Causes of the Trans Effect

A different explanation for the trans effect is based on the ability of a ligand to form multiple bonds to the metal ion. In Chapter 19, multiple bonding of such ligands as CO and CN- to metal ions by back donation of electron density from the d orbitals of the metal to vacant n orbitals on the ligands was described. The removal of electron density from the 

The removal of electron density from the Pt(II) by multiple bonding to CN causes the side of the Pt(II) toward X to be electron deficient and more susceptible to nucleophilic attack by strong nucleophiles. Thus, the activation energy for forming the transition state is lower and the reaction proceeds faster. For a large number of ligands the apparent order of the trans effect produced is 

It should also be pointed out that the trans effect is not limited to complexes of Pt(II) but should exist in any square planar complex. Generally, the trans effect is smaller in complexes of Pd(II) than in those of Pt(II) and this is exactly as would be expected from the fact that Pd(II) is less polarizable (harder) than Pt(II). In terms of the n bonding hypothesis, the Pd(II) is a harder acid than Pt(II) (and less likely to participate in back donation) and does not form covalent bonds as well. By either way of explaining the trans effect, the effect should be smaller in complexes of Pd(II) than in those of Pt(II). The trans 

Stabilization of the transition state by 7T bonding by a ligand (in this illustration it is CN ) trans to the leaving group. In this case, the shift of electron density away from X makes it easier to remove. 

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