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

Structures
Tbe boiling point of trimetbylaluminium ( 126 C) is ratber bigber tban tbat of
eitber Me 4 Si (2 7°C) or of Me,P (38°C) (Table 1.1 ). Trimetbylaluminium crystallizes
as a dimer (m.p. 15oC) and remains associated in tbe liquid state and also in
tbe vapour well above tbe boiling point. Tbe structure oftbe dimer. determined by
X-ray diffraction, is sbown in Fig. 3.1. As tbe aluminium atom is bigger than
boron tbere is no steric problem; steric constraints do arise. bowever, wben large
alkyl groups are introduced. Tbis appears in tbe entbalpy of dissociation; wbile
(Me,Al) 2 is still largely dimeric even at 150°C, tri-isobutylaluminium is
appreciably dissociated into monomer at 2 S'C.
Monomeric trimetbylaluminium bas a trigonal planar (D 1 h)* structure. Electron
diffraction on tbe vapour at 215°/30mm gives a value of 1.957 A for tbe
Al-C bond lengtb, very clase ta that in the dimer. Proton n.m.r. of
Me 6 Ai 2 in cyclopentane solution sbows two signals below - 55 oc witb intensity
ratia 2: 1. corresponding to terminal and bridging metbyl groups respectively. As
tbe temperature is raised tbese signals coalesce (at - 35°C) and at room
temperature tbere is only one averaged signal. Tbe activation energy for
excbange of metbyl groups in solution ( 6 5. 5 kJ moi- 1 ) is lower tban tbe
dissociation entbalpy into monomer eitber in tbe pure liquid ( 81. 5 kJ moi- 1 ) or in
tbe gas pbase ( 8 5. 7 kJ moi- 1 ). Excbange is tbougbt to occur by dissociation to
monomer units wbicb remain trapped in a solvent cage.
Tbe bonding in Me 6 Al 2 bas conventionally been discussed in terms of tbreecentre
molecular orbi tais. Ifthe valence orbi tais of Al are sp 1 hybridized. two ofthe
bybrids can form tbe terminal Al-CH, bonds. Tbese are classical electron pair
bonds and account for two of tbe three valence electrons of each Al. Tbe
remaining sp' bybrids are directed in tbe Al 2 C 2 plane and are well placed to
interact witb tbe Csp' orbitals of tbe bridging metbyl groups. Each of tbese three
centre bonding m.o.s contains two electrons. o ne from Al and o ne from a bridging
metbyl group (Fig. 3.2(a) ).
An alternative description is shown in Fig. 3.2(b). Here tbe Al orbitals are sp 2
bybridized, and tbe 3-centre bonds arise by interaction of Al{3p), C(sp 3 ) and
Al(3p). Tbis model is consistent with tbe wide Me 1 -Al-Me 1 angle wbich is
observed. but it overempbasizes tbe Al-Al interaction. Wbile tbe AJ .. ·Al
distance is short (2.60 A. cf. sum of covalent radii 2.36 A), it is not thought likely
tbat Al··· Al bonding is an important source of stabilization in these dimers.
Alkoxy, balogen. bydrogen and even aryl groups ali form stronger bridges to
aluminium tban alkyl groups. Some structures are illustrated opposite and also
on p. 8 5. In the first alkoxides and halides oligomers arise by Lewis acid/base
interactions. Tbe bonding in aryl bridges can be described by similar scbemes to
tbose sbown in Fig. 3.2(a), (b). witb tbe addition of some n:-bonding witb the
pbenyl groups. Alkenyl groups bridge in a similar fasbion. Tbe alkynyl bridges in
[Pb 2 AIC CPbL. bowever, are of a different type. Here it seems tbat tbere is
electron donation from tbe n:-system of tbe alkyne to aluminium. Purther
*Symmetry of AlC, skeleton. exclusive of hydrogen atoms.
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