Chemistry Reference
In-Depth Information
Fig. 2.37 Molecular structure and temperature dependent
57
Fe Mössbauer spectra of the mixed
valence complex compound [Fe
3
O(O
2
CCH
3
)
3
)
6
(3-Et-py)
3
]
S with S = 0.5 benzene (a), CH
3
CN
(b) and CH
3
CCl
3
(c). Solvates A and B are ''valence-trapped'' (localized) with two HS-Fe
III
ions
(red doublet) and one HS-Fe
II
ion (green doublet) in the molecular unit; the quadrupole doublets
are well resolved without significant line-broadening. c Shows a transition from electron
localization to delocalization (valence-trapped to detrapped) with increasing temperature. The
sharp doublets of the trapped species begin to broaden on heating and finally disappear at the favor
of a time-averaged new signal (blue) from a species with averaged oxidation state, ''Fe
2.67+
'', due to
fast electron fluctuation within the Fe
3
O triangle [
68
]
The resonance lines of the two quadrupole doublets begin to broaden around 50 K.
Further increase of temperature leads to valence-detrapping (delocalization) near
room temperature, where the red and green doublets have disappeared at the favor
of a new doublet (blue) which is a time-averaged resonance signal of a species
with an ''average'' oxidation state of ''Fe
2.67+
'' due to rapid electron circulation
within the Fe
3
O triangle of the complex.
2.3.3.3 Valence Fluctuation in a Trinuclear Cationic Complex
Another interesting example of valence fluctuation and temperature dependent
transition between localized and delocalized electronic structures in a trinuclear
transition metal compound was reported by Glaser et al. [
69
]. In this case the
electron fluctuation takes place between two iron centers of different oxidation states
and separated by a diamagnetic Co
III
ion. The
57
Fe Mössbauer spectra clearly show
that at sufficiently low temperatures, i.e. 5 K, the two iron centers are reflected as
localized oxidation states with a well resolved doublet for LS-Fe
III
(red) and a
poorly resolved doublet for LS-Fe
II
(light-blue). At higher temperatures the
