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

Molecular orbita! theory
n -(fi ll e d)
n' (empty)
( empty) (fi lled)
(a)
(b)
Fig. 6.4 Conventional representation of the metal-alkene bond. (a) Alkene--+ M a­
donation: (b) M::}lkene n:-back donation.
dx2~y2 orbitals is of 0'-rather than of 6-symmetry.
An older but stil! useful view of the bonding is given in Fig. 6.4. This shows 0'­
donation from the filled bonding (p,rr) molecular orbita! of ethene into an empty
hybrid metal orbita! and back-donation from a filled hybrid metal orbita! into the
empty antibonding (p,rr*) m.o. of ethene. This description resembles the situation
in metal carbonyls (p. 15 7). Like CO ethene is too weak a 0'-donor to form adducts
with typical Lewis acids such as BMe 1 , which complex with ammonia. Unlike
ammonia, however, CO and C,H 4 possess empty orbitals oflow energy which can
accept electrons from filled d-orbitals of transition elements. It is difficult to evaluate
the separate contributions from 0'- and rr-components in a metal-alkene
bond. Both removal of electrons from the bonding m.o. of ethene and also electron
donation into the antibonding m.o. lead to a weakening of the C=C bond. In
agreement with this the frequency of the C=C stretching vibration falls by 60 to
150 cm ~ 1 on complexing. In K+ [C,H 4 PtCI 3 r this hand is at 1511 cm ~ 1 in the
infrared spectrum; in the Raman spectrum of free ethene the corresponding hand
is at 1623cm~ 1 . Moreover the C=C bond Iength in Zeise's salt is l.354Â
(cf. 1.33 7 Â in ethene).
The relative proportions of alkene---> M tT-donation and M---> alkene rr-backdonation
are influenced by the electron density at the metal centre. In Zeise's salt
the 0'-component is thought tobe dominant. Where electron withdrawing ligands
\ f\
H------\:.,-->:.,---H
' '
'' 1 '
'
M
(el
'
Planar alkene
(sp 2 carbon)
u-donation dominant
Alkene substituents bent
up out of plane (sp 1 carbon)
n:-back donation dominant.
Fig. 6.5 Two extreme representations of the metal-alkene bond.
197