Sn1CB mechanism
Encyclopedia
The SN1CB mechanism describes the pathway by which many metal amine
complexes undergo substitution, that is ligand exchange. Typically, the reaction entails reaction of a polyamino metal halide with aqueous base
to give the corresponding polyamine metal hydroxide:
The rate law for the reaction is:
The rate law is deceptive: hydroxide serves not as a nucleophile but as a base to deprotonate the coordinated ammonia. Simultaneously with deprotonation, the halide dissociates. Water binds to the coordinatively unsaturated complex followed by proton transfer to give the hydroxy complex. The conjugate base resulting from deprotonation of the amine is rarely observed.
Amine
Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...
complexes undergo substitution, that is ligand exchange. Typically, the reaction entails reaction of a polyamino metal halide with aqueous base
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
to give the corresponding polyamine metal hydroxide:
- [Co(NH3)5Cl]2+ + OH- → [Co(NH3)5OH]2+ + Cl-
The rate law for the reaction is:
- rate = k([Co(NH3)5Cl]2+)([OH-])
The rate law is deceptive: hydroxide serves not as a nucleophile but as a base to deprotonate the coordinated ammonia. Simultaneously with deprotonation, the halide dissociates. Water binds to the coordinatively unsaturated complex followed by proton transfer to give the hydroxy complex. The conjugate base resulting from deprotonation of the amine is rarely observed.