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Substitution Reactions Dr L.L. Wong, Michaelmas Term, 3rd

Year_______________________________________________________________________________________________________________________________________________

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Substitution Reactions of Square Planar Complexes

Common metal oxidation states with square planar complexes. All have d8 configuration.

Ni2+

Rh+ Pd2+

Ir+ Pt2+ Au3+

Most details have come from studies on Pt2+ complexes. The reaction

occurs with retentionof configuration and has a general two-term rate law because of two parallel

mechanisms the solvent pathway and the nucleophilic pathway.

"d[ML3X]

dt= (ks + ky[Y])[ML3X]

The mechanisms:

The two vacant coordination sites in square

planar complexes mean that the mechanism is

almost invariably associative. The reactions show

large and negative values of "S and "V.

The incoming nucleophile donates its electrons

into the empty metal pzorbital.

The ks term arises from the solvent pathway in

which a solvent molecule displaces the leaving

group in a slow reaction followed by rapid

substitution of the coordinated solvent molecule by

the incoming nucleophile. The solvent does not

appear in the rate law.

The ky term is the direct attack by the incoming

group, hence the second order rate law.

Both mechanisms proceed via 5-coordinate

intermediates which have been detected for Rh+

and Ni2+ but not for Pt2+.

Reactivity at different metal centres will depend

on the readiness with which the centres move

away from the square planar geometry.

Hence the rate order: Ni2+ > Pd2+ > Pt2+.

With Au3+ the higher charge will attract the

incoming nucleophile, and Au3+ > Pt2+. Fig. 1: The parallel solvent andnucleophilic pathways.

M

L

XL

L

+ Y M

L

YL

L

+ X

M

L X

LL

M

L X

LL

+ S

S

M

L X

L

L

S

M

L X

LLY

+ Y

M

L S

LL

M

L X

L

L

Y

+ Y

M

L Y

LL

M

L Y

LL

X

M

L S

L

L

Y

- S

Reactant

Product

- X

- X

Substitution Reactions Dr L.L. Wong, Michaelmas Term, 3rd

Year_______________________________________________________________________________________________________________________________________________

_______________________________________________________________________________________________________________________________________________

Page 12 of 14

The rate law:

For the exchange reaction:

the plot of kobs = ks + ky[Y] shows that the

intercept is effectively zero in the non-polar

solvent hexane, i.e. the solvent pathway does

not operate and the slope gives ky. In the polar

solvent methanol, the rate is independent of

[Y], i.e. the solvent pathway dominates.

The entering group:

For the substitution reaction:

the plot of kobs = ks + ky[Y] shows that the

entering ligand has a significant effect on

the rate, in contrast to octahedral

complexes. The activity trends parallel

the nucleophilicity of the incoming ligand.

Hence:

I > Br > Cl >> F

PR3 >> NR3

R2S >> R2O

Overlap and solvation factors.

The leaving group:

In the reaction

Pt(dien)X+ + Py ! Pt(dien)Py2+ + X

the lability varies by 106 in the order:

NO3 > H2O > Cl

> Br > I >> N3 > SCN > NO2

> CN

This sequence broadly follows ligand basicity and ML bond strength, and suggests that bond

breaking must be an important factor even in an overall associative mechanism.

In general, a metal-halide bond is more labile than a MN bond. Hence all other things being

equal a PtCl bond will undergo substitution faster than a PtNH3 or PtPy bond.

Pt

PEt3

Cl Cl

NHEt2

*NHEt2Pt

PEt3

Cl Cl

NHEt2*

0#0.5#

1#1.5#

2#2.5#

3#

0# 0.05# 0.1# 0.15# 0.2# 0.25#

Pt

Py

Cl Cl

Py

Pt

Py

Cl Y

Py

Y

0#0.5#

1#1.5#

2#2.5#

3#3.5#

4#

0# 20# 40# 60# 80# 100# 120#

in hexane

in methanol

[Y] = [Et2NH] / M

105kobs / s1

103kobs / s1

SCN

I

[Y] / mM

Me2S

BrN3

NH3

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