Dissociative activation, square-planar

Discernible associative character is operative for divalent 3t5 ions through manganese and the trivalent ions through iron, as is evident from the volumes of activation in Table 4. However, deprotonation of a water molecule enhances the reaction rates by utilising a conjugate base 7T- donation dissociative pathway. As can be seen from Table 4, there is a change in sign of the volume of activation AH. Four-coordinate square-planar molecules also show associative behavior in their reactions. 

The reaction rate is primarily determined by the enthalpy of activation (A// ), which is usually the case in square planar nucleophilic substitution reactions. Of greater importance, so far as a dissociative versus an associative mechanism is concerned, are the entropies and volumes of activation, AS and AY, respectively. Note that the values are negative for both the fct and the steps. The observed decrease in entropy is what we would expect for a mechanism in which two particles come together to give an activated complex. The volume of activation is determined by doing the reaction under high pressure ... 

Ligand exchange by dissociative activation now appears to be well established for some 16-electron square-planar compounds, though examples are not common. The detail of the intimate mechanism is crude by comparison with associative reactions, but all the evidence indicates the involvement of T-shaped intermediates. 

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