Substitution Reactions at Sn IV

Correlations found for the six-coordinate complexes (SnR2X2Y2) reveal a possible mechanism for an (unprecedented) termolecular reaction involving a concerted double addition and elimination process in which the R ligands adopt transoid positions, and the X and Y groups are mutually cis in the intermediate (or transitional) octahedral conformer Britton and Dunitz dubbed this an Sn3 reaction  

At the time the authors knew of no experimental observations that indicated such an entropically disfavored pathway, but they speculated that it may be important for enzymatic reactions . Interestingly, a very similar Sn3 pathway may be operable in the base-catalyzed inversion of A-cis-(-f-)-[Co(en)2Cl2] to A-cis-(-)-[Co(en)2(OH)2], which Jackson and Begbie found to take place via a route that involves the simultaneous loss of the two Cl ions in a single observable step, and to be promoted by OH in some unprecedented way [51a]. The authors propose a complicated modification of the classical SnI conjugate base mechanism in order to avoid coming to the conclusion that the mechanism is indeed a termolecular one - and then concede that their account is not especially compelling [51b]. It remains to be seen 

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