Transition metal complexes pseudohalides

There is considerable and widespread interest in the metal complexes of these anions and current research topics comprise for example (i) the spectroscopic study of the binding in these anions (linkage isomerism) and their complexes, (ii) the synthesis of regular polymers of their transition metal complexes and study of the semiconducting properties of these polymers, (iii) the use of the pseudohalides in pharmacological (e.g. low toxicity of —SCN) and biochemical studies (easy complexation of SCN- to metals), and (iv) the use of the activation of these triatomic anions by coordination to metals for their selective conversion in organic synthesis. 

Innovations in the chemistry of aromatic compounds have occurred by recent development of many novel reactions of aryl halides or pseudohalides catalysed or promoted by transition metal complexes. Pd-catalysed reactions are the most important [2,29], The first reaction step is generation of the arylpalladium halide by oxidative addition of halide to Pd(0). Formation of phenylpalladium complex 1 as an intermediate from various benzene derivatives is summarized in Scheme 3.1. 

Diorganyl tellurides are starting materials for the preparation of diorganyltellurium dihahdes, pseudohalides, and carboxylates, of telluroxides, and of teUuronium salts, and are used as ligands for the synthesis of transition metal complexes. 

Detailed crystal structures of only a few of the other heavy metal pseudohalides are available in the literature. Among them are, cuprous azide which has a relatively simple tetragonal lattice and cupric azide, mercuric fulminate and a-lead azide which have increasingly complex orthorhombic lattices. a-Lead azide has four types of anion sites of varying amounts of as5nnmetry (33) while cupric azide (35) and mercuric fulminate (72) have two such sites. The structure of cupric azide which is built up of distorted octahedra of asymmetric Ns ions about the central cupric ion is analogous to that of a transition metal complex. 

Transition Metal Complexes Containing Bidentate Phosphine Ligands W. Levason and C. A. McAuliffe Beryllium Halides and Pseudohalides... 

Group IVB.—A survey of their reactions with electrophiles suggests that organo— tin-transition-metal complexes may be regarded as pseudohalides aliphatic C—Sn bonds are deactivated, and aromatic C—Sn bonds activated. Transition-... 

In the context of this review it is only possible to discuss a selected number of compounds. Based on these examples an idea of the scope and field of applications of these versatile triatomic anions can be provided. The fact that these and not other complexes are discussed is not because the latter lack importance. They can be found in reviews that are given in ref. 202-209. An account will be given of (i) the bonding aspects of the pseudohalides to transition metals based on reliable structural data and (ii) some synthetic aspects of the pseudohalides with special emphasis on their conversion in the coordination sphere of a metal. ... 

Examples of complexes of the various pseudohalides with transition metals for which the structure has been confirmed by X-ray analysis will be discussed in more detail in the next section. For preparative methods see Section 13.5.5.4. 

The cyanide ion is called a pseudohalide ion because it behaves like Cl- in forming an insoluble, white silver salt, AgCN. In complex ions such as Fe(CN)63-, CN - acts as a Lewis base (Section 15.16), bonding to transition metals through the lone pair of electrons on carbon. In fact, the toxicity of HCN and other cyanides is due to the strong bonding of CN- to iron(III) in cytochrome oxidase, an important enzyme involved in the oxidation of food molecules. With CN attached to the iron, the enzyme is unable to function. Cellular energy production thus comes to a halt, and rapid death follows. 

In this reaction an anionic leaving group is replaced by a neutral donor molecule. Consequently, if relative EPD strengths of both neutral and anionic EPD units are known, it should be possible to make at least qualitative predictions about the ionizability of substrates A—Bj, A—Bg,. .. in various EPD solvents. Approximate values of relative EPD strength of halide, pseudohalide, and neutral EPD units have been determined using vanadyl(IV)acetylacetonate, VO(acac)2, as reference EPA (46) and are listed in Table VII. These values have proved useful for the interpretation of numerous reactions involving complex formation between transition metal ions and halide or pseudohalide ions... 

The Bp ligand is the simplest of all the heteroscorpionate ligands. It is basically bidentate, forming the typical boat-shaped H2B( x-pz)2M ring, in which the pseudoaxial B-H may form an agostic bond with the metal.4,5 [M(Bp)2] complexes of first row transition metals are usually square planar or tetrahedral.6,7 Some octahedral anionic [M(Bp)3]-species of low stability can be also isolated.8,9 Five-coordinate anionic species [MX(Bp)2]- (M = Cr, Mn X = halides or pseudohalides) have been also known.10,11... 

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