Ligand substitution
The ligand substitution is a chemical reaction in which a ligand of a complex is replaced with a different ligand.
The general reaction equation of ligand exchange can be formulated as follows:
In it is the central atom (electrically neutral or positively charged) with a number of ligands and the ligand . The ligand ( leaving group ) is exchanged for the ligand ( entering group ) in the reaction .
Ligand substitution kinetics
Three reaction mechanisms of ligand substitution can be formulated as borderline cases . In fact, ligand exchange reactions seldom proceed exactly by one of the following mechanisms, and the course of the reaction lies somewhere in between.
Dissociative ligand exchange D
It is a monomolecular, two-step process .
In the first reaction step, the leaving group is split off from the complex , so the complex dissociates :
In the second reaction step the group entering is bound:
Both reaction steps are equilibrium reactions , with the first step, dissociation, determining the rate .
It is important with this reaction mechanism that an intermediate is formed which has a lower coordination number than the starting material .
This has the effect that, firstly, the coordination polyhedron of the remaining ligands around the central atom changes and, secondly, the size of the d orbital splitting at the central atom is reduced.
The dissociation is usually associated with a high crystal field activation energy and that is why the dissociative mechanism is mainly found in complexes that either have very many ligands or very large ligands, so that it is sterically favorable to split off such a ligand.
Associative ligand exchange A.
This mechanism is also a monomolecular, two-step process .
In the first reaction step, the group entering is bound, associated :
In the second step, the leaving group is split off:
Both reaction steps are equilibrium reactions , whereby the first step, the association, determines the rate .
It is important with this reaction mechanism that an intermediate is formed which has a higher coordination number than the starting material .
This has the effect that firstly extending coordination polyhedra of the ligand , and to the central atom changes and secondly, the size of the d -Orbitalaufspaltung raised on the central atom.
So the association is often with a small crystal field activation energy connected but the association is determined by the existing ligands and because they can block the association sterically difficult and shield the charge of the central atom.
This is why the associative mechanism is mostly found in complexes with few and small ligands.
Interchange Mechanism I.
Here is a bimolecular one-step mechanism in which bond formation and bond breakage take place simultaneously:
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In between, a non-isolable transition state forms , in which the group is not yet completely split off and the group is not yet fully bound. This is also an equilibrium reaction.
The interchange mechanism can have a more dissociative character, if it loosens earlier than it starts to bind, or more associative character, if it binds earlier than it starts to bind . Is the character of the interchange mechanism known to write I D or I A .
In general, the investigation of the kinetics of ligand substitutions is very difficult, since they often take place extremely quickly and, moreover, the solvent is often involved in the exchange reactions. For a long time, therefore, the focus was on investigating relatively inert platinum complexes.
Thermodynamics of ligand substitution
A ligand exchange is thermodynamically possible if the product of the reaction is energetically more favorable than the starting material . The decisive factor for this is mostly how big the difference in the ligand field stabilization energy is between the starting material and the product and this in turn depends mainly on whether the ligand (s) entering leads to an increase or decrease in this energy. Furthermore, entropy effects of importance to say essentially the number of recorded during ligand exchange / released ligand and the occurring solvent-dependent solvation .
It often happens that reactions that are thermodynamically very good do not take place or take place incredibly slowly. This is then due to the kinetic inertness of the educt when possible transition states or intermediate products are energetically very disadvantageous.
meaning
Ligand substitutions take place in almost all biological systems, e.g. B. the exchange of the gases oxygen , carbon monoxide and carbon dioxide in the hemoglobin of the red blood cells .
The chelation is usually a ligand exchange process in which many monodentate ligands , eg. B. water molecules are exchanged for a few multidentate ligands , z. B. EDTA .
The ligand exchange is of particular importance in the production and use of organometallic catalysts, e.g. B. for the synthesis of polymers .