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

Zinc. cadmium and mercury
two ha urs at - 3 5 °C. The dibenzyls of Ca, Sr and Ba are yellow ar yellow orange,
as are Ph 2 Sr and Ph 2 Ba.
Much early work an organic compounds of Ca, Sr and Ba was carried out with
solutions obtained by dissolving the metals in zinc dialkyls.
2ZnR 2 +M
benzene
~~--. MZnR 4 + Zn (M =Ca, Sr, Ba)
68 c
The complexes MZnEt 4 are monomeric in benzene. In cyclopentane as solvent,
1 H n.m.r. spectra over a range of temperatures indicate the presence of free
dialkyls and a complex CaZnEt 4 , which is probably present as a contact ion-pair.
The exchange reaction presumably occurs via an alkyl-bridged species:
[
,Et, ]
Et2Ca + Et2Zn ~ EtC<:, -~)nEt ~ Ca 2 + .ZnEt/-
Et contact ion- pair
3. 7 Zinc, cadmium and mercury
3.7.1 Derivatives of the type RMX
Ethylzinc iodide was first described by Frankland ( 1849) who made it from ethyl
iodide and zinc, at a stage in the development of chemistry at which the ethyl
group was represented C 4 H 5 an account of uncertainties about atomic weights.
Though diethylzinc, and other dialkyls R 2 Zn, obtained by the thermal disproportionation
of RZni were valuable synthetic reagents until they were almost
entirely superseded by Grignard reagents (from a baut 1900 ), the nature of RZni
long remained obscure. It was not until 1966 that its structure was unambiguously
established. The alkylzinc iodides and bromides are formed from the
alkyl halide and zinc which has been activated, for example, by copper. This may
be dane by alloying, formation of a zinc-copper couple by treating zinc powder
with aqueous copper(II) acetate, ar simply by mixing the two powdered metals. A
particularly active form of metal can be obtained by reaction of zinc chloride with
potassium in an ether solvent. The Schlenk equilibrium lies an the side of RZnX;
this may be applied ta the synthesis from the dialkyls.
R 2 Zn +ZnX 2
cthers
2RZnX (X= CI, Br. 1)
Ethylzinc chloride and bromide ha ve tetrameric structures in the crystal. which
are retained in solution in benzene. In contrast solid ethylzinc iodide forms
polymeric chains. In all these structures zinc is four-coordinate. In ethers RZnX
are monomeric through complex formation.
Organozinc halides are less re active than Grignard reagents and generally ha ve
been replaced by the latter for organic syntheses. There are. however, a few
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