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

Organometallic compounds of the transition elements
such as CI or CO are present the O"-component will be enhanced and the n­
component diminished relative to cases where the ligands are electron donating.
Alkenes with electron attracting substituents such as F, CN or COOH are poorer O"­
donors and better n-acceptors than the parent alkenes. The lengthening of the
C=C bond in an alkene on complex formation is particularly marked where
strong n-back donation is expected. This is well illustrated by measurements on
the complex (1] 5 -C 5 H 5 )Rh(C 2 H 4 )(C 2 F 4 ). A high degree of n-back donation is
accompanied by some bending back ofsubstituents out ofthe plane ofthe alkene
away from the metal. In the extreme (e.g. (Ph 3 P) 2 Pt{C 2 (CN) 4 }) the structure
approaches that of a metallacyclopropane (Fig. 6.5).
(a) ROTATION OF AN ALKENE ABOUT THE METAL~LIGAND BOND. The
ethene-metal bond thus consists of a O"- and a n-component. In this it formally
resembles the C=C double bond in ethene itself. Rotation about a C=C double
bond is restricted; Z and E geometrica! isomers can be isolated at ambient
temperatures as the activation energy barrier which separates them is about
150-250 kJ mol- 1 , depending on substituents. The energy barriers to rotation of
an alkene about the metal-alkene axis, however, are in general much lower than
this, falling in the range 30-llOkJmol- 1 • The O"-component of the bonding,
being cylindrically symmetrical about the rotation axis, does not contribute to
this barrier. The n-component, however, can be formed by interaction of either
Pxn* with d,x or pyn* with dy,· If d,x and dY, are degenerate in the complex, any linear
combination of the two orbitals is possible and this leads to equal metal-alkene
CH C2H4 CH 3
ţl
35°C
60& 2.0&
~l
-45°C
6.0& 20&
Fig. 6.6 lOOMHz 'Hn.m.r. spectra of PtCI(C 2 H 2 ) (acac). S. denotes 195 Pt-'H satellites.
(From C.E. Holloway, G. Hulley, B.F.G. Johnson and J. Lewis, ]. Chem. Soc. (A),
(1969). 53.)
198