SNlcB mechanisms

The fine balance between available mechanisms is seen in the hydrolysis of the related complex [Co(NH3)5(03POAr)]+, which also occurs by an SNlcb mechanism, with attack on the co-ordinated phosphate by co-ordinated amide. A similar mechanism involving... 

The second mechanism requires a preliminary displacement of chloride by the oxygen of the ester to give a chelated complex which may be attacked by external hydroxide as seen in Chapter 3. In practice, the displacement of chloride from cobalt(m) is very slow and this mechanism proceeds by the SNlcb mechanism, in which loss of chloride ion is aided by deprotonation of the amine. The first step involves deprotonation of the en ligand followed by chloride loss to give a five co-ordinate intermediate (Fig. 5-63). 

Volumes of activation of +14.7 and + 20.0 ml mol have been reported from high-pressure studies of the rates of isomerization of j5-[Co(edda)(AA)]+ ions (AA = tmd and en respectively) to the a-isomers under basic conditions (0.2 mol dm NaaCOs-NaHCOs). An. SNlcb mechanism is favoured. ... 

Partial molar volumes, and AK measurements for the base hydrolysis of a series of [CrCl(3,3-tri)(AA)] complexes suggest an associative interchange conjugate base mechanism may be more applicable to Cr(III) systems than the dissociative SNlcb. ... 

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