Chemistry Reference
In-Depth Information
Fig. 2.15 [Fe
II
(phen)
3
]X
2
(phen = 1,10-phenanthroline) is a typical LS compound with
characteristic Mössbauer spectra shown on the left. If one of the relatively strong phen ligands
is replaced by two monofunctional NCS
-
groups, the average ligand field strength becomes
weaker such that now the condition for thermally induced SCO, viz. DE
HL
& k
B
T is met and the
compound [Fe
II
(phen)
2
(NCS)
2
] adopts HS character at room temperature. The temperature
dependent Mössbauer spectra shown on the right confirm that [Fe
II
(phen)
2
(NCS)
2
] undergoes ST
between S = 2 and S = 0 near 180 K
The influence of the ligand molecules on the spin state of the central Fe
II
ions is
demonstrated with the two examples and their temperature dependent Mössbauer
spectra shown in Fig.
2.15
.Fe
II
X
2
(phen = 1,10-phenanthroline) is a
typical LS compound at all temperatures under study as confirmed by the char-
acteristic Mössbauer spectra on the left with isomer shift of ca. 0.2 and quadrupole
splitting ca. 0.5 mm s
-1
independent of temperature. If one of the relatively strong
phen ligands, which occupies two coordination positions of the octahedron, is
replaced by two monofunctional isothiocyanato groups, the average ligand field
strength becomes weaker than the mean spin pairing energy and the compound
Fe
II
phe
ð Þ
3
adopts HS character at room temperature. The Mössbauer
spectrum at 300 K shows the typical features of an Fe
II
HS compound with isomer
shift of ca. 1 mm s
-1
and a large quadrupole splitting of ca. 3 mm s
-1
. However,
the compound
ð
phen
Þ
2
NC
ð Þ
2
fulfils the condition for thermal SCO to occur,
viz. DE
HL
& k
B
T. As the temperature is lowered, the compound changes spin state
from HS to LS near 180 K as is well documented by the Mössbauer spectra as a
function of temperature on the right hand side of Fig.
2.15
, which was first
reported by Dezsi et al. in 1967 [
27
]. Since then more than 200 SCO compounds of
Fe
II
Fe
II
ð
phen
Þ
2
NC
ð Þ
2
and Fe
III
have been studied by Mössbauer spectroscopy (see e.g. [
25
]).
