Substitution reactions at octahedral complexes: the search for mechanism.
Substitution reactions of square planar complexes
description
Transcript of Substitution reactions of square planar complexes
![Page 1: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/1.jpg)
Substitution reactions of square planar complexes
![Page 2: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/2.jpg)
especially d8: Ni(II), Rh(I), Pd(II), Ir(I), Pt(II), Au(III)
![Page 3: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/3.jpg)
General mechanism
A
P
B
k2 [Y]
k3 [Y]
k1
+Sk-1
-S
General rate law:1 2Rate ( [Y])[A]k k
Reaction:[ML3X] + Y [ML3Y] + X
[ML3X] A
![Page 4: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/4.jpg)
A
P
B
k2 [Y]
k3 [Y]
k1
+Sk-1
-S
A
P
B
k2 [Y]
k3 [Y]
k1
+Sk-1
-S2 3
[P][A][Y] [B][Y]
dk k
dt
Assume [B] is in steady state
1 1 3
1
1 3
[A] [B] + [B][Y]
[A][B] =
( [Y])
k k k
k
k k
Substituting into (1)
(1)
12 3
1 3
[A][P][A][Y] + [Y]
( [Y])
kdk k
dt k k
1 2Rate ( [Y])[A]k k
![Page 5: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/5.jpg)
A
P
B
k2 [Y]
k3 [Y]
k1
+Sk-1
-S
A
P
B
k2 [Y]
k3 [Y]
k1
+Sk-1
-S
12 3
1 3
[A][P][A][Y] + [Y]
( [Y])
kdk k
dt k k
Two situations usually arise for the solvent pathway
The rate of attack of solvent on A is rate limiting
k3[Y] >> k-1
1 2Rate ( [Y])[A]k k
12 3
3
2 1
1 2
[A][P][A][Y] + [Y]
( [Y])
[A][Y] + [A]
( [Y])[A]
kdk k
dt k
k k
k k
which is in agreement with the experimental rate law
![Page 6: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/6.jpg)
A
P
B
k2 [Y]
k3 [Y]
k1
+Sk-1
-S
A
P
B
k2 [Y]
k3 [Y]
k1
+Sk-1
-S
12 3
1 3
[A][P][A][Y] + [Y]
( [Y])
kdk k
dt k k
Two situations usually arise for the solvent pathway
The rate of attack of solvent on A is much faster than attack of Y on the intermediate B
k3[Y] << k-1
1 2Rate ( [Y])[A]k k
12 3
1
1 32
1
[A][P][A][Y] + [Y]
+ [A][Y]
'[A][Y]
kdk k
dt k
k kk
k
k
![Page 7: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/7.jpg)
1 32
1
[P]+ [A][Y]
'[A][Y]
k kdk
dt k
k
1 2
[P]( [Y])[A]
dk k
dt
Study the rate of the reaction as a function of [Y]
ko b s
[Y ]
ko b s
[Y ]
k1
k2
1 32
1
+ k k
kk
0
![Page 8: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/8.jpg)
The k2 pathwayA
P
B
k2 [Y]
k3 [Y]
k1
+Sk-1
-S
A
P
B
k2 [Y]
k3 [Y]
k1
+Sk-1
-SDefine
k2o as the 2nd order rate constant when Y = MeOH
in the reactiontrans-[PtCl2(py)2] + Y trans-[PtClY(py)2] + Cl
then compare the rate for any other ligand Y to the rate when Y = MeOH
2Pt o
2
(Y)log
k
k
nucleophilicity parameter
The greater Pt, the greater the
nucleophilicity of the ligand
![Page 9: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/9.jpg)
nucleophilicity increases with
softness of the donor ligand
![Page 10: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/10.jpg)
2Pt o
2
(Y)log
k
k
oPt 2 2
o2 Pt 2
2 Pt
log (Y) log
log (Y) log
log (Y)
k k
k k
k C
where C = log k2o
Now generalise for any square planar Pt complex [PtL3X]
[PtL3X] + Y [PtL3Y]+ X
![Page 11: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/11.jpg)
2 Ptlog (Y)k S C
S is the nucleophilic discrimination factor and gives the sensitivity of the
rate constant to the nucleophilicity of the
incoming ligand
![Page 12: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/12.jpg)
lo g k 2
P t
CH3OH
CH3OH
NO2
Cl
Br
I
SCN
SeCN
NH3
N3
I
SCN
thiourea
trans-[PtCl2(PEt3)2]
[PtCl2(en)]
S is larger
![Page 13: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/13.jpg)
lo g k 2
P t
Usually...
As
reac
tivi
ty t
owar
ds
the
com
mon
liga
nd
, MeO
H, i
ncr
ease
s
Th
e discrim
ination
factor,S
, decreases
![Page 14: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/14.jpg)
All values are significantly > 0, i.e., all complexes undergo substitution
reactions that are quite sensitive to the
nucleophilicity of the entering ligand
This sensitivity is expected for
reactions under associative activation
![Page 15: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/15.jpg)
As softness of ligands on Pt increases, S increases
– the complexes becomes less reactive
and more discriminating
2Cl, 2 aliphatic N
Cl, 3 aliphatic N
2Cl, 2 aromatic N
2 Cl, 2 P
![Page 16: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/16.jpg)
ExampleCalculate the second-order rate constant for the reaction of trans-[PtCl(CH3)(PEt3)2] with NO2
, for which Pt = 3.22. For this complex, I (Pt = 5.42) and N3 (Pt = 3.58),
react at 30 oC with k = 40 M-1 s-1 and 7 M-1 s-1, respectively.
2 Ptlog (Y)k S C
log 40 5.42 (1)
log 7 3.58 (2)
S C
S C
(1) (2)
0.755 1.84
0.410
S
S
1.60 5.42 0.410
0.62
C
C
![Page 17: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/17.jpg)
Hence, for NO2
2log 0.410 3.22 0.62
0.70
k
0.702
-1 -1
10
5.0 M s
k
![Page 18: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/18.jpg)
For the generalised reaction
L1
PtL2 Cl
Cl
L1
PtL2 Cl
Y
L1
PtL2 Y
Cl
Y
Y
A
B
whether the predominant product is A or B depends on the relative trans effect of the spectator ligands L1 and L2
Two important observations:
The nature of the transition state
![Page 19: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/19.jpg)
The rate of the reaction
depends significantly on the nature of the trans ligand, T, but hardly at all on the cis ligands C
C
PtT Cl
C
Y
C
PtT Y
C
![Page 20: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/20.jpg)
The trans effect order is
For donor ligands
H- > PR3 > SCN- > I- > CH3-, CO, CN- > Br-, Cl- > NH3, py > OH-, H2O
For acceptor ligands
CO, C2H2 > CN- > NO2- > NCS- > I- > Br-
Stronger σ donors Weaker σ donors
Stronger π acceptors Weaker π acceptors
![Page 21: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/21.jpg)
Observations consistent with a trigonal bipyramidal transition state in which the cis ligands are axial, and T, X and Y are equatorial
‡
‡
MT
C X
C
T M
S
X
C
C
T M
Y
X
C
C
T M
Y
X
C
C
MT
C Y
C
S
Y
Y
S
![Page 22: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/22.jpg)
‡
T M
Y
X
C
C
If T is a good donor ligand, it is readily polarisable...
...and polarises electron density from M towards it (i.e., the TM bond has significant covalency...
...and this weakens and labilises the MX bond.
i.e., T destabilises the ground state
![Page 23: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/23.jpg)
‡
T M
Y
X
C
C
as X departs in the transition state, there is a build-up of electron density on the metal...
...which can be accommodate by donation to the T ligand.
i.e., T stabilises the transition state
If T is a good π acceptor ligand…
![Page 24: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/24.jpg)
AN ASIDE
The trans effect order can be exploited in synthesis
Example
Given that the trans effect order is PPh3 > Cl- > NH3, explain how to synthesise trans-[PtCl2(NH3)(PPh3)] starting from [PtCl4]2-
Pt
Cl
Cl
Cl
Cl
2-
PPh3Pt
PPh3
Cl
Cl
Cl
2-
NH3Pt
PPh3
Cl
NH3
Cl
2-
How would you synthesise the cis complex?
![Page 25: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/25.jpg)
Steric Effects
Steric crowding at a metal centre will retard an associative reaction, but speed up a dissociative reaction
Pt
L
Cl PEt3
PEt3
+
Pt
L
H2O PEt3
PEt3
2+
H2O N N N
L =
k = 8 x 10-2 2 x 10-4 1 x 10-6 s-1
![Page 26: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/26.jpg)
![Page 27: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/27.jpg)
![Page 28: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/28.jpg)
Stereochemistry
The stereochemistry at the metal centre is preserved, consistent with a transition state in which the entering (Y), leaving (X) and trans (T) ligands are in the plane of a trigonal bipyramidal complex
C
T C
XC
CX
Y
T
C
T C
YC
T C
X
Y
X
![Page 29: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/29.jpg)
The intermediate must be shortlived, else scrambling of stereochemistry would be expected
C
T C
X C
CX
Y
T
Y
T C
C
Y
C
C
T
X X
T
CC
Y
Berry pseudo rotation througha square pyramidal intermediate
![Page 30: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/30.jpg)
Activation parameters
Both the k1 and the k2 pathways have S‡ and V‡ values that are negative. For example:
PtEt2P
Br PEt2Pt
Et2P
I PEt2I-
k1 k2
S‡ /J K-1 mol-1 -59 -121V‡ /cm3 mol-1 -67 -63
+ +
![Page 31: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/31.jpg)
The k1 pathwayA
P
B
k2 [Y]
k3 [Y]
k1
+Sk-1
-S
A
P
B
k2 [Y]
k3 [Y]
k1
+Sk-1
-S
B is a solvento intermediate.
The solvento intermediate has been trapped and isolated in some cases
![Page 32: Substitution reactions of square planar complexes](https://reader033.fdocuments.in/reader033/viewer/2022061314/5681446b550346895db0fdd7/html5/thumbnails/32.jpg)
The solvento intermediate has been trapped and isolated in some cases
Pt
N
N
N
I Pt
N
N
N
H2O Pt
N
N
N
Y
+ 2+ +
k1 H2O Y-
Pt
N
N
N
OH
+
base
kinetically inert