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

Organometallic compounds of the transition elements
axis with a-symmetry with respect to this axis, and a degenerate pair of two n­
orbitals, which are essentially the metal d zx and dyz orbitals with some p character
Molecular orbita! calculations using the extended Hiickel (EHMO) method on
M(CO), fragments lead to the general energy level diagram shows in Fig. 6.13.
It will be noted that 18 electrons are required to fill the nine orbitals shown in
Fig. 6.13 for the M(CO), fragment. When M = Cr, six come from M and six from
the three COs, requiring six more from the ligand L (e.g. C 6 H 6 ). When M =Pe,
eight come from M and six from the three COs, so that four more are needed from L
( e.g. C 4 H 4 or C 4 H 6 ). This is merely a restatement ofthe 18-electron rule. Fragment
analysis, however, enables us to see clearly the nature ofthe interactions between
the frontier orbitals ofthe M(CO), group (labelled xz, yz and hy in Fig. 6.12) and
the available 2pzn orbitals of the ligand. As an example consider the interaction
between the frontier orbitals ofthe Mn(CO), group and the 2pn m.o.s ofthe C 5 H 5
ligand to form the molecule (IJ 5 -C 5 H 5 )Mn( CO),. In the conformation shown, there
is a plane of symmetry passing through the metal atom and one CO group, which
bisects the C 5 H 5 ligand and also the angle between the other two carbonyl
ligands. This defines the xz plane; the point group is C 5 • The metal and ligand
orbitals can be classified as symmetric (a') or antisymmetric (a") with respect to
reflection across this plane, depending on whether or not the wave function
changes sign on carrying out this symmetry operation.
6rr*orbitols ~ e ===;;
~
of the ~· ~
01
/,/::::::==, 4 P
3 CO ligonds o1 hy----(.._
\\~4s
e \
xz,yz~
3 o--orbitols !'
/';41=!t ~ 3 d
',,
z2__jj___/
of the \ -rr-a1
'
3 CO tigonds ~ --~,,, '\~ e
' ' '
'---ft-- 01
3CO
,M~
,/J
o
o
c
Fig. 6.13 lnteraction diagram for an M(CO), fragment. Orbital occupancy for Cr(CO),
shown.
208
M