4 From Small Molecules to Complex Systems: A Survey of Chemical …
189
means of the Mössbauer effect [52, 53]. Iron(II) containing SCO complexes are
mostly in an octahedral ligand field which is caused by six nitrogen ligands. When
the iron is in its ferrous low spin state the three t 2g orbitals are fully occupied with 6
electrons (see Fig. 4.3). This situation causes a symmetric charge distribution around
the iron nucleus and thus only a small or sometimes even zero quadrupole splitting
is observed. On the other hand, when the complex is switched to its S = 2 state
the iron ligand distances increase. The partial negatively charged ligands are farer
away from the iron ion and thus the crystal field splitting between the t 2g and the e g
orbitals decreases. This is caused by the occupation of the σ–antibonding e g d-orbitals
which leads to a reduced bond strength in comparison to the t 2g
6 low spin state. As
a consequence the electronic population of the orbitals changes. Now the iron ion
faces a weaker ligand field and the electrons populate the d-orbitals according to
Hund’s rule with a maximum spin multiplicity resulting in a S = 2 state.
In order to understand the increase in quadrupole splitting it is advantageous to
consider first the symmetry of the negative charge density caused by the five spinup electrons. This charge density is spherical symmetric around the iron ion and
thus does not significantly contribute to the quadrupole splitting. This is actually the
reason that octahedral coordinated iron(III) ions with S = 5/2 in most cases have a
small quadrupole splitting <0.8 mms
−1 . For ferrous high spin ions the main electronic
contribution comes from the excess spin down electron in the double occupied t 2g
orbital. The charge distribution caused by one electron has been estimated to app. 3–
4.5 mms
−1 [4, 54] which easily explains the high value of the quadrupole splitting
in ferrous high spin ions.
Fig. 4.8 Mössbauer spectra at indicated temperatures of a trinuclear iron(II) complex undergoing
SCO analyzed with three doublets having lorentzian line shape. The purple doublet represents the S
= 0 fraction of the central iron (Fe2). The blue doublet is due to the S = 2 fraction of the two terminal
irons (Fe1 and Fe3) which stay in their high spin state at all temperatures. Raising the temperature
leads to the green doublet representing the high spin fraction of the central iron (Fe2). Also shown
is the labelled core of the complex with the iron(II) sites (spheres) being colored differently to
distinguish the spin state (S = 2: red, S = 0: purple). Adapted from [55]
189
means of the Mössbauer effect [52, 53]. Iron(II) containing SCO complexes are
mostly in an octahedral ligand field which is caused by six nitrogen ligands. When
the iron is in its ferrous low spin state the three t 2g orbitals are fully occupied with 6
electrons (see Fig. 4.3). This situation causes a symmetric charge distribution around
the iron nucleus and thus only a small or sometimes even zero quadrupole splitting
is observed. On the other hand, when the complex is switched to its S = 2 state
the iron ligand distances increase. The partial negatively charged ligands are farer
away from the iron ion and thus the crystal field splitting between the t 2g and the e g
orbitals decreases. This is caused by the occupation of the σ–antibonding e g d-orbitals
which leads to a reduced bond strength in comparison to the t 2g
6 low spin state. As
a consequence the electronic population of the orbitals changes. Now the iron ion
faces a weaker ligand field and the electrons populate the d-orbitals according to
Hund’s rule with a maximum spin multiplicity resulting in a S = 2 state.
In order to understand the increase in quadrupole splitting it is advantageous to
consider first the symmetry of the negative charge density caused by the five spinup electrons. This charge density is spherical symmetric around the iron ion and
thus does not significantly contribute to the quadrupole splitting. This is actually the
reason that octahedral coordinated iron(III) ions with S = 5/2 in most cases have a
small quadrupole splitting <0.8 mms
−1 . For ferrous high spin ions the main electronic
contribution comes from the excess spin down electron in the double occupied t 2g
orbital. The charge distribution caused by one electron has been estimated to app. 3–
4.5 mms
−1 [4, 54] which easily explains the high value of the quadrupole splitting
in ferrous high spin ions.
Fig. 4.8 Mössbauer spectra at indicated temperatures of a trinuclear iron(II) complex undergoing
SCO analyzed with three doublets having lorentzian line shape. The purple doublet represents the S
= 0 fraction of the central iron (Fe2). The blue doublet is due to the S = 2 fraction of the two terminal
irons (Fe1 and Fe3) which stay in their high spin state at all temperatures. Raising the temperature
leads to the green doublet representing the high spin fraction of the central iron (Fe2). Also shown
is the labelled core of the complex with the iron(II) sites (spheres) being colored differently to
distinguish the spin state (S = 2: red, S = 0: purple). Adapted from [55]