Graduate Center
Inorganic Chemistry
(Fall 2012)
Unit 5
Coordination Chemistry
Part 3
Reactions of Metal Complexes
Suggested reading:
Miessler & Tarr: Chapter 12
Chemical reactions move from one energy minimum (reactants) through
a higher energy structure (transition state) to another energy minimum (products)
Free
Energy
Free
Energy
ΔG#
MX + Y
MXY
ΔG#
MX + Y
MY + X
no intermediate
intermediate
Extent of the reaction
(a)
MY + X
Extent of the reaction
(b)
Principle of microscopic reversibility
Steady-state
approximation
Parameters that can be obtained by
kinetic experiments
• Order of the reaction
• Rate constant (speed of the reaction)
By studying the reaction at different temperatures:
• Free energy of activation
• Enthalpy (heat)
• Entropy
Inclusion of pressure dependence:
• Volume of activation (whether the transition state is larger or smaller
than the reactants)
Main types of reactions
- Substitution Reactions
- Oxidation-Reduction Reactions
- Reactions of the ligands that do
not change the attachments to
the metal center
Substitution reactions
MLn + L'
MLn-1L' + L
Labile complexes <==> Fast substitution reactions (< few min)
Inert complexes <==> Slow substitution reactions (>h)
a kinetic concept
Not to be confused with
stable and unstable (a thermodynamic concept ΔGf <0)
Inert
Intermediate
Labile
d3, low-spin d4 , d4, d6
strong-field d8 (sq-pl)
d8 (weak-field)
d1, d2, high-spin d4, d5, d6
& d7, d9 , d10
Mechanisms of ligand exchange reactions
in octahedral complexes
MLn X + Y
MLn Y + X
Detection of intermediates
not possible
Ia if association
is more important
Id if dissociation
is more important
No clear-cut evidence to distinguish them
Kinetics
of dissociative reactions
Steady-state
approximation
Rates of formation and reaction
of the intermediate must be equal
Detectable
Intermediate!
Kinetics
of interchange reactions
Fast equilibrium
K1 = k1/k-1
k2 << k-1
For [Y] >> [ML5X]
common experimental
condition!
Dissociative and interchange reactions
rate =
K[M]0[Y]
[X] + K'[Y]
Dissociative
rate =
K[M]0[Y]0
1 + K'[Y]0
Interchange
At low conc. of Y both reactions become second order (first order
in M and Y) with the rate of the dissociative reaction slowing as
more free X is forming
At high conc. of Y (more common), second term in denominator larger,
reaction first order in complex and zero order in Y
Kinetics of associative reactions
Rate determining
step
At any [Y]
Change from one rate law to the other depends on the specific values of the rate constants
Similarity of the rate laws limits their usefulness in determining the mechanism
Mechanisms of ligand exchange reactions in square planar complexes
L
L
K2 = KY
X
L
L
M
X
-X
Two-term rate laws
rate = K1[ML3X] + K2[ML3X][Y]
Y
L
-d[ML 3X]/dt = (ks + ky [Y]) [ML3X]
M
X
L
L
L
+Y
L
S
+S
M
L
L
M
S
L
+Y
Y
L
-X
L
Rh(I)
Ir(I)
Ni(II)
Pd(II)
Pt(II)
L
Au(III)
L
L
M
Y
-S
L
M
S
K1 = KS
Principal mechanisms of ligand exchange in octahedral complexes
Dissociative
ML5 X
k1
slow
ML5 + X
+Y
k2
fast
r = k 1 [ML5X]
ML5 Y
Associative
ML5 X + Y
k1
slow
ML5 XY
-X
k2
fast
r = k 1 [ML5X][Y]
ML5 Y
Dissociative pathway
(5-coordinated intermediate)
MOST COMMON
Associative pathway
(7-coordinated intermediate)
Experimental evidence for dissociative mechanisms
Rate is independent of the nature of L
Experimental evidence for dissociative mechanisms
Rate is dependent on the nature of L
Inert and labile complexes
Some common thermodynamic and kinetic profiles
Exothermic
(favored, large K)
Large Ea, slow reaction
Exothermic
(favored, large K)
Large Ea, slow reaction
Stable intermediate
Endothermic
(disfavored, small K)
Small Ea, fast reaction
Labile or inert?
L
L
L
M
L
L
Ea
L
L
L
L
M
L
L
M
L
L
L
X
L
X
ΔG
LFAE = LFSE(sq pyr) - LFSE(oct)
Why are some configurations inert and some are labile?
Inert !
Substitution reactions in square-planar complexes
the trans effect
L
X
M
T
+X, -Y
L
L
M
T
(the ability of T to labilize X)
Strong Π acceptors followed by strong σ donors
Y
L
Synthetic applications
of the trans effect
g) h) Greater lability of Cl-
Explanations of the trans effect (σ-bonding and Π bonding)
Trans effect is dominated by 2 factors: weakening of the Pt-X bond and
stabilization of the presumed 5-coordinate transition state
σ- bonding effect
• Pt-X bond is influenced by the Pt-T bond (both use the Pt px and dx2-y2 orbitals)
• If Pt-T σ bond strong, it uses part of the orbitals and leaves less for Pt-X bond.
Then Pt-X bond weaker and its ground state (σ-bonding orbital) is higher in E
leading to smaller Ea for breaking this bond. This ground state effect is called
the trans influence and applies primarily to the leaving group (thermodynamic
effect) contributes to the overall kinetic result by changing the reactant ground
state.
• This part of the explanation predicts the order for the trans effect based on the
relative σ-donor properties of ligands
H- > PR3 > SCN- > I- similar CH3- sim CO sim CN- > Br- > Cl- > NH3 > OH-
Explanations of the trans effect (σ-bonding and Π bonding)
Π- bonding effect
When the T ligand forms a strong Π-acceptor bond with Pt, charge is removed
from Pt and the entrance of another ligand to form a 5-coordinate species
more likely. dx2-y2 and dxz and dyz orbitals can contribute to Π-bonding in
the trigonal-bipyramidal transition state.
Effect on the ground state of the reactant is small, but the energy of the TS is
lowered and then reducing Ea
Order of Π-acceptor ability of ligands:
C2H4 sim CO > CN- > NO2- > SCN- > I- > Br- > Cl- > NH3 > OHCombination of the two effects:
Electron transfer (redox) reactions
-1e (oxidation)
M1(x+)Ln + M2(y+)L’n
M1(x +1)+Ln + M2(y-1)+L’n
+1e (reduction)
Very fast reactions (much faster than ligand exchange)
May involve ligand exchange or not
Very important in biological processes (metalloenzymes)
Outer sphere mechanism
[Fe(CN)6]3- + [IrCl6]3-
[Fe(CN)6]4- + [IrCl6]2-
[Co(NH3)5Cl]+ + [Ru(NH3)6]3+
[Co(NH3)5Cl]2+ + [Ru(NH3)6]2+
Reactions ca. 100 times faster
than ligand exchange
(coordination spheres remain the same)
A
B
"solvent cage"
r = k [A][B]
Tunneling
mechanism
Ea
A
+
Ligands with Π or p e- or orbitals that can be
used in bonding good for tunneling
NH3 (no low-lying antibonding orbitals) do not
B
A'
ΔG
+
B'
Inner sphere mechanism
[Co(NH3)5Cl)]2+ + [Cr(H2O)6]2+
[Co(NH3)5Cl)]2+:::[Cr(H2O)6]2+
Step 1: formation of a bridging ligand
[CoIII(NH3)5(μ-Cl)CrII(H2O)6]4+
[Co(NH3)5Cl)]2+:::[Cr(H2O)6]2+
Step 2: transfer of e- (frequently accompanied by transfer of the ligand)
[CoII(NH3)5(μ-Cl)CrIII(H2O)6]4+
[CoIII(NH3)5(μ-Cl)CrII(H2O)6]4+
Step 3: separation of products
[CoII(NH3)5(μ-Cl)CrIII(H2O)6]4+
[CoII(NH3)5(H2O)]2+ + [CrIII(H2O)5Cl]2+
[CoII(NH3)5(H2O)]2+
Also tunneling mechanism by a single ligand
[Co(H2O)6]2+ + 5NH3
Inner sphere mechanism
Ox-X + Red
k1
Ox-X-Red
k2
Reactions much faster
than outer sphere electron transfer
(bridging ligand often exchanged)
k3
k4
Ox(H2O)- + Red-X+
Ox-X-Red
Tunneling
through bridge
mechanism
r = k’ [Ox-X][Red] k’ = (k1k3/k2 + k3)
Ea
Ox-X
+
Red
Ox(H2O)- + Red-X+
ΔG