Chemistry Reference
In-Depth Information
2.3.3 Mixed-Valence Compounds, Intramolecular
Electron Transfer
Mixed-valence compounds are known to possess two or more transition metal ions
with different oxidation states. Intramolecular electron transfer may take place
between (generally two) heterovalent metal centers over suitable bridging atoms or
ligand molecules. There may be three cases observed in the Mössbauer spectrum.
(1) If the electron fluctuation rate s f is less than the reciprocal of the Mössbauer
time window (given by the lifetime s n of the nuclear excited state of the
Mössbauer probe), s f \ s n , the spectrum will show two resolved resonance signals
arising from the static (''localized'') oxidation states. (2) If s f [ s n , the spectrum
will show a single time-averaged resonance signal which is different from the two
signals of case (1) and points at an oxidation state in between those of the two
static components. One speaks of a ''delocalized'' mixed-valence compound. (2) If
s f & s n , the Mössbauer spectrum will be the sum of overlapping heavily broad-
ened resonance lines of cases (1) and (2); the spectrum will be very complicated
and difficult to analyze. As the fluctuation rate s f is temperature dependent, one
often observes a transition between the localized and delocalized cases during
variation of temperature.
In the following, we shall briefly discuss three examples of studies of mixed-
valency systems with Mössbauer spectroscopy.
2.3.3.1 Mixed-Valence Biferrocenes
In Fig. 2.36 is schematized the molecular structure of biferrocenes which possess
two iron atoms with different valence electron structures. Depending on the nature
of the substituent R, the iron centers have either static but different electronic
structures referred to as ''localized'' ferrocene and ferricinium, respectively. Or a
rapid electron transfer between the two iron centers takes place and as a result of
the fast fluctuation one observes a time-averaged species with an electronic
structure between the two localized species. With the help of 57 Fe Mössbauer
spectroscopy one has studied the biferrocenes containing ethyl groups as R sub-
stituents as a function of temperature. At 115 K, the spectrum shows two well
resolved quadrupole doublets, the light-blue one being characteristic of ferrocene-
like iron, the red one of ferrocinium-like iron. With increasing temperature a fast
electron fluctuation sets in and the doublets of the two localized species turn into
one quadrupole doublet (shown in green at 287 K) of a time-averaged species, the
parameter values of which are different from those of the two localized species.
From the sharpness of the quadrupole doublet of the time-averaged species one can
conclude that the electron fluctuation rate must be faster than 10 7 s -1 corre-
sponding to the time-window of 57 Fe Mössbauer spectroscopy. This example
shows a thermally induced transition between localized and delocalized electronic
structures in a mixed-valence organometallic compound.
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