Chemistry Reference
In-Depth Information
Table 7.7
Comparison of pairing energy (
P
) versus ligand field splitting energy (
o
) for some
metals ions with d
4
-d
6
electronic configurations, and consequences in terms of observed spin state.
d
n
Ion
Ligands
P
o
Spin State
d
4
Cr
4
+
(OH
2
)
6
282
166
HIGH (P
o
)
Mn
3
+
(OH
2
)
6
335
252
HIGH (P
o
)
d
5
Mn
2
+
(OH
2
)
6
306
94
HIGH (P
o
)
Fe
3
+
(OH
2
)
6
360
164
HIGH (P
o
)
d
6
Fe
2
+
(OH
2
)
6
211
125
HIGH (P
o
)
(CN
−
)
6
211
395
LOW (P
o
)
Co
3
+
(F
−
)
6
252
156
HIGH (P
o
)
(NH
3
)
6
252
272
LOW (P
o
)
(CN
−
)
6
252
404
LOW (P
o
)
Low spin and high spin arrangements
exemplified for d
6
[
o
(low spin)
o
(high spin)]:
'
o
o
low spin
high spin
example. However, this definition is significant in terms of the variable number of unpaired
electrons that can be present in a transition metal complex. Since we are dealing with five d
orbitals, the maximum number of unpaired electrons that can be associated with one metal
ion is five - simply, there can be only from zero to five unpaired electrons, or six possibilities.
Experimentally, dia- and para-magnetism can be detected by a
weight change
in the
presence of a strong inhomogeneous magnetic field. With the sample suspended in a
sample tube so that the lower part lies in the centre of a strong magnetic field and the upper
part outside the field, the experimental outcome is that diamagnetic materials are heavier
whereas paramagnetic ones are lighter than in the absence of a magnetic field. Even the
number of unpaired electrons can be determined by experiment, based on theories that we
shall not develop fully here. However, we will explore the factors that contribute to the
magnetic properties, usually expressed in terms of an experimentally measurable parameter
called the magnetic moment (
).
We are dealing in our model with electrons in orbitals, which are defined to have both
orbital motion and spin motion; both contribute to the (para)magnetic moment. Quantum
theory associates quantum numbers with both these motions. The spin and orbital motion
of an electron in an orbital involve quantum numbers for both
spin momentum
(
S
), which is
actually related to the number of unpaired electrons (
n
)asS
=
n
/2, and the
orbital angular
momentum
(
L
). The magnetic moment (
) (which is expressed in units of Bohr magnetons,
B
) is a measure of the magnetism, and is defined by an expression (7.1) involving both
quantum numbers.
1)]
1
/
2
=
[4
S
(
S
+
1)
+
L
(
L
+
(7.1)
The introduction of an equation involving quantum numbers may be daunting, but we
are fortunately able to simplify this readily. Firstly, for
first-row
transition metal ions,
the effect of
L
on
is small, so a fairly valid approximation can be reached by ne-
glecting the
L
component, and then our expression reduces to the so-called 'spin-only'
