Chemistry Reference
In-Depth Information
As a guide to how absorbance band intensity varies with selection rules that allow or
forbid the transition, experimental results for some simple compounds can be compared.
The high-spin octahedral d
5
[Mn(OH
2
)
6
]
2
+
ion, which is spin forbidden and Laporte for-
ε
max
0.1 M
−
1
cm
−
1
); the octahedral d
1
[Ti(OH
2
)
6
]
3
+
ion, which is spin allowed and Laporte forbidden, has a moderate-sized band (
bidden, has a very low intensity band (
ε
max
10 M
−
1
cm
−
1
); the d
7
tetrahedral [CoCl
4
]
2
−
ion, which is spin allowed and partially Laporte al-
lowed, has a markedly greater band intensity (
ε
max
500 M
−
1
cm
−
1
), whereas the octahedral
d
0
[TiCl
6
]
2
−
ion, which is spin allowed and Laporte allowed (i.e. a charge-transfer spec-
trum), has a very large band intensity (
ε
max
10 000 M
−
1
cm
−
1
). Above, a Laporte allowed
transition is one that occurs between different orbital types, such as s
→
p, p
→
dord
→
f;
as a consequence, a d
d transition is Laporte forbidden, although some symmetry-based
relaxation rules may operate.
It is also noted that the absorbance bands we see in the electronic spectra of d-block
complexes are broad. This arises because complexes are constantly undergoing an array
of molecular vibrations and rotations that, for example, are changing bond lengths slightly
and thus influencing the size of
→
o
in the process. Because absorption of a photon of
light is an extremely fast process compared with these minor internal structural changes in
the complex, the form of the complex at the particular instant of photon capture is itself
'captured', leading to a range of energies associated with different vibrational and rotational
states, so that we see an averaged outcome, and a broad peak. It is notable that f-block
elements display sharp absorbance bands, as the f orbitals involved are more 'buried' and
overall there is little influence of rotational and vibrational motion in that block of the
Periodic Table.
7.3.3
A Magnetic Personality? - Paramagnetism and Diamagnetism
As discussed earlier in Chapter 3.3, if we examine the set of d electrons for d
4
to d
7
, there is
choice available as regards the arrangement of electrons in the octahedral d subshells. These
configurations display options for electron arrangement, namely
high spin
and
low spin
.
The differentiation depends on the size of the energy gap (
o
) compared with the amount
of spin pairing energy (
P
), with a large
o
favouring low spin arrangements. As discussed
in the last section, the size of
o
is ligand-dependent, especially depending on the type of
donor atom, and also is influenced appreciably by the charge on the metal ion;
P
varies
dominantly only with the metal centre and its oxidation state. In general, where
P
o
,
the complex will be high spin, whereas where
P
o
, the complex will be low spin (Table
7.7). Note how, as the ligand changes from F
−
to NH
3
for d
6
Co(III), and from OH
2
to CN
−
for d
6
Fe(II) the spin state switches; if this model is correct, this should be experimentally
observable. It is the magnetic properties that most clearly illustrate this behaviour.
In terms of magnetism, there are two classes of compounds, distinguished by their
behaviour in a magnetic field:
diamagnetic
-
repelled from
the strong part of a magnetic
field; and
paramagnetic
-
attracted into
the strong part of a magnetic field.
Ferromagnetism
can be considered a special case of paramagnetism.
All
chemical substances are diamagnetic, since this effect is caused by the interaction of
an external field with the magnetic field produced by electron movement in filled orbitals.
However, diamagnetism is a much smaller effect than paramagnetism, in terms of its
contribution to measurable magnetic properties. Paramagnetism arises whenever an atom,
ion or molecule possesses
one or more unpaired electrons
. It is not restricted to transition
metal ions, and even non-metallic compounds can be paramagnetic; dioxygen is a simple
