Chemistry Reference
In-Depth Information
Table 7.4
Electrochemical and thermodynamic/kinetic analysis techniques.
Method
Sample and
device
requirements
Outcome expected
Electrochemical Methods
Electrochemistry:
voltammetry
(particularly cyclic
voltammetry)
Solution (aqueous or non-aqueous)
of compound.
Potentiostat and electrochemistry
cell.
Metal-centred and ligand redox
behaviour. Potentials and
reversibility of reduction and
oxidation processes.
Information on kinetics of
electron transfer and any
decomposition processes.
Electrochemistry:
coulometry
Solution (aqueous or non-aqueous)
of compound.
Potentiostat/galvanostat and
coulometry cell.
Number of electrons in a
particular redox process.
Kinetic and Thermodynamic Analysis Techniques
Potentiometric and
spectrophotometric
titration
Solutions of a metal ion and of a
pure ligand.
Auto-titration assembly and
detector system (pH and/or
UV-Vis spectrum).
Metal-ligand speciation in
solution, and actual complex
thermodynamic formation
constants.
Stopped-flow and
conventional
kinetics
Solutions of a known complex.
Stopped-flow instrument with
UV-Vis detector (for fast
reactions), or standard UV-Vis
spectrophotometer;
temperature-controlled cell
holder.
Information on mechanisms of
reactions via measurement of
reaction kinetics; rates of
particular reactions and
activation parameters.
Calorimetry
Solutions of a metal ion and a pure
ligand.
Calorimetry cell with temperature
change detection electronics.
Heat of reaction. Information on
complexation
thermodynamics.
species directly under most circumstances. Thus to probe the life of complexes, or their
behaviour as molecular species, we must resort to far more elaborate instrument-based
techniques. Two of the most accessible spectroscopic techniques for chemists, because
instrumentation is relatively cheap and readily accessible in most laboratories, are UV-Vis
or electronic spectrophotometry and IR or vibrational spectroscopy. Of the more expen-
sive instrumentation available, NMR is perhaps the most commonly employed. For both
electronic and vibrational spectroscopy, the observation of a molecule happens extremely
rapidly (
10
−
11
s), so we get an 'instant' view, and any rearrangements are too slow to
be seen or influence outcomes. For NMR, timescales are of the order of 10
−
2
-10
−
5
s, so
that rearrangements where the activation energy is small (as in some fluxional molecules)
may be probed, such as by varying temperature. We call these aspects the spectroscopic
timeframe of the techniques; each method will have one, which influences the tasks it can
perform somewhat.
Whereas IR spectroscopy is often employed in coordination chemistry to identify func-
tional groups or ligand types, and thus differs little from its application in organic chem-
istry, UV-Vis spectroscopy for coordination complexes relies heavily on the special theories
(crystal and ligand field theory) developed for these complexes. Thus, it will be a target
for particular attention here. Another physical method closely tied up with these theories is
