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

Aluminium. gallium. indium and thallium
Table 3.11. Enthalpies of dissociation (kJ moi- 1 ) of adducts Me ,ML
Basejacid Me,B Me,Al Me,Ga Me,In Me,Tl
Me,N 74 Undisso- 88 83 Appreciably
ciated at
dissociated at ooc
l 35°C
Me,P 69 Undisso- 76 72 m.p. 2Î-28 C Probably
ciated at
more stable than Me,N
13 S"C adduct
Me,As Appreciably 42
dissociated
at ooc
Me,O 80 38
Me,S 80 33
Me,Se 68 42
Measurements on trimethylaluminium are complicated by its dimerization.
Formation of adducts with Me,N, Me 2 0 and Me,P goes to completion
below at least 135°C. because the enthalpy of the reaction
Me,Al(g) + Me,N(g);::::':Me,AINMe 3 (1J) exceeds that of the dimerization
2Me,Al(g) ;::::':Me,Al 2 (1J); L:\He= - 8 S kj moi- 1 (assuming that thc entropy
changes in both reactions are similar). Complicated equilibria result when the
enthalpy of dissociation of the adduct is compara bie with that of the dimer. This
happens with Me 2 S and Me 2 Se. Steric factors are much less important for
aluminium and the other Group III metals than for boron.
Not only are the organo derivatives of Al. Ga and In better acceptors than their
boron analogues but they also are much more reactive towards cleavage by
protonic acids. probably on account of the greater polarity of the metal-carbon
bonds. This bas already been noted in connection with their easy hydrolysis.
Adducts with primary and secondary amines eliminate methane an heating.
Me 2
110°C /N\
e.g. 2Me3AL- NHMe 2-- Me 2 Al ALMe 2
""-w*
Similar dimers are formed by gallium and indium.
3.9.3 MOCVD
Mixed Group III and Group V compounds such as gallium arsenide (GaAs) and
indium phosphide (InP) find important applications as semiconductors in
optoelectronic devices such as solid state lasers. light-emitting diodes, solar cells
and photodetectors. These devices require single crystal layers of the semiconductor
material of exactly controlled thickness. The compounds MY (M =
Al, Ga, In; Y = P. As) form' solid solutions with each other over a wide range of
compositions. By changing the composition of the mixed compounds in a layer.
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