Group 17 elements Methyl halides

The key elements of these carbonylation processes is the ability of a metal complex to undergo facile oxidative addition with methyl halide (especially iodide), carbon monoxide (CO) insertion into the methyl-metal bond, and reductive elimination of the acetyl group as the acetyl halide [3]. 

Figure 2.1 shows that the main driving force for these reactions is, in general, the strongly exothermic formation of the metal halide. This is particularly striking for the Group IIB elements, Zn, Cd and Hg, where the methyls are in each case... 

Phosphorus is the second element in Group 5A of the Periodic Table and, like nitrogen, has five electrons in its valence shell. Examples of trivalent phosphorus compounds are phosphine, PH3, and triphenylphosphine, PhjP. Phosphine is a highly toxic, flammable gas. Triphenylphosphine is a colorless, odorless solid. Because phosphorus is below nitrogen in the Periodic Table, phosphines are weaker bases than amines and good nucleophiles (Section 9.3E). Treatment of a phosphine with a methyl, primary, or secondary all l halide gives a phosphonium salt by an Sj 2 pathway. 

Copper is a group 11 element, so Cu" contributes 10 valence electrons, and the two methyl anions contribute 4 more, for a total of 14. With fewer than 18 valence electrons, copper is coordinatively unsaturated and can accommodate additional ligands. When dimethylcu-prate reacts with an alkyl halide RX, both R and X become ligands on copper. 

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