Chemistry Reference
In-Depth Information
Fig. 2.19 a View of the crystal structure of [Fe(mtz)
6
](BF
4
)
2
showing two lattice sites, A and
B at 300 K occupied by Fe
II
ions in the HS state with metal to ligand bond distances Fe(A) -
N = 2.181(5), 2.181(5), 2.181(7) Å and Fe(B)-N = 2.161(5), 2.197(5), 2.207(4) Å [
30
].
b Temperature dependent Mössbauer spectra of [Fe(mtz)
6
](BF
4
)
2
over the temperature range
160-60 K [
31
]. From room temperature down to ca. 160 K, only one quadrupole doublet arising
from iron(II) ions in HS(A) and HS(B) state is observed, i.e. A and B site ions are not
distinguishable in the Mössbauer spectrum. On further cooling, the A site ions undergo HS to LS
transition whereas the B site ions remain in the HS state at all temperatures under study
2.3.2.3 Mechanism of Spin State Switching in Fe
II
Dinuclear Compounds
The dinuclear Fe
II
compound [Fe(bpym)(NCS)
2
]
2
bpym (bpym = bipyrimidine)
belongs to a family of materials that can combine both antiferromagnetic (AF)
coupling and SCO phenomena. Its crystal structure, shown in Fig.
2.21
a, consists
of two Fe
II
centers, each of them coordinated to a bidentate bpym and two
monodentate isothiocyanato anions, and bridged by a bipyrimidine ligand to form
a dinuclear unit [
32
]. This compound does not show thermal ST, but the magnetic
susceptibility measurements reveal weak AF coupling (J & -4.1 cm
-1
).
Replacing the NCS group by NCSe with slightly stronger ligand field strength
causes temperature induced ST near 120 K as seen in Fig.
2.21
b, which shows the
temperature dependent magnetic properties of [Fe(bpym)(NCSe)
2
]
2
bpym as a plot
of v
M
T vs. T, where v
M
is the magnetic susceptibility corrected for diamagnetic
contributions. It reveals the presence of weakly coupled HS Fe
II
ions in the region
between room temperature and ca. 120 K. Around this temperature, a dramatic
decrease of v
M
T due to thermally induced ST is observed to reach a plateau with a
