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

Molecular orbital theory
6.2 Molecular orbital theory
The bonding of unsaturated organic ligands to transition metals is conveniently
discussed in terms of molecular orbital theory. For significant interaction to occur
(a) metal and ligand orbitals must be of similar energies ( this energy requirement
is met by metal ns, np and (n- l)d and ligand C(2p)n orbitals) and (b) the
symmetry properties of the metal and ligand orbitals must be the same. By
considering the symmetry of a molecule and by applying group theory, it is
possible to deduce which orbitals of the metal and which of the ligands fulfil the
second condition. We concentrate first on one symmetry element, an axis of
rotation which passes through the metal atom and lies perpendicular to the plane
of the ligand. This is defined as the z-axis. Metal and ligand orbitals can be
classified as having CJ, n or 3 symmetry (the last applies only if the rotational
symmetry ofthe molecule is > 2) with respect to this axis. The phase ofthe wave
function of a CJ-orbital does not change sign on rotation through 180° about this
symmetry axis. With a n-orbital the sign changes once, while with a 3-orbital it
changes twice. This division into CJ, n and 3-orbitals is further illustrated in
Fig. 6.7.
As only ligand and metal orbitals of the same symmetry can combine to form
bonding and antibonding molecular orbitals it is often possible through symmetry
considerations to developa qualitative picture ofthe bonding in a molecule. We
contine ourselves here to the valence orbitals of the metal and the p molecular
orbitals of the ligand. Orbitals of similar energy interact more strongly than those
of disparate energy. Estimates of the energies of the interacting metal and ligand
orbitals are obtained from spectroscopic measurements (UV /visible spectra, UVphotoelectron
spectra). The ordering of molecular orbitals in a complex is
deduced from the results of molecular orbital calculations in conjunction with
spectroscopic (especially UV-photoelectron spectral and magnetic data. Even
though more and more powerful computers are becoming available, molecular
Fig. 6.2 Structure of the
anion [C,H.PtCI.J-.
Table 6.2. Symmetry of metal orbitals with respect to
rotation about z-axis (metal-ligand axis).
Symmetry
(J
1[
b
Metal Orbitals
S, p2 , d"z
Px• Py• dzx, dyz
dx'-y'• dxy
Cl y'
1
!
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