Tuning Reactivity of Platinum(II) Complexes

inductive donor capacity and weak π-acceptor ability. Hence, the stronger the trans-
influence, the weaker is the bond. 53 The weakening of the M–X bond accelerates rates of
substitution at the square-planar Pt(II) centre. 54
+
+
+
L
-
-
-
+
+
Pt - C l
-
- +
-
The chloride ion trans to the polarisable ligand L is easily
released due to the extra repulsive forces betw een its
negative charge and the induced dipoles of the P t(II) cation
Scheme 2.4: Distribution of charge in induced dipoles in the L–Pt–Cl coordinate of
trans-[PtA2LX] 35 due to polarisation (where L = non-leaving trans ligand, X
= leaving group e.g. Cl ¯).
Therefore, ligands with stronger trans influence weaken the bond between Pt and the
trans ligand to be displaced and increase the energy of ground state accordingly
(polarisation theory by Griberg). 55
In the Pt(II) complexes both the σ-donor and π-acceptor capacities are involved in the
trans-effect as illustrated in Figure 2.12. A ligand which exhibits a strong trans-effect is
either a good σ-donor, e.g. H ¯ , CH3 ¯ (Figure 2.12) (a) or a good π-acceptor such as C2H4
(Figure 2.12 (b)) or has both σ-donor and π-acceptor properties like CN ¯ and CO.
Figure 2.12: (a) σ-donation from the filled ligand px orbital to vacant metal 5dx 2 -y 2
orbital. (b) π-back-donation from the filled metal dxz orbital to the
antibonding linear combination of carbons’ px orbitals in C2H4. 49
37