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

Complexes with unsaturated hydrocarbons
3 .11.2 Arene complexes
Magnesium reacts with anthracene in tetrahydrofuran to give a brilliant orange
complex Mg( anthracene) (THF) 3 • This compound dissociates above room temperature
to give a very active form of magnesium. Organic halides react with it to
form Grignard reagents; allyl bromide affords a 98% yield, whereas rather low
yields are obtained by the conventional method on account of coupling. While
bulk magnesium reacts reluctantly with hydrogen, the complex rapidly forms
MgH 2 in the presence of catalytic amounts of TiCl 4 or CrCl 3 • The hydride has a
high surface area and is very reactive, adding to alkenes to give magnesium alkyls
and reducing organic halides to hydrocarbons.
On heating to 250°C hydrogen is liberated reversibly. The Mg/MgH 2 system
could prove useful as a storage system for hydrogen; it can contain up to 7.65%
hydrogen by weight. Hydrogen is a possible alternative fuel to petrol but there are
problems in its transport and storage, particularly as a liquid. Metal hydride
systems could provide a solution to these difficulties.
THF H
Mg + anthracene ~ Mg(anthracene) ~ MgH 2 + anthracene
- H 2 /250oC
Magnesium also reacts with 1, 3-dienes in THF. Derivatives of composition
Mg(C 4 H 6 ), Mg(C 4 H 6 ) 2 and Mg(C 4 HJ 3 have been isolated as solvates. They are
almost certainly polymeric.
Salts ofGa(I), In(I) and Tl(I) are often readily soluble in aromatic hydrocarbons.
Ga[GaCl 4 ], for instance, forms a 70% solution in benzene at room temperature. In
spite ofthe similar ionic radii ofGa + (1.20 A), In+ (1.40 Al and Tl + (1. 50 A) and
K+ (1.38 A), KGaCl 4 is quite insoluble. Similarly complex halides of tin(II) and
lead(II), such as M(AlCl 4 ) 2 also dissolve in arenes. Crystalline materials (ry 6 -
C6HJSnCl.AlCl4 and (ry 6 -C 6 H6)M(AIC1 4 ) 2 • C 6 H 6 (M = Sn. Pb) have been isolated
and their structures determined by X-ray diffraction.
If hot benzene is saturated with Ga[GaCI 4 ] and the solution cooled to
20°C, colourless crystals can be grown which have the composition
[(C 6 H 6 ) 2 Ga 2 Cl 4 ] 2 .3C 6 H 6 • These crystals Iose benzene readily under vacuum or on
purging with nitrogen. The mesitylene complex, however, is somewhat more
stable. In these compounds, bent sandwiches, (Arene) 2 Ga, in which the arenes
are symmetrically bonded to gallium, are linked together through bridging GaCl 4
units to form rings (arene= benzene) or chains (arene= mesitylene) (Fig. 3.24).
Some related indium and thallium complexes with even more complicated
structures have also been studied.
The ions Ga +,In+, Tl +, Sn 2 + and Pb 2 + ha ve the electronic configurations Core
( n-1 )d 1 0 ns 2 • The three p-orbitals are therefore vacant and can accept electrons
from the filled n-orbitals of the arene. Transition elements are also capable of 1] 6 -
coordination with arenes (p. 204), but here there are important additional
interactions involving the metal d-orbitals. Copper(I), silver(I) and gold(I) form
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