Chemistry Reference
In-Depth Information
TABLE 6.3
Electronic Configurations in Octahedral Configuration
Electron configuration
Number of
d
electrons
t
2g
e
g
1
[
2
[
[
3
[
[
[
4 high-spin
[
[
[
[
4 low-spin
[Y
[
[
5 high-spin
[
[
[
[
[
5 low-spin
[Y
[Y
[
6 high-spin
[Y
[
[
[
[
6 low-spin
[Y
[Y
[Y
7 high-spin
[Y
[Y
[
[
[
7 low-spin
[Y
[Y
[Y
[
8
[Y
[Y
[Y
[
[
9
[Y
[Y
[Y
[Y
[
10
[Y
[Y
[Y
[Y
[Y
arrangement with the paired electrons is the low-spin (or strong field) state. Alternative electronic configurations
are also possible for d
7
ions in octahedral complexes.
When bridging ligands connect two or more magnetic centres, their electron spins can either cancel each other
out or reinforce one another
5
, d
6
, and d
this constitutes antiferromagnetic and ferromagnetic coupling, respectively. This
phenomenon is frequently encountered in biological inorganic chemistry, particularly the former, and we will
encounter a good example in the case of the iron core stored within the interior of the protein core of the iron-
storage protein, ferritin (Chapter 19).
e
ELECTRON PARAMAGNETIC RESONANCE (EPR) SPECTROSCOPY
The bioinorganic chemistry of V, Mn, Fe, Co, Ni, Cu, Mo, W, as well as of a number of nonbiological transition
elements, is permeated by their paramagnetism, and EPR spectroscopy is a particularly useful tool for their
analysis (
Hagen, 2006
)
. It can be used with frozen dilute solutions of metalloproteins, and is quite sensitive (high-
spin ferric ions can be detected in the
M range), and it has the potential to establish the stoichiometries of
complex mixtures of paramagnets. EPR detects unpaired electrons in a sample by their absorption of energy from
continuous microwave irradiation (X-band, ca.9
m
10 Hz) when the sample is placed in a strong magnetic field
(around 0.3 Tesla). In standard EPR practice, the EPR absorption is detected by varying the magnetic field at
constant microwave frequency, because in order to get the resonance condition, the wavelength of the microwave
frequency must be tuned to the dimensions of the resonator cavity. EPR spectra are usually represented as the first
derivative of the measured absorption spectrum and are characterised by the four main parameters: intensity,
linewidth, g-value (which defines position), and multiplet structure.
e
