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Chapter 3
Thermodynamic and kinetic
aspects of metal complexes
Conclusions from these examples.
Stable complexes have a large POSITIVE GoRXN for ligand substitution and
Inert complexes have a large POSITIVE G‡ (activation).
Stability and Coordination Complexes ([MLn]x+)
Typically expressed in terms of an overall formation or stability constant.
(This is Kst on the Chemistry Data sheet you receive with exams)
[M]x+ + nL
[MLn ]x 
K st 
x
[M(aq)
][L]n
[MLn]x+
BUT, this does not occur in one fell swoop!!
Water molecules do not just all fly off and are immediately replaced by nL ligands.
[M] x+(aq) + L
[ML(n-1)]x+ + L

[ML]x+
[MLn]x+
K1
Kn
Ks are the stepwise formation constants and provide insight into
the solution species present as a function of [L].
Stepwise formation constants
These formation constants provide valuable information given that different species may
have VERY DIFFERENT properties…including environmental impact. Such information
provides selective isolation of metal ions from solution through reaction with ligands.
For formation of divalent alkaline earth
and 3d M2+ TM ions the IrvingWilliams Series holds true.
Ba<Sr<Ca<Mg<Mn<Fe<Co<Ni<Cu>Zn
What is contributing to this trend?
1.
2.
3.
Charge to radius ratio.
CFSE (beyond Mn2+)
Jahn-Teller Distortion
Hard-Soft Acids/Bases
See R-C p 450-451.
Associative Mechanism
ML5X + Y
ML5Y + X
Step 1. Collision of ML5X with Y to yield a 7-coordinate intermediate. (slow)
K1
ML5X + Y
ML5XY
(slow, rate determining)
X
Y
L
L
M
L
L
Capped
Octahedron
Pentagonal
Bipyramid
L
L
M
L
L
Y
L
L
X
Step 2. Cleavage of the M-X bond. (fast)
ML5XY
ML5Y + X
(fast)
The rate law for this process is rate = K1[ML5X][Y] (the units of K1 are sec-1Mole-1)
If we find a reaction follows this rate law we conclude it is associative.
Substitution reactions
MLn-1L' + L
MLn + 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 Gf <0)
Inert
Intermediate
d3, low spin d4-d6& d8
d8 (high spin)
Labile
d1, d2, low spin d4-d6& d7-d10
Mechanisms of ligand exchange reactions
in octahedral complexes
MLnY + X
MLnX + Y
Dissociative (D)
MLn Y
MLn X
X
Associative (A)
MLn
MLn Y
MLn X
Y
Y
MLn XY
Interchange (I)
MLn X
MLn Y
Y
Ia if association
is more important
[ML n]°
X
Y
X
Id if dissociation
is more important
X
Kinetics
of dissociative reactions
Kinetics
of interchange reactions
Fast equilibrium
K1 = k1/k-1
k2 << k-1
For [Y] >> [ML5X]
Kinetics of associative reactions
Principal mechanisms of ligand exchange in octahedral complexes
Dissociative
Associative
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
Labile or inert?
L
L
L
M
L
L
Ea
L
L
L
L
M
L
L
M
L
L
L
X
L
X
G
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
L
+X, -Y
L
Y
M
T
(the ability of T to labilize X)
L
Trans Effect Strengths
I. Trans effect is more pronounced for s donor
as follows:
OH-<NH3<Cl-<Br-<CN-,CO, CH3-<I-<PR3
• Trans effect is more pronounced for a ð
acceptor as follows:
Br-<Cl-<NCS-<NO2-<CN-<CO
Synthetic applications
of the trans effect
B.
Substitution in trans complexes
1) 3 possible substitution reactions for trans-[M(LL)2BX] + Y
a) Retention of configuration with
a square pyramidal intermediate
b) Trigonal bipyramidal intermediate
with B in the plane gives a mixture
of products
c) Trigonal bipyramidal intermediate
with B axial leads to cis product
2)
Experimental Data
a) Many factors determine the mixture of isomers in the product
b) Example: Identity of X
c)
C.
Prediction is very difficult without experimental data on related complexes
Substitution in cis complexes
1) The same 3 possibilities exist as for trans
1) The products are just as hard to predict
D.
Isomerization of Chelate Complexes
1) One mechanism is simple dissociation and reattachment of one donor of the
ligand. This would be identical to any other substitution reaction
2)
Pseudorotation
a) “Bailar Twist” = Trigonal twist = all three rings move together through a
parallel intermediate
b) Tetragonal Twists = one ring stays the same and the others move
Bailar Twist
Tetragonal Twist
Tetragonal Twist
Bailar Twist
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]
Ea
Tunneling
mechanism
A
+
B
A'
G
+
B'
Inner sphere mechanism
[Co(NH3)5Cl)]2+ + [Çr(H2O)6]2+
[Co(NH3)5Cl)]2+:::[Çr(H2O)6]2+
[CoIII(NH3)5(m-Cl)ÇrII(H2O)6]4+
[CoII(NH3)5(m-Cl)ÇrIII(H2O)6]4+
[CoII(NH3)5(H2O)]2+
[Co(NH3)5Cl)]2+:::[Çr(H2O)6]2+
[CoIII(NH3)5(m-Cl)ÇrII(H2O)6]4+
[CoII(NH3)5(m-Cl)ÇrIII(H2O)6]4+
[CoII(NH3)5(H2O)]2+ + [ÇrIII(H2O)5Cl]2+
[Ço(H2O)6]2+ + 5NH4+
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 +
G