Transcript Slide 1

Substitution reactions of square planar
complexes
especially d8: Ni(II), Rh(I), Pd(II), Ir(I), Pt(II), Au(III)
Reaction:
[ML3X] + Y  [ML3Y] + X
General rate law:
[ML3X]  A
Rate  ( k1  k 2 [Y])[A]
General mechanism
A
k1
+S
k2 [Y]
P
k-1
-S
k3 [Y]
B
A
d [P ]
dt
 k 2 [A ][Y ]  k 3 [B ][Y ]
(1)
k1
+S
k 2 [Y ]
k -1
-S
P
Assume [B] is in steady state
k 3 [Y ]
B
k 1 [A ]  k  1 [B ] + k 3 [B ][Y ]
[B ] =
Rate  ( k1  k 2 [Y])[A]
k 1 [A ]
( k  1  k 3 [Y ])
Substituting into (1)
d [P]
dt
 k 2 [A][Y ] + k 3 [Y ]
k1 [A]
( k  1  k 3 [Y ])
d [P]
dt
 k 2 [A][Y ] + k 3 [Y ]
A
k1 [A]
( k  1  k 3 [Y ])
k1
+S
k 2 [Y ]
k -1
-S
Two situations usually arise for the solvent pathway
P
k 3 [Y ]
B
The rate of attack of solvent on A is rate limiting
Rate  ( k1  k 2 [Y])[A]
k3[Y] >> k-1
d [P ]
dt
 k 2 [A ][Y ] + k 3 [Y ]
k1 [ A ]
( k 3 [Y ])
 k 2 [A ][Y ] + k 1 [ A ]
 ( k 1  k 2 [Y ])[A ]
which is in agreement with the experimental rate law
d [P]
dt
 k 2 [A][Y ] + k 3 [Y ]
A
k1 [A]
( k  1  k 3 [Y ])
k1
+S
k 2 [Y ]
k -1
-S
Two situations usually arise for the solvent pathway
P
k 3 [Y ]
B
The rate of attack of solvent on A is much faster
than attack of Y on the intermediate B
k3[Y] << k-1
d [P ]
dt
 k 2 [A ][Y ] + k 3 [Y ]

k1 k 3
  k2 +
k 1

 k '[A ][Y ]

 [A ][Y ]

k 1 [A ]
k 1
Rate  ( k1  k 2 [Y])[A]
d [P ]
dt
d [P ]
 ( k 1  k 2 [Y ])[A ]
dt

k1 k 3
  k2 +
k 1


 [A ][Y ]

 k '[A ][Y ]
Study the rate of the reaction as a function of [Y]
k o bs
k2
k o bs

k1 k 3 
k
+
 2

k 1 

k1
[Y ]
0
[Y ]
A
The k2 pathway
k1
+S
Define
k2o as the 2nd order rate constant when Y = MeOH
in the reaction
k 2 [Y ]
k -1
-S
P
k 3 [Y ]
B
trans-[PtCl2(py)2] + Y  trans-[PtClY(py)2] + Cl
then compare the rate for any other ligand Y to the rate when Y = MeOH
 P t  log
nucleophilicit
y parameter
k 2 (Y )
o
k2
The greater Pt, the
greater the
nucleophilicity of
the ligand
nucleophilicity increases with
softness of the donor ligand
 P t  log
k 2 (Y )
o
k2
 P t  lo g k 2 (Y )  lo g k 2
o
lo g k 2 (Y )   P t  lo g k 2
o
lo g k 2 (Y )   P t  C
where C = log k2o
Now generalise for any square planar Pt complex [PtL3X]
[PtL3X] + Y  [PtL3Y]+ X
log k 2 (Y)  S Pt  C
S is the nucleophilic
discrimination factor and
gives the sensitivity of the
rate constant to the
nucleophilicity of the
incoming ligand
trans-[PtCl2(PEt3)2]
SeCN
S is larger
SCN
I
thiourea
log k 2
SCN
N3
I
NH3
Br
Cl
CH3OH
NO2
CH3OH
 Pt
[PtCl2(en)]
log k 2
The discrimination factor,
S, decreases
As reactivity towards
the common
ligand, MeOH, increases
Usually...
 Pt
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
2 Cl, 2 P
2Cl, 2 aromatic N
Cl, 3 aliphatic N
2Cl, 2 aliphatic N
As softness of ligands on
Pt increases, S increases
– the complexes
becomes less reactive
and more discriminating
Example
Calculate 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.
log k 2 (Y)  S Pt  C
log 40  S  5.42  C
(1)
log 7  S  3.58  C
(2)
(1)  (2)
0.755  1.84 S
S  0.410
C  1.60  5.42  0.410
C   0.62
Hence, for NO2
log k 2  0.410  3.22  0.62
 0.70
k 2  10
0.70
 5.0 M
-1
s
-1
The nature of the transition state
Two important observations:
For the generalised reaction
L1
L2
L1
Pt
Y
Y
L2
Pt
Cl
A
Cl
L1
Cl
Y
L2
Pt
Cl
Y
B
whether the predominant product is A or B depends on the relative
trans effect of the spectator ligands L1 and L2
The rate of the reaction
C
C
Y
T
Pt
C
Cl
T
Pt
Y
C
depends significantly on the nature of the trans ligand, T, but hardly at
all on the cis ligands C
The trans effect order is
For  donor ligands
H- > PR3 > SCN- > I- > CH3-, CO, CN- > Br-, Cl- > NH3, py > OH-, H2O
Stronger σ donors
Weaker σ donors
For  acceptor ligands
CO, C2H2 > CN- > NO2- > NCS- > I- > BrStronger π acceptors
acceptors
Weaker π
Observations consistent with a trigonal bipyramidal transition state in which
the cis ligands are axial, and T, X and Y are equatorial
‡
C
X
T
C
X
M
Y
Y
M
T
C
C
S
C
Y
M
C
T
X
T
M
C
Y
S
C
‡
X
T
M
Y
S
C
C
‡
C
If T is a good  donor ligand, it is
readily polarisable...
T
X
M
...and polarises electron density
from M towards it (i.e., the TM
bond has significant covalency...
...and this weakens and
labilises the MX bond.
Y
C
i.e., T destabilises the ground state
If T is a good π acceptor ligand…
as X departs in the transition state,
there is a build-up of electron
density on the metal...
C
‡
X
T
M
Y
...which can be accommodate by
 donation to the T ligand.
C
i.e., T stabilises the transition state
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]22-
Cl
2-
PPh3
NH3
PPh3
Cl
Pt
Cl
Cl
2-
PPh3
Cl
Pt
Cl
Cl
How would you synthesise the cis complex?
Cl
Pt
NH3
Cl
Steric Effects
Steric crowding at a metal centre will retard an associative reaction,
but speed up a dissociative reaction
+
Cl
P E t3
Pt
L
P E t3
L =
H 2O
2+
H 2O
P E t3
Pt
L
N
N
P E t3
k =
8 x 10
-2
2 x 10
N
-4
1 x 10
-6
s
-1
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
T
C
C
X
T
T
C
C
X
X
X
C
C
Y
Y
T
C
C
Y
The intermediate must be shortlived, else scrambling of
stereochemistry would be expected
T
T
C
C
X
X
C
T
Y
C
C
C
C
T
C
X
Y
B erry pseudo rotation through
a square pyram idal interm ediate
X
Y
T
Y
C
C
Activation parameters
Both the k1 and the k2 pathways have S‡ and V‡ values that are
negative. For example:
Br
P E t2
Pt
+
+
I
I
-
P E t2
Pt
E t2 P
E t2P
k1
S‡ /J K-1 mol-1
V‡ /cm3 mol-1
-59
-67
k2
-121
-63
A
The k1 pathway
k1
+S
k 2 [Y ]
k -1
-S
B is a solvento intermediate.
P
k 3 [Y ]
B
The solvento intermediate has been trapped and isolated in some cases
The solvento intermediate has been trapped and isolated in some cases
2+
+
k1 H 2O
N
Pt
I
Y
N
Pt
H 2O
-
N
b a se
+
N
N
Pt
Pt
N
N
N
+
N
N
N
OH
N
kinetically inert
Y