the possibilities of radical mechanisms involving homolytic cleavage of the C-Cl bond. All of these different processes are known to be operative in various reaction conditions. In other cases, organocopper intermediates are thought to be involved. 

A full account of the crystal structure determination of potassium heptacyano-vanadate(m) dihydrate has appeared.387 Apart from clearing up the uncertainty surrounding the nature of the vanadium(m) cyanide complex, this study has established the V(CN)7- ion as one of the very few examples of a seven-co-ordinate complex containing only simple unidentate ligands. The choice of this co-ordination number has been rationalized on the basis of the nine-orbital rule. The two electrons of the paramagnetic V(CN)4 ion (pef = 2.8 BM) occupy two separate orbitals,... 

According to Werner s theory (see this series, Vol. X.), the formulae of hydrogen ferro- and ferri-cyanide should be written [Fe(CN)6]H4 and [Fe(CN)6]H3 respectively, the six cyanogen groups being co-ordinated with the iron atom, and constituting the nucleus around which hover the replaceable hydrogen atoms. 

Cyanides.—A concise review of the seven- and eight-co-ordinate molybdenum complexes and related molybdenum(vi) 0x0 complexes, with cyanide and isocyanide ligands has appeared and Group Via cyanide complexes have been reviewed within the broader scope of transition metal cyanides. ... 

The spinel structure is found among a very large number of oxides AB2X and also in a limited number of sulphides, selenides, fluorides and cyanides of the same composition, some of which are recorded in table 8.09. The cubic unit cell of this structure is shown in fig. 8.21 and contains 32 X ions. Each A ion is tetrahedrally co-ordinated by four and each B ion is octahedrally co-ordinated by six X neighbours, and each X ion is bound to one A and to three B ions. The co-ordination may therefore be summarized thus ... 

Cyanide Complexes.—The i.r. and Raman spectra of KsfMfCN) ] (M = Mn, Tc, or Re) have been recorded from 40 to 4000 cm, and the frequencies observed assigned with the aid of normal-co-ordinate analyses. The low C— force constants obtained suggest that the M—C bond has considerable ir-character, especially in the case of the technetium complex. 

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