Chemistry Reference
In-Depth Information
bridging structure.
589
On the other hand the monomeric tetra-
tert
-butoxide possesses a
distorted tetrahedral configuration having D
2d
symmetry which is well in accordance
with the requirements of the magnetic and electronic spectral data.
Adams
et al
.
219
determined the magnetic moment of chromium triethoxide and the
value was
eff
D 3
.
56 B.M. with
D 270
Ž
.Brown
et al
.
547
found the same value of
magnetic moment and also observed the temperature-dependent behaviour of magnetic
moments for chromium trimethoxide and triethoxide. These magnetic moment values
are less than the spin only value of 3.88 B.M. Assuming this low value to be due
to antiferromagnetic behaviour, they extrapolated the curve obtained by plotting
1
vs T
from which a high
value between 270 and 320
Ž
was obtained, leading to
a value of 3.88 B.M. for the magnetic moment of triethoxide; this value is again
close to the requirement of d
3
spin only value. Chromium dimethoxide shows
219
temperature-dependent paramagnetic behaviour having a magnetic moment of 5.16
B.M. with
D 160
Ž
and it belongs to the d
4
system. The above data for chromium
dimethoxide appears to indicate that it exhibits strong antiferromagnetic interaction.
Dubicki
et al
.
592
observed the temperature-dependent magnetic behaviour of chromium
dichloride monomethoxide monomethanolate; the magnetic moments of this product in
the temperature range 177 to 20
Ž
C was of the order of 3.32 - 3.66 B.M. These values
are less than the spin only value of 3.88 B.M. required for a d
3
ion. The magnetic
moment measurements in solvents like acetonitrile, dioxane, and acetone also showed
low values and these low values have been assumed to be due to the spin interaction of
neighbouring Cr
3C
ions in solution. The magnetic susceptibility measurements corre-
sponded to its dimeric behaviour and this was supported by the observed molecular
weight of the compound. The tetrameric structure shown earlier for this dimeric product
has been assumed on the basis that two dimeric molecules will combine to form such
a tetrameric structure, thus satisfying the stoichiometric and geometric requirements.
Manganese dimethoxide also shows a temperature-independent magnetic moment
of value 5.96 B.M. Thus a comparison of the results indicate that the dimethoxides
of divalent metal (manganese, iron, nickel, and cobalt) show temperature-independent
magnetic moments between 193 and 77
Ž
C and thus follow the Curie - Weiss law.
The compounds are weakly antiferromagnetic and the crystal lattices involve MO
6
octahedral geometry. The dimethoxides of divalent chromium and copper on the other
hand exhibit marked temperature-dependent magnetic moments and are strongly anti-
ferromagnetic.
Ferric alkoxides exhibit significant temperature-dependent magnetic behaviour. The
magnetic moments for solid trimethoxide and triethoxide, and liquid tri-
n
-butoxide at
ambient temperature are found to be 4.51, 4.37, and 4.35 B.M., respectively (against
5.90 B.M. required for a high-spin d
5
system), but these reduce considerably at lower
temperatures. For example at 181
Ž
C, these alkoxides show moments of the order of
3.24, 3.25, and 3.44 B.M., respectively.
474
It has also been noted that these alkoxides
follow the Curie - Weiss law with
values between 170
Ž
and 200
Ž
C as shown
by the curve of the variation of magnetic susceptibility and magnetic moments with
temperature (Fig. 2.8a). From the curve it may be expected that at higher temperature,
the higher spin levels become densely populated and the slope would correspond to
(
eff
2
D 105
B
.
M
.
2
whilst at sufficiently low temperatures the magnetic moment
would decrease to the value corresponding to
eff
2
D 1
.
0
B
.
M
.
2
per Fe
3
core.
