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

Organotransition metal chemistry
In this elementary step, the electron count at the metal is reduced by two. This
mechanism is followed in the decomposition of the octahedral complexes fac
[ (PhMe 2 P) 2 PtMe)]-----+ trans[ (PhMe 2 P) 2 PtMei] + C 2 H 6 • Ethane is the only hydrocarbon
product. The reaction is first order in starting material and has an
activation enthalpy of 12 9 kJ moi- 1 • If homolysis to radicals were the rate
determining step, AHt would be near to the Pt-Me bond energy of
ca. 2 50 kJ moi- 1 • The intramolecular nature of the process is verified by
decomposing together a mixture of fac [(PhMe 2 P) 2 Pt(CD 3 ) 3I] and fac
[ (PhMe 2 P) 2 Pt(CH 3 ))]. C 2 H 6 and C 2 D 6 only are produced; there is no crossproduct
CH 3 CD 3 •
7.1.5 Carbon-hydrogen bond activation
Normally intramolecular elimination of alkane from alkyl(hydride) complexes
occurs readily and is favoured thermodynamically. There is interest, however, in
the possibility of carrying out the reverse reaction, the addition of a C-H bond to
an unsaturated transition metal centre. Alkanes are susceptible to electrophilic
attack, for example by Lewis or Br~nsted acids which convert linear alkanes into
their branched isomers via carbonium ion intermediates. Linear and cyclic
alkanes can be converted into aromatic hydrocarbons and hydrogen over metal
surfaces such as platinum. These reactions are carried out on a large scale
industrially in the reforming of petroleum.
Intramolecular cyclometallations are special cases of the desired reaction. They
are likely to compete successfully on entropy grounds over an intermolecular
process involving an alkane. Cyclometallation is favoured by steric congestion at
the metal centre. Such congestion should therefore be reduced as far as possible in
designing a system which might activate alkanes. Alkanes lack both lone pairs
and low lying empty orbitals. Systems have been discovered, however, with
which they do react. Interaction with a metal centre may occur initially through
M-H-C bridges ('agostic' hydrogens).
R
"'-H
In a reaction R-H + [M] --+ [M]/ the alkyl hydride will be stable only if the
fragment [M] is of high energy content, that is, is itself unstable. When Cplr( C0) 2
or the related compound (IJ 5 -C 5 Me 5 )1rH 2 (PMe 3 ) is photolysed in the presence of an
alkane, loss of CO or H 2 respectively occurs ta form very reactive, unsaturated
species CpirL (L = CO ar PMe 3 ). These can add to an alkane forming an alkyl
hydride.
R-H
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