Dr. Geeta Tewari discusses factors that affect the rates of substitution reactions in square planar complexes. (1) The nature of the entering and leaving ligands impacts the rate, with more polarizable and soft ligands being better nucleophiles for Pt(II) complexes, and the leaving group bond strength determining the rate. (2) Steric effects of non-leaving ligands can slow the rate, as bulky ligands create hindrance. (3) For associative mechanisms common in square planar complexes, the charge on the metal center does not impact the rate.

1. Dr. Geeta Tewari
Department of Chemistry
D. S. B. Campus
Kumaun University, Nainital
Email: geeta_k@rediffmail.com
Substitution Reactions in Square
Planar Complexes: Part 2, Factors
affecting Rates of Substitution
Reactions

2. (i) Nature of entering ligand
• Nucleophilicity of incoming ligand.
trans-[Pt(py)2Cl2] + Y → trans-[Pt(py)2Cl(Y)] + Cl-
• Nucleophilicity: Basicity (Soft Lewis) of Y towards Pt(II)
Oxidation potential of Y
Polarizability
 Soft ligands (more polarizable) more effective nucleophile for
soft complexes.
 Hard ligands (OH-) are more effective nucleophile for hard
complexes.

3.  For Pt(II) complexes, more polarizable or soft ligands are good
nucleophile (Pt2+ ).
 Therefore, increasing order of nucleophilicities of incoming
ligands for Pt (II) square planar complexes is:
OH- < H2O < Cl- < NH3 < alkenes < C6H5NH2 < Br- < NO2
- < N3
-
< SCN- < I- < R3P
(i) Nature of entering ligand

4. In dissociative mechanism, the bond between metal and leaving
group breaks in transition state. Therefore, these reactions
depend upon leaving group.
In associative mechanism, the leaving group effect depends
upon the degree of bond breaking in the transition state.
[Pt(dien)X]+ + py → [Pt(dien)py]2+ + X-
Diethylenetriamine ligand occupies three sites in the complex
(inert sites) and the leaving group is X-.
(ii) Nature of leaving group

5. The order of rate of reaction as per leaving
groups (X-):
CN-<NO2
-<SCN-<N3
-<I-<Br-<Cl-<H2O<NO3
-
 Pt-X bond breaking is important as
compared to Pt-py bond formation.
(ii) Nature of leaving group

6. Octahedral complexes: dissociative mechanism
Square planar complexes: associative mechanism.
Steric hindrance favours dissociative mechanism
(bulky groups ceases the attack of incoming ligands).
In SP complexes, bulky ligands decreases the rate of
substitution reaction (steric hindrance).
Overcrowding opposes the increase in coordination
number.
(iii) Steric effect of non-leaving group

7. Complex X = Pyridine 2-methyl 2,6-dimethyl
pyridine pyridine
cis-[Pt(PEt3)2X Cl] 8x10-2 2x10-4 1x10-6
trans-[Pt(PEt3)2X Cl] 1.2x10-2 1.7x10-5 3.4x10-6
(iii) Steric effect of non-leaving group
Rate constants (s-1)

8. (iv) Effect of charge on metal complex
Fast
 Martin and coworkers had provided the information regarding
the effect of charge on the metal complex on the substitution
reaction rate.
 For [Pt(NH3)3Cl]+, [Pt(NH3)2Cl2], [Pt(NH3) Cl3]- and [PtCl4]2-
, the value of k1 remains roughly constant.
 If the reaction is proceeding via associative mechanism, the
substitution reaction should not be affected by the charge on
the metal complex.
 In associative mechanism, bond breaking and bond making
starts simultaneously and if increase in the charge on metal
complex make the Pt-X bond breaking difficult, it also make
the formation of M-H2O bond easy.
 So, the rate of overall reaction will remain unaffected.

9.  In the solvent path mechanism of substitution, direct
displacement of X- takes place with water in the slow step.
 If the solvent has more tendency to coordinate with the metal
ion, the contribution of the solvent path mechanism will
increase in the overall rate of reaction.
trans-[Pt(py)2Cl2] + 36Cl-
 DMSO, H2O, CH3OH, ROH : Good coordinating solvents, the
rate of substitution does not depends upon concentration of
36Cl-.
 CCl4 and C6H6 : Poor coordinating solvents, the rate of
substitution depends upon the concentration of 36Cl-.
(v) Effect of solvent

10. (vi) Trans-effect
 Besides entering and leaving ligands, the other ligands present
in the square planar complexes affect the rate of substitution
reaction.
 The spectator ligand (not lost in the reaction), affects the rate of
substitution of the ligands trans to it. This effect is called as
trans-effect.
 Therefore, trans-effect of a ligand (L) increases the rapid
substitution of ligand (X) that is present trans to it.
M
L
X
A
A
M
L
Y
A
A
+
Y
-
X