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

Five electron ligands
a pair of non-bonding electrons. (In the 16-electron compound Cp 2 TiCl 2 the
equivalent orbital is vacant.) Cp 2 MoH 2 acts as a base, being protonated by
trifluoroacetic acid to give Cp 2 MoH; and forming adducts Cp 2 MoH 2 .BF 3 and
Cp 2 MoH2'AlMe 3 with Lewis acids. The tungsten complex is similar.
Mean bond dissociation energies D(M-H) and D(M-Me) in the compounds
Cp 2 MX 2 are as follows: D(M-H)/kJmol- 1 : Mo, 252; W, 305. D(M-Me): Mo,
159; W, 209. Consider the hypothetical addition of methane to Cp 2 M:
Cp 2 M + H-Me~Cp 2 M(H)Me
in which a C-H bond is broken and M-H and M-Me bonds are formed. Given
that D(C-H) in methane is 435kJmol-1, AHreaction is +24kJmol- 1 (M=Mo)
and -79 kJmol- 1 (M = W). TASB-is estimated tobe about 55 kJmol- 1 at 298K
owing to the loss of one mole of gaseous methane. Although this reaction has not
been observed in this system with methane, other hydrocarbons have been
shown to react with Cp 2 WH 2 when they are irradiated with ultraviolet light.
It is believed that the addition proceeds through a labile tungstenocene species
'Cp 2 W'. Spectroscopic investigations of the species produced on photolysis of
Cp 2 WH 2 in an argon matrix at 20 K are consistent with this interpretation. Cp 2 W
apparently has a triplet ground state like chromocene, but is much more reactive
behaving more like a diradical. This radical character is shared by other bis
cyclopentadienyls of the heavy transition elements (apart from Ru, Os). Where
they have been observed at ali the monomers are very short lived and dimerize
rapidly to products rather complicated structures.
9.3 Cyclopentadienylmetal carbonyls
The cyclopentadienyl metal carbonyls form a large group of compounds the
chemistry of which has been explored in detail. Consequently only a few general
features can be covered here. Most ofthese compounds obey the 18-electron rule.
This is clearly illustrated by the series of mononuclear complexes CpV(C0) 4 ,
CpMn(C0) 3 and CpCo(C0) 2 (Fig. 9.9). The simplest derivatives ofthe intervening
elements Cr, Pe and Ni are binuclear; here the electron count is attained by
metal-metal bonding or by electron sharing through bridging carbonyls. The
formation of binuclear systems by alternate members of the transition series
resembles the similar formation of binuclear carbonyls in the series Cr(C0) 6 ,
Mn 2 (C0) 10 , Fe(C0) 5 , Co 2 (C0) 8 and Ni(C0) 4 •
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