Effect of Ligands Already Present

The group trans to the leaving ligand appears to have a more pronounced influence than the two cis to it, on the rate of its departure. It has been known for many years that a ligand can be assigned an order of trans effect which denotes its tendency to direct an incoming group in the position trans to itself. In Pt(II) complexes, this power decreases approximately in the order 

In an associative mechanism, increasing the steric hindrance in the complex should lead to decreasing rates (steric retardation). This has been realised with a number of systems, that of Pd(Rjdien)X + being the most explored. Activation parameters for the reaction 

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