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

Cluster compounds
It is extremely stable both thermally (m.p. 320°C) and to oxidation in air. On
heating to 4 70°C in the gas phase for several hours or by flash pyrolysis to 600°C
for 30 s, 1, 2-C2B10H12 isomerizes to the 1, 7-isomer (m.p. 265°C). Further
isomerization to the 1,12-isomer (m.p. 261 °C) occurs on heating 1, 7-C2B10H 12
for a few seconds to 700°C.
The C-H bonds in closo-carboranes are appreciably acidic. Reaction of 1, 2-
C2B10H12 with butyllithium yields lithio derivatives which provide a starting point
for an extensive chemistry of organo-substituted carboranes. Heat resistant
silicone polymers have been developed commercially which include carboranes
in their backbones. The equilibrium lies to the left in benzene, so the mono-
HC-CLi
\1
8 10H10
-....----- +
lithiated compound is prepared in that solvent, whereas tetrahydrofuran is used
to make the dilithio derivative.
Degradation of closo carboranes such as closo-1, 2- or -1, 7-C2B10H12 with base
affords corresponding isomers of the nido anion
p.e.c. 50
(4n + 2)
EtOH NaH 2 _
----+ C 2 B 9 H; 2 ------> C 2 B 9 H,,
EtO
p.e.c. 48
p.e.c. 48
(4n + 4)
(4n + 4)
11.4 Metallocarboranes and metalloboranes
In these reactions the BH vertex adjacent to the CH vertices is removed. In the
C 2 B 9 H~; ion five orbitals are directed from the five atoms ofthe pentagonal face
towards the site vacated by this BH group. Six electrons can be assigned to these
orbitals, so that the ion strongly resembles the cyclopentadienyl anion C,H;.
(C2B 9 H;1 is a five-electron ligand). Transition metal complexes have been
prepared which have sandwich structures analogous to that of ferrocene.
Reaction between C2B 9 Hi; and FeCl2 in tetrahydrofuran yields the pink
diamagnetic complex [Fe(C2B 9 Hn)2)2- which is reversibly oxidized to the red
paramagnetic anion [Fe(C2B 9 Hn)2r, which corresponds to Fe(C,H,);. Carborane
complexes of a wide range of transition elements are now known. They
include mixed sandwiches e.g. [CpFe{1] 5 -C 2 B 9 Hn)1 and [CpCo(1] 5 -C 2 B 9 Hn)1 as
well as half sandwiches e.g.
344
[M(CO),(IJ 5 -C 2 B 9 H 11 )F- (M = Cr, Mo, W)