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

Organotransition metal chemistry
( Cb)- CH2CH3
(Cb)- COR
RX hl)
(Cb)----R-fCb)- R'+ (Cb)'
Fig. 7.6 Some reactions of reduced cobalamins and cobaloximes.
hydrochloric acids. The methyl group is cleaved by the electrophile HgCI 2 giving
MeHgCI.
7.2 Alkylidene and alkylidyne (carbene and carbyne)
complexes
Alkylidene and carbene complexes contain the grouping [M]=CRR'. Free
carbenes CRR' are ~ery reactive and short lived species (p. 61). The isolation of
complexes ofthese ligands is a good illustration ofthe stabilization oflabile species
by coordination to a transition element (cf. cyclobutadiene, p. 269 and trimethylenemethane
p. 2 70).
Carben~ is considered tobe a good a-donor and a weak n-acceptor. In its singlet
state it has a lone pair of electrons in an orbita! of sp 2 character which is coplanar
with the two C-H bonds; this acts as the donor orbita!. There is also an empty
C(2p) orbita! which can accept electron density from metal orbitals of n­
symmetry. A M=CH 2 bond would therefore be expected to show significant
double bond character. This is observed in alkylidene complexes such as
Cp 2 Ta(=CH 2 )CH 3 in which the Ta=CH 2 bond is considerably shorter than the
single Ta-CH, bond.
!Il\ 22}'cH3
Ta
@ 2.0~C-H
1/1070
In a large group of complexes (Fischer carbenes) one of the substituents on the
carbene contains a heteroatom X such as nitrogen, oxygen or sulphur which itself
232
H