Download Reaction Mechanism

Reaction Mechanism
By Prof Yeap Guan Yeow
Earlier study on:
-- Kinetically inert metal centres such as Co(III)
-- Organometallic reaction
Ligand substitution
MLaX + Y → MLaY + X
X is a leaving group and Y is the entering group.
Labile
Metal complexes that undergo reactions with t1/2 ≤ 1 min
Innert
Metal complexes that undergo reactions with t1/2 ≥ 1 min
Ligand Ligand Substitution Reactions in
Substitution Reactions in Octahedral Compounds
Kinetics of H2O exchange
LnM(H2O) + H2O* = LnM(H2O*) + H2O
Trends in Water Exchange
The rate of water exchange for main group elements
increases:
◆ As the metal ion size increases
◆ The coordination number increases
◆ The surface charge density decreases
Kinetics of Water Exchange
Group I
The ions are alkali and alkaline earth (except Be2+, Mg2+)
and Cr2+ and Cu2+.
 Very fast exchange k ≥ 108 s-1･
Group II
The ions are 1st row divalent transition metal
(except V2+, Cr2+,and Cu2+) as well as Mg2+ and
trivalent lanthanide ions.
 Exchange 104 < k > 108 s-1
Group III
This group includes Be2+, V2+, Al3+, and Ga 3+
as well as trivalent 1st row transition metals.
 Exchange rate 1 < k > 104 s-1
Group IV
This group includes ions classified as inert and include
Cr3+, Co3+, Rh3+, Ir 3+, and Pt2+.
 Exchange rate 10-3 < k > 10-10 s-1
Types of Substitution Mechanism
Dissociative (D)
- Intermediate has a lower coordination number than the starting
complex
Associative (A)
- Intermediate has a higher coordination number than the starting
complex
Interchange (Id)
- Bond breaking is dominate over bond formation
Interchange (Ia)
- Bond formation is dominate over bond breaking
(1) Dissociative (D)
A reaction in which the intermediate has a lower
coordination number than the starting complex.
MLaX
MLa
→
+
MLa
int.
+
X
leaving
group
Y
→ MLaY
entering
group
(2) Associative (A)
A reaction in which the intermediate has a higher
coordination number than the starting complex.
MLaX
+
MLaXY →
Y
→
entering
group
MLaY
+
MLaXY
int.
X
leaving
group
(3) Interchange (I)
In most metal complex, substitution pathways, bond formation
between the metal and entering group is thought to be
concurrent – Interchange mechanism.
MLaX + Y
→ Y…MXa…X
→ MLaY + X
leaving
transition
leaving
group
state
group
In I mechanism, there is no intermediate but various
transition states are possible.
◆Dissociative interchange (Id) in which bond breaking
dominates over bond formation.
◆Associative interchange (Ia) in which bond formation
dominates over bond breaking.
An interchange (I) mechanism is a concerted process
in which there is no intermediate species with a
coordination number different from that of the starting
complex.
Activation parameters
The Eyring equation – relationship between the rate constant,
temperature and activation parameters
ΔG‡= Gibbs
energy of
activation
k = rate constant
T = temperature (K)
ΔH‡=enthalpy of activation (Jmol-1)
ΔS‡ = entropy of activation (JK-1mol-1)
R = molar gas constant
k’= Boltzmann constant
h = Planck constant
 From the above equation, a plot of In(k/T) against 1/T
(an Eyring plot) is linear.
 The activation parameters ΔH‡ and ΔS‡ can be determined
as shown in the following figure.
Activation Parameters
¬ H, enthalpy of activation is a measure of the height of
the energy barrier, particularly bond strengths within
and between reactants, which must be overcome to attain
the transition state;
¬ If H < D (M-X), then bond is broken during
activation where D = dissociation energy.
¬ S, useful in distinguishing between associative and
dissociative mechanisms.
¬ The more negative the value of S, indicative of an
associative mechanism.
Substitution in square planar
Square planar complexes:
→ Rh(I), Ir(I), Pt(II), Pd(II), Au(III)
Tetrahedral or Square planar complexes:
→ Ni(II)
 In general, the nucleophilic substitution reactions in square planar
Pt(II) complexes normally proceed by associative mechanisms (A or Ia).
 Negative values of ΔS and ΔV support the following proposal.
Activation parameters for substitution in selected
square planar complexes
Reactants
[Pt(dien)Cl]+ + H2O
[Pt(dien)Cl]+ + N3trans-[PtCl2(PEt3)2] + py
trans-[PtCl(NO2)(py)2] + py
ΔH‡/kJmol-1 ΔS‡/JK-1mol-1 ΔV‡/cm3mol-1
+84
+65
+14
+12
-63
-71
-25
-24
-10
-8.5
-14
-9
Trans Effect
In 1926, Chernyaev introduced the concept of the
trans effect in Pt chemistry.
An effect from a coordinated group upon the reaction rate of
substituted ligand located at the opposite site.
 Pt(II) compounds, ligands trans to chloride are more easily
replaced than those trans to ligands such as ammonia.
 One contributing factor to the trans effect is the trans influence.
Cl
Cl
2-
Pt
NO2-
NO2- 2-
Cl
Cl
Cl
NH3
2-
Pt
Pt
Cl
NH3
Cl
Cl
Cl
Cl
NO2-
NH3
NO2- 2-
Cl
Pt
H3N
Pt
Cl
I
NH3
Cl
Cl
NO2-
II
2-
In the formation of complex I:
◆ Cl- ion which is trans to NO2- group become more
labile
In the formation of complex II:
◆ Cl- ion which is trans to another Cl- ion become more
labile
Trans effect order:NO2-
>
Cl-
> NH3
In general:CO ≅ CN- ≅ C2H4 ≅ PR3 ≅ H- > NO2- ≅ I- ≅ SCN- >
Br-- > Cl- > NH3 ≅ py > OH- > H2O
The above order is important particularly in the
synthesis of isomer.
For example:
Synthesis of [PtClBr(NH3)(py)]
Cl
Cl
2-
Pt
Cl
Cl
Br
NH3
Pt
Cl
py
2-
Cl
Cl
2-
NH3
Br
Pt
Pt
Cl
Cl
Cl
py
NH3
NH3
Cl
py
2-
Br-
NH3
Br
Pt
Cl
2-
Pt
Cl
Cl
Cl
2-
Redox Reactions
 Electron transfer
 A + B- → A- + B
 Inner sphere
 Transition state involve interpenetrationg coordination sphere
or bridging ligands or interaction between reactants which is
common to both coordination sphere & serves as channel
through which electron flows.
Taube & Meyer Cr2+& Cr3+
Co(H2O)62+ (Labile) & [Cr(NH3)5Cl]2+ (inert))
H+
Co(H2O)62+ + [Cr(NH3)5Cl]2+ →[Co(H2O)6]2+ + [ClCr(H2O)6]2+ + 5NH4+
Inner sphere
[Co(NH3)5Cl]2+ + Cr2+ = [Co(NH3)5ClCr]4+
[Co(NH3)5ClCr]4+ → CrCl2+ + [Co(NH3)5]2+
[Co(NH3)5]2+ + 5H = Co2+ + 5NH4+
Cr3+ inert to substitution with k = 2.9x10-8 M-1 sec-1 for Cl- anation Cr3+
For the reduction k = 6x105 M-1 sec-1
Cr-Cl could not have been formed from substitution of free free Cl-
Outer-sphere Electron Transfer
The interaction between the oxidant and reductant at the
time of electron transfer is small, therefore the coordination
shells are intact;
i.e. through space , diffusion control intact.
Example of Outer-sphere Transfer
[Fe(me2bipy)3]2+ + [Fe(bipy)3]3+ = [Fe(me2bipy)3]3+ + [Fe(bipy)3]2+
Rate = k[Fe(me2bipy)3]2+[Fe(bipy)3]3+ = 108M-1 sec-1
Rate of substitution is ~ 104 M-1 sec-1
For outer sphere the redox rate must be faster than the substitution rate.