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

Alkyls and aryls
corresponds to J)-hydrogen transfer, o ne of the most important pathways for the
decomposition of transition metal alkyl derivatives, which is discussed below.
7.1.4 Decomposition pathways
In Table 7.2 pathways for decomposition of alkyl and aryl derivatives of the
transition elements are summarized. Only the initial step of the decomposition is
shown. The metal-containing species produced in this step can sometimes be
isolated or detected spectroscopically, but it may also decompose further.
(a) /3-HYDROGEN TRANSFER (/3-ELIMINATION). J)-Hydrogen transfer is well
recognized in Main Group chemistry as a pathway for decomposition of alkyl
derivatives (e.g. Be, p. 54, B, p. 66 and Al, p. 80). It also provides a major
mechanism for transition metal alkyls. Chatt found that the ethyl platinum
complex below eliminates ethene on heating, leaving an isolable hydride. The
latter takes up ethene again under pressure to regenerate the ethyl complex. This
shows that the reaction is reversible.
IXOT
trans[ (Et,P) 2 PtC!(CH 2 CH,)] ~ trans[ (Et,P) 2 PtCIH] + C 2 H 4
(16e)
YS"C/-J-Oatm
For the elimination to proceed. a vacant coordination site at the metal centre is
required. In the example quoted the square planar platinum complex has only a
16-electron configuration, so it is coordinatively unsaturated. In 18-electron
compounds this site has to be provided by dissociation or by change of hapto
number of a ligand.
The equilibrium involving J)-hydrogen transfer has been observed directly by
proton n.m.r. spectroscopy. The complex [Ru(IJ 6 -C 6 H 6 ){1J 2 -C 2 H 4 ) (PMe,)] is a
strong base which is readily protonated to give a hydride [RuH(IJ 6 -
C6H6) (C 2 H 4 ) (PMe 3 )] +. At - 78°C separate signals from the ethyl complex and
the alkene hydride with which it is in equilibrium are observed, as the
interconversion is slow. As the temperature is raised the signals from the
individual species coalesce and emerge as an averaged spectrum. The ethyl
compound can be trapped by addition oftrimethylphosphine, which occupies the
free coordination site which is required for the hydrogen transfer to proceed. It is
~ (i)CF3C02H
Ţ (iilNH~PFs-
/Ru"'-
~ PMe 3
~ PMe3
~-
/Ru+~
H2\ ~ PMe 3
CHf
(16e;18eif
(18e) (18e) agostic H)
217