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

Structures
monomer units are stiU easily distinguishable. Similar weak association may also
occur in solid triphenylindium. Proton n.m.r. studies show that exchange of
methyl groups between Me,M molecules (M = Ga, In. TI) occurs readily,
presumably through methyl bridged dimers.
The Group IIB dialkyls and diaryls are ali monomeric, having linear skeletons
in which the metal is 2-coordinate. The change in bond length in the dimethyls is
worth nating; Zn-C 1.929 Â: Cd-C. 2.1 J 2 Â; Hg-C, 2.094 Â, ali ± 0.005 Â.
The anomalous position of cadmium is reflected also in the boiling point sequence
(p. 6) and is attributed to bigger van der Waals forces between Me 2 Cd than
between Me 2 Hg molecules. Proton n.m.r. spectra ofmixtures ofMe 2 Zn and Me 2 Cd
with each other or with Me,Al indicate that alkyl exchange occurs readily. The
methyl groups in Me 2 Hg, however, exchange only slowly. In solid Ph 2 Hg the two
phenyl rings are coplanar. In the di-o-tolyl compound, however, they are twisted
through 59° relative to each other, on account of interactions between the ortho
substituents on different rings.
The dialkyls and diaryls ofberyllium and magnesium are generally associated.
Steric effects play an important part in determining the degree of association in
beryllium compounds. Dimethylberyllium. which forms colourless crystals
decomposing above ca. 200cC, is a long chain polymer in which the metal atoms
are surrounded by four methyl groups in a nearly tetrahedral arrangement. It is
soluble only in donor solvents which are able to break down the polymeric
structure. It has an appreciable vapour pressure above l00°C (0.6 mm at l00°C
and 760 mm at 220°C extrapolated), and the vapour contains mainly the
monomer, with some dimer and trimer. Diethylberyllium, b.p. 9 3-5" /4 mm is
dimeric in benzene, whereas the very hindered di-tert-butylberyllium is monomeric
both in the vapour and in solution. The Be-C bonds in this linear
monomer are l. 70 A long (electron diffraction).
The magnesium dialkyls are colourless solids, which react readily with oxygen
and water and which usually decompose without melting at high temperatures.
The pathways followed in their thermal decomposition are typical of many alkyl
derivatives of main group and of transition elements. Where a /3-C-H is
present, as in Et 2 Mg, /3-hydrogen transfer predominates with loss of alkene:
1711
~
21 (1
nMgH 1 + nH 1 C=CH 1
,~370
nMg + nH 1
where such a function is absent, as in Me 2 Mg, ~-hydrogen transfer may occur:
[Mg(CH, ) 2 ]"
Donor solvents tend to break down the polymers by coordination to the Lewis
35