Organic reaction mechanisms unimolecular/bimolecular substitutions

Early attempts to establish mechanistic models for substitution at metal centers used the labels Sjp and SN2 (substitution, nucleophilic, unimolecular or bimolecular) inherited from Ingold s attempts26 to extend his classic studies in organic reaction mechanisms to substitution at elements other than carbon. Unfortunately, Ingold s attribution of the discontinuity in reaction rates in the replacement of one Cl in m-Co(en)2CI2 1 by various anions A in methanol in the sequence... 

Christopher Ingold and Edward D. Hughes, in the middle of the last century, used such kinetic properties to devise nomenclature for the mechanisms of organic reactions. For the nucleophilic substitutions. Ingold and Hughes have proposed labels SnI, and Sn2, where S and N refer to substitution and nucleophilic, respectively. Numbers 1 or 2 are used to designate unimolecular or bimolecular mechanisms, respectively. 

The symbols SnI and Sn2 were introduced by Hughes and Ingold for organic reactions. The term SnI is used for dissociative mechanisms and means substitution nucleophilic unimolecular. The term Sn2 is used for mechanisms in which bond making is important and means substitution nucleophilic bimolecular. Let us now look at several systems to see how mechanistic information on substitution reactions of coordination compounds was obtained. 

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