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

Structures
both strongly exothermic,
Me,B(l) + iH 2 0(1) = ~B 2 0,(c) + 3CH 4 (g): ~H~ =- 289 kJmol- 1
Me,Al(l) + fH 2 0(/) = iAI 2 0,(c) + 3Cil 4 (g): ~H~ = - 497 kj moi~ 1
If the entropy terms are included, however, the free energy changes would be
even more favourable, on account of the formation of three moles of gaseous
methane. The fact that Me,Al is hydrolysed with explosive violence, whereas
Me 3 B is not attacked by water at room tempera ture is due to kinetic rather than to
thermodynamic factors.
As noted in Chapter 1, the ability to form complexes with unsaturated organic
ligands is a property of d-block transition elements rather than of main group
elements. Alkyls ofthe alkali metals and especially of aluminium are able to add to
alkenes and hence to initiate oligomerization or polymerization (p. 80). Some of
these reactions are used industrially on a very large scale. There is evidence for
weak interactions between the n-electron systems of unsaturated hydrocarbons
and these electropositive metal centres. This is illustrated by the structure of
benzyllithium, although there is some controversy about the extent of covalent
bonding between metal orbitals and the n-orbitals of the ligand. The cyclopentadienyl
derivatives have a wide range ofstructures. In some ofthese, e.g. Cp 2 Mg
the rings are symmetrically bonded to the metal in a sandwich (p. 91 ). In others,
e.g. solid CpTl the individual units are linked in chains through bridging C 5 H 5
groups. While a case could perhaps be made that these compounds are ionic, this
is difficult to envisage in the intriguing arene complexes ofmain group elements,
such as (C 6 H 6 ) 2 Ga,Cl 4 (p. 9 3 ). Formation of n-complexes between copper(I) and
silver(I) and alkenes and arenes has been known for many years. Similar n­
complexes are implicated in the electrophilic attack by Hg 2 + or Tl 3 + on alkenes or
arenes (p. 62 ). The polymeric alkynyls of Cu(l), Ag(l) and Au(I) are associated by
n-interactions.
3.2 Structures
A wide range of structural types is found among the organic derivatives of the
elements of the first three groups. On the one hand the alkyls and aryls of the
heavy alkali metals are essentially ionic in character. On the other, those of the
least electropositive elements form covalently bonded monomers e.g. R 3 B, R 2 M
(M = Zn, Cd, Hg). Intermediate between these are the derivatives of Li, Be, Mg and
Al which are associated through alkyl or aryl bridges.
In contrast to diborane, B 2 H 6 , trimethylborane is monomeric in the vapour and
also in the condensed states. The methyl groups are arranged trigonally in a plane
about the boron atom. The boron atom has an empty 2p orbita) and consequently
acts as a Lewis acid, forming complexes with electron pair donors such as amines,
phosphines or hydride ion. Hydrogen readily forms bridges between boron atoms,
for instance in tetramethyldiborane or in diborane itself. Stable alkyl bridges,
however, have not been found, although they are presumably present in the
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