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

Organometallic compounds of the transition elements
C2H4 C3H5 C4H4 C5H5 CsHs C7H7 Ca Ha
-2~ 5--
--
-~ TT-- --
+=
o
+ =#= *o
~
()--%-- + *
* + *TT
2r cr+ + + + +()
-- Non bonding
Fig. 6.8 Hiickel energies of ligand orbitals.
decrease with increasing si:r.e of the delocalized system. Their occupancy also
increases. The n m.o. in C 2 H 4 is empty, so that it is a n-acceptor. In benzene,
however, both n-m.o.s are filled, so that this ligand acts as an-donor as well as a rrdonor.
c5-Interactions are rather unimportant for small ligands where the
relevant orbitals are ofhigh energy compared with metal dx2_Y2 and dxy, but they
play a bigger part in the binding of larger ligands such as 1] 8 -C 8 H 8 to metals.
~ ~
Ligand
(b2gl Cr Cr (b1ul
orbi fals
non-bonding
(empty in complex)
~ ~
202