Diazonium salts, aryl with metal halides

Nesmeyanov has extended this reaction to the synthesis of aryl derivatives of the heavier B-elements, but the yields are often low. Often the double salts of the metal halide with the diazonium salt are first isolated and then decomposed by a metal such as copper or zinc, e.g. 

Replacement of diazonium groups by halide is a very valuable synthetic alternative to direct halogenation for preparation of aryl halides. There are three broad types of procedures decomposition of aryl diazonium-halide adducts with expulsion of nitrogen, copper-mediated redox processes, and thermal processes proceeding via aryl radicals. The first type of process is probably involved in the reaction of aryl diazonium salts with iodide ion. Smooth high-yield transformation takes place in the absence of any metal catalyst. The mechanism of the reaction... 

Aromatic and heteroaromatic halides fail to react with alkali metal cyanides under the conditions normally used for aliphatic nucleophilic displacement reactions. Traditionally, the preparation of aryl [ CJnitriles has been accomplished either by the Sandmeyer reaction or the Rosenmund-von Braun reaction. In the former, cyano-dediazoniation of aryl diazonium salts is accomplished with Cu CN or a mixture of and CuCN (or CuCl)... 

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