_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Metal exchange
are produced.
Si+ 2RCI = R 2 SiCI 2 (R = alkyl or aryl)
The chlorides are almost always used; methyl fluoride and methyl iodide do not
react according to the equation given above. This reaction is often called the
'Direct' synthesis and, with its applications, is discussed in Chapter 4. It is not
restricted to silicon chemistry. however, although it is in this field that its main
importance lies. Similar reactions occur between alkyl halides and metals and
metalloids such as Ge, Sn, P, As and Sb. As long ago as 18 70 Cahours observed
that arsenic reacts with methyl iodide in a sealed tube,
4Mel + 2As = Me 4 As+I- +Asi,
More recently it has been shown that the following reaction occurs when methyl
bromide is passed over a heated mixture of arsenic and copper
3MeBr + 2As = MeAsBr 2 + Me 2 AsBr
2 .1. 3 Alloy methods
The alkyls and aryls ofthe heaviest elements (e.g. Hg, Tl. Pb. Bi) are endothermic
compounds. Even the reactions between the element and organic halide may be
endothermic, in spite of a high heat of formation of the metal halide tending to
drive the reaction to the right. In these cases the direct synthesis can still be used
by taking an alloy ofthe metal with an alkali metal such as sodium, instead ofthe
free metal. Here the formation ofthe strongly exothermic sodium halide provides
the driving force. Alkyls and aryls ofHg, Tl. Sn, Pb and Bi can be made in this way.
2Hg(l) + 2MeBr(l) = Me 2 Hg(l) + HgBr 2 (s) LiW" = - 1 kJ mol- 1
HgBr 2 (s) + 2Na(s) = Hg(J) + 2NaBr(s) LiH' = - 5 SI kJ moi- 1
Hg(l) + 2MeBr(l) + 2Na(s) = Me 2 Hg(l) + 2NaBr(s) LiH~~ = - 552 kJ moi- 1
The alloy method is used in the manufacture of tetramethyl- and tetraethyllead,
antiknock additives to petrol (p. 1 00). Similarly bismuth aryls were first made by
Michaelis in 18 77 by the reaction of powdered bismuth-sodium alloy with aryl
halides,
3ArX + Na,Bi = Ar,Bi + 3NaX
2.2 Metal exchange: the reaction between a metal and an
organometallic compound of another metal
M+RM'=M' +RM
This method depends on the difference in the free energies of formation ofthe two
species RM' and RM. Owing to the lack of L\G 0 values, enthalpies of formation are
used to indicate the thermodynamic feasibility of such reactions. Endothermic or
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