This document discusses substitution reactions in square planar complexes, primarily focusing on platinum(II) complexes. It describes the associative mechanism of these reactions, detailing types like oxidative addition, electrophilic substitution, and nucleophilic substitution, as well as the involvement of stable intermediates. Additionally, the document outlines the rate laws for these reactions and differentiates between solvent path and direct displacement mechanisms.

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

2. Square planar complexes
Square planar complexes are formed by of the transition
metal complexes having d8 configuration [Ni(II), Pd(II),
Pt(II), Rh(I), Ir(I).
Pt(II) complexes are most studied SP complexes:
Easy synthesis.
Pt(II) tetracoordinated complexes are always SP.
Most stable complexes.
Slow rate of reaction of Pt(II) square planar complexes.

3. Substitution reaction occurs via associative mechanism (SN2).
Three types:
1. Oxidative addition followed by reductive elimination
[PtII (PR3)2 (R)Cl] + Cl2 → [PtIV (PR3)2 (R)Cl3] → [PtII (PR3)2Cl2]
+ RCl
2. Electrophilic substitution
[PtII (PR3)2 (R)Cl] + HgCl2 → [PtII (PR3)2 Cl2] + RHgCl
3. Nucleophilic substitution
[PtII (PR3)2 (R)Cl] + NH3 → [PtII (PR3)2 (R)NH3]+ + Cl-

4. Occur via associative mechanism (no problem of steric
hindrance).
Stable trigonal bipyramidal intermediate.
Pt(II) has pz empty orbital that can accept electrons from the
incoming ligand (Square pyramidal intermediate formed which
transforms into TBP intermediate).
Mechanism of substitution reaction
P t
A X
A
L
P t
A X
A
L
P t
A
L
P t
A Y
A
L
P t
A Y
A
L
Y Y
X
A
X
+ Y -X
S q u a re p la n a r S q u a re p y ra m id a l S q u a re p y ra m id a l S q u a re p la n a r
T rig o n a l b ip y ra m id a l

5. Mechanism of substitution reaction
Stereochemistry
Pt
A X
A
L
Pt
A Y
A
L
Pt
L X
A
Pt
L Y
A
A
A
Pt
A
L
Y
X
A
Pt
A
A
Y
X
L
+Y
+Y
-X
-X
cis-[Pt(A)2XL] cis-[Pt(A)2YL]
trans-[Pt(A)2YL]
trans-[Pt(A)2YL]

6. The mechanism is complicated.
Two path mechanism
Solvent path mechanism
Direct displacement path mechanism
Mechanism of substitution reaction

7. [Pt(A)2LX] + H2O → [Pt(A)2L(H2O)]+ + X-
[Pt(A)2L(H2O)]+ + Y- → [Pt(A)2L(Y)] + H2O
Rate of reaction (r1) = k1 [Pt(A)2LX] [H2O]
k1 [Pt(A)2LX]
Mechanism of substitution reaction
Slow
Fast
Solvent path

8. Mechanism of substitution reaction
Fast
Direct path
[Pt(A)2LX] + Y- → [Pt(A)2LXY]-
[Pt(A)2LXY]- → [Pt(A)2L(Y)] + X-
Rate of reaction (r2) = k2 [Pt(A)2LX] [Y-]
Slow
k2
Fast

9. Rate of substitution reaction = r1 + r2
= k1 [Pt(A)2LX] + k2 [Pt(A)2LX] [Y-]
= [Pt(A)2LX] (k1 + k2 [Y-])
Two term rate law
1. k1 = first order rate constant (solvent path)
2. K2 = second order rate constant (direct path)
The reaction is pseudo first order (concentration of Y- is constant)
Mechanism of substitution reaction

10. Mechanism of substitution reaction
Pt
L X
A
Pt
A Y
A
L
A
Pt
A
A
Y
X
L
+Y
-X
Pt
L H 2 O
A
A
-X
+H 2O
+Y
-H 2O
Slow
(Slow )
Fast
Fast