Lecture 27: Substitution in Octahedral Complexes

Read section 25.4 from your textbook.

Water Exchange

All metal cations form aquo complexes and many of these are octahedral. A simple substitution reaction is the exchange of coordinated water for free water in the solution. This reaction occurs at different rates depending on the nature of the metal.


  • Class 1: Exchange of water is very fast and is essentially diffusion controlled (k > 108 sec-1) Ions include Groups 1A , IIA (except Be2+, Mg2+), and IIB (except Zn2+), plus Cr2+, and Cu2+.
  • Class 2: Rate constants are in the range 104-108 sec-1. This includes most of the 1st row T.M divalent ions (except V2+, Cr2+, Cu2+) and Mg2+ and the lanthanide M3+ ions.
  • Class 3: Rate constants are in the range 1 - 104 sec-1. This includes Be2+, Al2+, V2+, and some 1st row T.M. tri-valent ions.
  • Class 4: Rate constants are in the range 10-3-10-6 sec-1. This includes Cr3+, Co3+, Rh3+, Ir3+, Pt2+

Kinetics

Substitution reactions at octahedral complexes can proceed through 3 main pathways. Kinetics allows us to probe the mechanism of an observed reaction. Of course, more than one of these can go on with any particular set of metal complex and ligand. Different reaction pathways typically have different rates of reactions.



  1. Associative substitution: rate = k[ML][L']
  2. Dissociative substitution: rate = k[ML]
  3. Interchange: rate = k[ML][L']
What does this look like? To simplify analysis, we typically examine reactions under pseudo first order conditions. That is, we use a very large excess of the incoming ligand L' and monitor the disapperance of the starting complex over time. Because we use a large excess, the concentration of L' doesn't change much over the course of the reaction. We monitor the rate of reaction with several large concentrations of L'.

For an associative reaction, the natural log of the concentration of starting complex is inversely proportional to time. For each concentration of L', the kobs is the slope of this line. The observed rate constant is directly proportional to the concentration of L' and the slope of the line is the second order rate constant.



The difference between associative and dissociative reactions is in the dependence of the rate of reaction on the concentration of the incoming ligand. When there is no effect of the concentration of L' on the rate, the kobs is the first order rate constant.



Another very common mechanism involves the reversible loss of a ligand (such as water) followed by addition of the incoming ligand.



Effect of the Metal

In the last lecture we discussed labile and inert metal complexes. Because most octahedral metal complexes undergo dissociative substitution, we can explain relative rates of substitution for different metal oxidation states by comparing the crystal field stabilization energy for the starting complex (octahedral CFSE) and the intermediate (square pyriamidal CFSE).

Electron
Configuration 		Octahedral
CFSE 		Sq. Pyramidal
CFSE 		Change
in CFSE
d0 0 0 0
d1 4 4.6 -0.6
d2 8 9.1 -1.1
d3 12 10 2
d4 (strong field) 16 14.6 1.4
d4 (weak field) 6 9.1 -3.1
d5 (strong field) 20 19.1 0.9
d5 (weak field) 0 0 0
d6 (strong field) 24 20 4
d6 (weak field) 4 4.6 -0.6
d7 (strong field) 18 19.1 -1.1
d7 (weak field) 8 9.1 -1.1
d8 12 10 2
d9 6 9.1 -3.1
d10 0 0 0

Effect of the Ligands

  1. Chelate effect Bidentate ligands are substituted more slowly than monodentate ligands. This is because one end of the ligand can dissociate but it remains tethered close to the metal so reattachemnt is favored. Tridentate and tetradentate ligands are even more difficult to substitute. For example, the first order rate constant for dissociation of pyridine from one nickel complex is 38.5 while the dissociation of bipyridine from a related complex is 3.8 x 10-4! A few chalating ligands are shown below.



  2. Leaving group effect

    For dissociative and interchange mechanisms, the rate depends on the nature of the leaving group. The leaving group order is opposite to the bond strength order.

    NO3- > I- > Br-, H2O > Cl-, SO42- > CH3CO2- > N3-, NCS-, NH3 > NO2-, HO-


  3. Effect of the nucleophile

    The nature of the incoming ligand is important in associative and interchange mechanisms. The relative rate and ordering of incoming ligands depends very much on the nature of the metal. You should refer back to the hard-soft acid/base section. Pt(II) is a "soft" metal center and the observed order for substitution with this metal is:

    PR3 > I-, SCN-, N3- > NO2- > Br- > py > NH3, Cl- > H2O > HO-

Acid-Base Catalysis

Acids can catalyze substitution reactions by protonating ligands and making them better leaving groups.



In base catalyzed substitution reactions, the base deprotonates the incoming ligand making it a better nucleophile.



Stereochemistry

The trans effect can be used to predict which ligand will dissociate in an octahedral complex but it isn't easy to determine the stereochemistry of the product. That is because the intermediate square pyramidal complex is comparable in energy to a trigonal bipyrimidal complex. Addition of a ligand will typically give a mixture.