Associative and Dissociative Substitution

Associative substitution describes a pathway by which coordination and organometallic complexes interchange ligands. Associative mechanism resembles the SN2 mechanism in organic chemistry. The opposite pathway is dissociative substitution, being analogous to SN1 pathway. Intermediate pathways exist between the pure associative and pure dissociative pathways, these are called interchange mechanisms.


Associative mechanism

• Metal size should be large
• Ligand size should be small
• Incoming group should have pi bonding ability (CN- is a pi acid ligand)
• Rate depend on the nature of nucleophile
• Trigonal bipyramidal intermediate 

    L5MX + Y =   L5MXY        (SLOW)
    L5MXY =    L5MY + X        (FAST)

    Rate = k [L5MX] [Y]
 

Dissociative mechanism

• Metal size should be small
• Ligand size should be large
• Rate does not depend on nature of the nucleophile

    L5MX =  L5M + X   (SLOW)
    L5M + Y =  L5MY   (FAST)

    Rate = k [L5MX]