Cyanides involving metal complexes

The cyanide exchange on [M(CN)4]2 with M = Pt, Pd, and Ni is a rare case in which mechanistic comparisons between 3d, 4d, and 5d transition-metal complexes. Surprisingly, the behavior of these metal square-planar centers leads to mechanistic diversity involving pentacoordinated species or transition states as well as protonated complexes. The reactivities of these species are strongly pH-dependent, covering 15 orders of magnitude in reaction rates.85... 

In this context it is interesting to note that benzonitrile, Ph—C=N, trimerizes to a triazine on a Raney nickel surface. It was assumed that Jt-bonded nitriles were involved in the reaction mechanism.10 This reaction resembles the well-known template synthesis of phthalocyanine complexes from phthalodinitrile. Formation of linear polymers [—C(R)—N—] occurs on heating aryl or alkyl cyanides with metal halides.11... 

The tendency of cyanide ion to bond with iron(III) is responsible for its toxicity in that it bonds with iron(III) in one of the enzymes involved in the utilization of molecular oxygen in respiration processes.) This prevents utilization of oxygen with potentially fatal results, as discussed in Chapter 11.) The species that bonds to the metal ion, cyanide in this case, is called a ligand, and the product of the reaction is a complex ion or metal complex. The overall process is called complexation. 

Several indirect methods for determination of cyanide involve the displacement by cyanide of metals from their complexes with organic reagents, with subsequent change in colour of the solutions, such as the methods using Hg(II) complexes with diphenylcarbazone [21] and Methylthymol Blue [22]. 

HCN also adds to alkenes in the presence of an appropriate catalyst (Arthur et al., 1954 Jackson and Level, 1982). Thus, cobalt carbonyl leads to Markownikov addition, for example, 1-propene yields isopropyl cyanide in approximately 75% yield. HCN adds to alkynes in the presence of metal complexes, and the use of a nickel complex may lead to syn addition (Jackson and Level, 1983 Jackson et al., 1988). Hydrogen cyanide reacts with conjugated dienes, the mechanism involving a TT-allyl intermediate. The course of addition is complex and may lead to more than one product (Keim et al., 1982). 

Some birds may not die immediately after drinking lethal cyanide solutions. Sodium cyanide rapidly forms free cyanide in the avian digestive tract (pH 1.3-6.5), whereas formation of free cyanide from metal cyanide complexes is comparatively slow. A high rate of cyanide absorption is critical to acute toxicity, and absorption may be retarded by the lower dissociation rates of metal-cyanide complexes. In Arizona, a red-breasted merganser (Mergus senator) was found dead 20 km from the nearest known source of cyanide, and its pectoral muscle tissue tested positive for cyanide. A proposed mechanism to account for this phenomenon involves weak-acid dissociable (WAD) cyanide compounds. Cyanide bound to certain metals, usually copper, is dissociable in weak acids such as stomach acids. It has been suggested that drinking of lethal cyanide solutions by animals may not result in immediate death if the cyanide level is... 

Emil Fischer s result involving cyanide additions to carbohydrates had demonstrated the power of diastereoselective synthesis early as the 1890s (Equation 1) [4, 34,162]. The corresponding enantioselective formation of cyanohydrins has been the subject of immense efforts. It has long been appreciated that optically active cyanohydrins are synthetically useful intermediates that can be elaborated into a number of chiral building blocks, such as hydroxy acids. In general, there are three main classes of catalysts for the preparation of chiral cyanohydrins enzymes, cyclic dipeptides, and transition metal complexes [163-166]. 

Cyanides are strong Lewis bases that form a range of complexes with d-block metal ions. They are also famous as poisons. When they are ingested, they combine with certain protein molecules—the cytochromes—involved in the transfer of electrons and the supply of energy in cells, and the victim dies. 

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