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

Complexes of arenes
For chromium this can be reduced by alkaline aqueous dithionite or, with loss of
half the product, allowed to disproportionate in aqueous solution.
or
2(C 6H 6 ) 2 Cr+ + S 2 o;- + 20H- -+2(C 6 H 6 ) 2 Cr + 2HSO;
2(C 6 H 6 ) 2 Cr+---+ (C 6 H 6 ) 2 Cr + Cr2 + + 2C 6 H 6
The green complex bis(benzene )molybdenum can be prepared similarly in up to
70% yields
3MoCI 5 +AICI,+ 4Al + 6C 6 H 6 ---+ 3(C 6 H 6 ) 2 Mo + AICI 4 -
6(C 6 H 6 ) 2 Mo+ + 80H----+ S(C 6 H 6 ) 2 Mo + Moo;- + 4H 2 0 + 2C 6 H 6
The method has been applied to many ofthe transition elements. The cations first
produced canin some cases be converted into the neutra! complexes by reduction
or disproportionation. It suffers from the disadvantage that arenes with
functional groups such as halogen, acyl etc. cannot be used as they interact with
the aluminium trichloride.
10.1.2 Metal atom synthesis
Bis( arene )complexes are, like the metallocenes, endothermic compounds. The
standard enthalpy of formation of bis(benzene )chromium from the elements is
141.4 kJ moi - 1 • Nevertheless they do often exhibit a fair degree of thermal
stability. The mean bond dissociation energies D(M-arene) in Cr(C 6 H 6 ) 2 ,
Mo(C 6 H 6 ) 2 and W(C 6 H 5 Me) 2 defined according to
M(arene) 2 (g)---+ M(g) + 2arene(g); ~H 9 = 2D(M-arene)
have been estimated as 165, 247 and 304kJmol- 1 respectively. The trend of
increasing bond strength down a group, previously noted for transition elements
(p. 5) is thus maintained here.
Can M(arene) 2 be prepared from the metal and arene? At first sight this might
appear tobe ludicrous, bearing in mind the unreactive nature both ofbulk metals
and also of arenes. Ferrocene, however, was prepared by Miller, Treboth and
Tremaine by passing cyclopentadiene over hot metallic iron. The reaction
Cr(s) + 2C 6 H 6 (g)-+ Cr(C 6 H 6 ) 2 (s) has ~H"' = - 11.6 kJ mol- 1 ( + 69.5 kJ mol- 1
for Mo), although the entropy term is unfavourable. This reaction does not occur
at room temperature, however, and becomes increasingly disfavoured thermodynamically
as the temperature is raised.
Transition metals have high energies of atomization (Cr, 397; Mo, 658; W,
860 kJ mol- 1 ). In other words, the gaseous atoms possess high energy content. A
reaction such as Cr(g) + 2C 6 H 6 (g)-+ Cr(C 6 H 6 ) 2 (s) is strongly favoured thermodynamically
~H 0 = - 408 kJ moi- 1 (- 5 8 9 kJ moi- 1 for Mo ). The 'metal atom'
synthesis takes advantage of this stored energy. While compounds such as
Cr(C 6 H 6 ) 2 cannot survive at the temperatures required to vaporize chromium
metal. it is possible by rapid chilling of the vapour to trap the atoms ( or small
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