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

Molecular orbital theory
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Ferrocene Dibenzenechromium 0sh
Symmetry (a) ( b) Symmetry
Fig. 6.10 Schematic illustrations of the important orbita! interactions in (a) Fe(1]-C,H,) 2
and (b) Cr(1]-C 6 H 6 ) 2 • (After Mingos, D.M.P .. 1982, Comprehensive Organometallic
Chemistry, Vol. 3. Ch. l.)
applies both to the neutra! molecule and to the molecular ion. Molecular orbita!
calculations of good quality reveal that this is not always true. Nevertheless the
PE spectrum ofbis(benzene)chromium (Fig. 6.11) can be assigned on the basis of
the m.o. diagram in Fig. 6.10. The first sharp bandA is assigned to ionization
from the essentially non-bonding d,2 (a 1 g) orbita!. The second band B is of
approximately double the intensity ofthe first and is broad. This is consistent with
ionization from the doubly degenerate e,g orbita! (reason for intensity) which is
bonding in character (reason for breadth). The large group of ionizations at
ca. 9-10 eV are from essentially ligand orbitals e 1 u, e,g' while those at still higher
energies are associated with the a 2 u and a,g orbitals and the a-skeleton (C-H and
C-C bonds) of the ligands.
If Koopmans' theorem is not obeyed, the order of m.o.s indicated by the
spectrum will not correspond to that present in the neutra! molecule. Even so,
much useful information can be derived from studying the photoelectron spectra
of series of closely related complexes. In this way the effect of changing ligand
substituents for example, on the relative energies of molecular orbitals can be
investigated.
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