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

2
Methods of formation of
metal-carbon bonds of the
main group elements
2.1 The reaction between a metal and an organic halogen
compound
2M + nRX = R 11 M + MX 11 (or R,MX!I: x + y = n)
The reaction between a metal and an organic halogen compound provides a fairly
general route to organometallic compounds. One of the earliest organometallic
compounds ever to be prepared, ethylzinc iodide. was made in this way from
zinc/copper couple and ethyl iodide.
2 .1.1 Thermochemical considerations
It is useful to examine the scope of the reactions between metals and organic
halides in the light of their thermochemistry. Some typical values of the heats of
reaction between methyl halides and main group elements are given below
(Figs. 2.1. 2.2). Within one series of reactions involving compounds of the
same group of the Periodic Table where the stoichiometry is the same for
each element, the neglect of the entropy term should not, to a first approximation,
seriously alter the gross pattern of changes within the Group.
These data show that, apart from the elements C, N and O and the B-elements of
the last row of the Periodic Table (Hg, TI. Pb and Bi), the reaction between an
element and methyl chloride to yield the chloride and methyl of the element is
exothermic, and therefore might be expected to provide a method of synthesizing
the methyls. In nearly ali the examples listed where a negative value of ~H is
found, such reactions have been observed and many are important synthetic
routes. The mechanisms of the reactions and the conditions under which they
occur, however, vary greatly with different elements. Kinetic as well as
thermodynamic factors are thus significant in controlling the reactions.
Figure 2.1 shows that the main driving force for these reactions is, in general.
the strongly exothermic formation ofthe metal halide. This is particularly striking
for the Group IIB elements, Zn. Cd and Hg, where the methyls are in each case
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