A ligand exchange (also called ligand substitution) is a  chemical reaction in which a ligand in a compound is replaced by another. Two general mechanisms are recognized: associative substitution or by dissociative substitution.
Associative substitution closely resembles the S2 mechanism in organic chemistry. A typically smaller ligand can attach to an unsaturated complex followed by loss of another ligand.  Typically, the rate of the substitution is first order in entering ligand L and the unsaturated complex.
Dissociative substitution is common for octahedral complexes. This pathway closely resembles the S1 mechanism in organic chemistry.  The identity of the entering ligand does not affect the rate.