1. Thermodynamic and kinetic aspects of metal complexes
Chapter 3
Thermodynamic and kinetic aspects of metal complexes

2. 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+
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]x K1
[ML(n-1)]x+ + L [MLn]x Kn
Ks are the stepwise formation constants and provide insight into the solution species present as a function of [L].

3. 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 Irving-Williams Series holds true.
Ba<Sr<Ca<Mg<Mn<Fe<Co<Ni<Cu>Zn
What is contributing to this trend?
Charge to radius ratio.
CFSE (beyond Mn2+)
Jahn-Teller Distortion
Hard-Soft Acids/Bases
See R-C p

4. 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)
Capped
Octahedron
Pentagonal
Bipyramid
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.

5. Substitution reactions
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 DGf <0)

6. Mechanisms of ligand exchange reactions in octahedral complexes
Dissociative (D)
Associative (A)
Interchange (I)
Ia if association
is more important
Id if dissociation
is more important

8. of dissociative reactions
Kinetics
of dissociative reactions

9. of interchange reactions
Fast equilibrium
K1 = k1/k-1
k2 << k-1
Kinetics
of interchange reactions
For [Y] >> [ML5X]

10. Kinetics of associative reactions

11. Dissociative Associative
Principal mechanisms of ligand exchange in octahedral complexes
Dissociative
Associative

12. (5-coordinated intermediate)
MOST COMMON
Dissociative pathway
(5-coordinated intermediate)
Associative pathway
(7-coordinated intermediate)

13. Experimental evidence for dissociative mechanisms
Rate is independent of the nature of L

14. Experimental evidence for dissociative mechanisms
Rate is dependent on the nature of L

15. Labile or inert?
LFAE = LFSE(sq pyr) - LFSE(oct)

16. Why are some configurations inert and some are labile?

17. Substitution reactions in square-planar complexes
the trans effect
(the ability of T to labilize X)

18. Trans Effect Strengths
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

19. Synthetic applications
of the trans effect

20. 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

21. The same 3 possibilities exist as for trans
Experimental Data
Many factors determine the mixture of isomers in the product
Example: Identity of X
Prediction is very difficult without experimental data on related complexes
Substitution in cis complexes
The same 3 possibilities exist as for trans
The products are just as hard to predict

23. Isomerization of Chelate Complexes
One mechanism is simple dissociation and reattachment of one donor of the ligand. This would be identical to any other substitution reaction
Pseudorotation
“Bailar Twist” = Trigonal twist = all three rings move together through a parallel intermediate
Tetragonal Twists = one ring stays the same and the others move
Bailar Twist
Tetragonal Twist
Bailar Twist

24. 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)

25. Reactions ca. 100 times faster than ligand exchange
Outer sphere mechanism
[Fe(CN)6] [IrCl6]2-
[Fe(CN)6] [IrCl6]3-
[Co(NH3)5Cl]+ + [Ru(NH3)6]3+
[Co(NH3)5Cl] [Ru(NH3)6]2+
Reactions ca times faster
than ligand exchange
(coordination spheres remain the same)
r = k [A][B]
Tunneling
mechanism

26. Inner sphere mechanism
[Co(NH3)5Cl)]2+:::[Çr(H2O)6]2+
[Co(NH3)5Cl)] [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+
[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+
[CoII(NH3)5(H2O)]2+
[Ço(H2O)6] NH4+

27. than outer sphere electron transfer (bridging ligand often exchanged)
Inner sphere mechanism
Reactions much faster
than outer sphere electron transfer
(bridging ligand often exchanged)
r = k’ [Ox-X][Red] k’ = (k1k3/k2 + k3)
Tunneling
through bridge
mechanism