Ligand cleavage - ligand attachment
By splitting off the ligand, complexes which are unsaturated by coordination are generated. The vacant coordination site may be occupied by a solvent molecule in solution. If this is not included in the electron balance of M and when determining the coordination number, the number of electrons in the valence shell of M decreases by two and the coordination number of M decreases by one unit.
Ligand substitution reactions can occur after a dissociative (D.) or an associative mechanism (A.), according to the sequential execution of ligand cleavage and addition. With the mechanism D., dissociative mechanism, ligand cleavage takes place before attachment, with mechanism A., associative mechanism, ligand attachment prior to cleavage.
Ligand substitution can also be an exchange mechanism I., interchange. Ligand substitution reactions that follow an exchange mechanism (I.) play a lesser role in catalytic cycles, since no intermediate complexes can be detected.
Ligand cleavage, which plays a role in homogeneous catalysis, is mostly reversible. The coordinatively unsaturated complexes formed are often able to coordinate a substrate molecule (ligand attachment). This can be an indispensable prerequisite for catalysis because it involves activation of the substrate. Examples of this are the formation of π complexes with unsaturated hydrocarbons (olefins, alkynes, dienes, ...) and of σ complexes with dihydrogen or hydrocarbons (HH or CH as ligand). In many cases, ligands are formed from the precatalyst the actual catalyst complexes.
In homogeneous catalysis, particles with 16 and 14, but also those with 12 valence electrons, often appear as intermediates. In precatalysts with 18 valence electrons, the catalytically active complex (16 ve) is generally only formed by ligand cleavage.