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Chemical Reactions of Transition Metal Complexes

The most common reaction is the substitution reaction:

\[ML_6 + X \rightarrow  ML_5X + L\]

In solution phase, all reactions are substitution reactions: if there is no "ligand", then the initial complex is a solvate.

MX2(s) [M(OH2)6]2+(aq) + 2 X(aq)

MX2(s) [M(NCCH3)6]2+(sol) + 2 X(sol)

Water is a good choice for solvent because it dissolves many salts and it is a weak ligand, thereby acting as a good leaving group. Remember: strong ligands will replace weaker ligands to increase the total LFSE.

Thermodynamics

Substitution reactions are equilibria:

\[ [M(OH_2)_6]^{2+}(aq) + L \rightleftharpoons [M(OH_2)_5L]^{2+}(aq) + H_2O(l)\]

\(K_f\) is called a formation constant. There is a formation constant for each addition of a ligand:

[M(OH2)6]2+(aq) + L [M(OH2)5L]2+(aq) + H2O(l)

Kf1

 

 

[M(OH2)5L]2+(aq) + L [M(OH2)4L2]2+(aq) + H2O(l)

Kf2

 

 

[M(OH2)4L2]2+(aq) + L [M(OH2)3L3]2+(aq) + H2O(l)

Kf3

etc.

Kfi are called step-wise formation constants. The overall equilibrium constant for the formation of a complex is β = Kf1Kf2Kf3 ... The size of β depends upon a number of factors:

  • the ligand field strength of the ligand
  • the charge/size of the metal ion
  • the presence of chelation and ring formation
  • the degree of substitution
  • steric effects

Kinetics

The rates of substitution reactions can vary over many orders of magnitude, from ns to years.

  • Complexes that undergo fast substitution reactions are called labile.
  • Complexes that undergo slow substitution reactions are called inert.

The cutoff time between the two categories is somewhat arbitrary, but usually taken to be around a minute. Ions with large LFSE (especially d3 and low field d6) are inert. d0, high spin d5, d10 ions are usually very labile. Other ions fall in between.

There are two common, limiting mechanisms of substitution: Associative and Dissociative.

Associative

happens for 4- or 5-coordinate complexes; less often for 6-coordinate.

Substituting ligand adds to an open coordination site, then pushes leaving group and replaces it in the complex. The rate law is first order in both complex and L.

Dissociative

common for octahedral complexes but also can be found for lower coordination numbers.

The leaving group exits, then the substituting ligand bonds. The rate law is first order in complex, zero order in ligand.

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