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Compression is preferred because in this case the ground state is an orbital singlet
with the doubly occupied xy orbital being lowest in energy. Suppose all d-orbitals
are singly occupied, as for instance in the case of
3 þ H ð ; EF ð Þ val
would be zero. But the sixth electron placed in the lowest xy orbital in the case of
Fe H 2 ð Þ 6
Fe H 2 ð Þ 6
2 þ accumulates more charge in the xy plane than along the z-axis and,
thus, causes a large (EFG) val = 0 and the observed quadrupole splitting.
2.3.1.2 Structure of Fe 3 (CO) 12
On the basis of single crystal X-ray diffraction measurement three possible
molecular structures had been suggested for the metalorganic compound
Fe 3 (CO) 12 [ 18 ]. In all cases the iron atoms form a triangle, but with different
surroundings by the CO groups. In the upper two structures of Fig. 2.11 the three
iron atoms have identical surroundings, the Mössbauer spectrum is expected to
show only one type of resonance signal. The lower structure has two identical iron
positions and a different one for the third iron atom. In this case the Mössbauer
spectrum is expected to show two different types of resonance signals with an area
ratio of 2:1. A Mössbauer effect study performed by Greatrex and Greenwood in
1969 [ 19 ] indeed showed two types of resonance signals, a quadrupole doublet A
and a singlet B with an area ratio of 2:1 confirming the presence of two types of
iron positions in Fe 3 (CO) 12 (Fig. 2.11 ).
3 þ n
ð
Þ
Fe C ð 5 X n
2.3.1.3 Effect of p-Backdonation in
The following example demonstrates that Mössbauer spectroscopy can help to
characterize chemical bond properties. Taking from the literature [ 7 ] the isomer
shift data for the pentacyano complexes of Fe II with a different sixth ligand X and
normalizing the isomer shifts to that of the pentacyanonitrosylferrate complex as
Fig. 2.11 The Mössbauer spectrum of Fe 3 (CO) 12 shows a quadrupole doublet A and a singlet
B with area ratio 2:1, which confirms that the structure with two inequivalent lattice sites of iron,
A and B, is the correct structure [ 19 ]
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