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Stability of coordination complexes - Coggle Diagram
Stability of coordination complexes
Thermodynamics of complex formation
must always consider both the kinetics and the thermodynamigs
4.1.1 formation constant
relate the strength of a ligand to the interaction strength of the solvent.
we use the equilibrium constant Kf (also called the formation constant).
Just note the [H2O] is considered a constant in dilute solutions. ([H2O] always present)
following reaction and Kf:
If Kf is large :arrow_right: conc products > conc reactants. - If Kf is small :arrow_right: conc. of reactants > conc. of products
Step wise formation constant (replacing more than 1 ligand)
happens when we replace more than 1 ligand. we tend to write 1 equation, however we consider it a step wise process.
we move 1 H2O and replacing it (and we ignore the formation of isomers).
we are retaining same oxidation state and the geometry
we have Kf1, Kf2, Kf3...etc.
4.1.2 Overall formation constant
Kfn = [MLn] / [ML(n-1)[L]
finding the
overall formation constant:
beta(n)
4.1.3 Dissosiation constant (Kd):
Just the inverse of Kf
4.1.4 Trends in formation constant
In general : Kf1> Kf2 > kf3 ....
Look at the log(Kf) vs n (the different steps). We get a linear graph that looks something like this (drawn below):
Why do we see this trend in the graph?
this trend can be accounted for by considering the decrease in the no. of H2O molecules.
also consider the increase in the no. of NH3 molecules making the reverse reaction more likely.
Statistical arguments:
a reversal of the trend (or large difference in successive Kf values) suggests a major change in the structure of the complex as more ligands are added.
Example (mercury)
just note we are assuming that mercury is acting as a transition metal (it isn't one but it is acting like one).
for the
3rd step
we go from an octahedral complex to a tetrahedral complex, in stead of kicking off 1 H2O it kicks off 3.
this is an onset of 4 coordination.
we see an increase in entropy - a decrease in Gibbs free energy - thus a more spontaneous reaction.
delta G is substantially negative indicating stable complexes
4.2.1 The Chelate effect
:key:"The greater the stability of a complex containing a polydentate ligand compared with a complex containing an equivalent number of analogous monodentate ligands"
Examples : [Cd(NH2Me)2]2+ and [Cd(en)2]2+
-enthalpy is similar
one has a more favourable entropy witch leads to Gibbs free energy
Entropic effects
chelating reaction results in more independent molecules present in the solution so will have a more positive entropy
more donor sites on the ligand leads to a greater stability of the complex
Steric effects
once 1 of the ligating groups (or atom) is bound to the metal, the other will be held in close proximity so has a high chance of binding
5- or 6- membered rings are particularly stable bc they result in near ideal bond angles
Google search
- the chelating effect is the enhanced affinity of a chelating liggand for an metal ion compared to its monodentate ligand counter part(s). When a polydentate ligand contains donor atoms position in such a way that when they coordinate with the central metal ion, a 5- or 6- membered ring is formed.
4.2.2 The Macrocyclic effect
:key:thought to be a combination of the entropic effects described below with additional energetic contribution that comes from the preorganized nature of the ligating groups (no additional strains induced in the ligand upon coordination).