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3. The Nature of other Ligands in the Complex

The Trans Ligands

Definition;

The trans effect is best defined as the effect of a coordinated ligand upon the rate of substitution of ligands opposite to it.

For example, in the substitution reactions of Pt(II) square planar complexes, labilizing effect is in the order;

H2O ~ OH- ~ NH3 ~ amines ~ Cl- < SCN- ~ I- < CH3- < Phosphines ~ H- < Olefins < CO ~ CN-

Note that the "labilizing effect" is used to emphasise the fact that this is a kinetic phenomenon.

This labilization may arise because of destabilisation (a thermodynamic term) of the ground state and/or a stabilisation of the transition state.

The trans influence is purely a thermodynamic phenomenon. That is, ligands can influence the ground state properties of groups to which they are trans. Such properties include;

(i) Metal-Ligand bond lengths

(ii) Vibration frequency or force constants

(iii) NMR coupling constants

The trans influence series based on structural data, has been given as;

R- ~ H- >= PR3 > CO ~ C=C ~ Cl- ~ NH3

* The cis ligands

In cases where a relatively poor nucleophile act as the entering group

Note: Compare with the trans series below, it acts in the same way.