Lecture 26: Substitution Reactions

Read Chapter 25 from your textbook.



The reactions of coordination complexes are important in both catalysis and in biochemistry. This lecture will examine reactions in which one donor molecule substitutes for another at a metal center. These ligand substitution reactions can proceed through dissociative, associative, or interchange mechanisms. External electrophiles or nucleophiles can modify ligands. Photolysis can result in dissociation of ligands or in other ligand transformations. The recent chemical literature will provide us with examples of each reaction type.

The picture above shows data for the substitution of water molecules on [Co(H2O)6]2+ for 2 histidine and 2 cysteine ligands of a polypeptide. The researchers followed the reaction by UV-visible spectroscopy.

What is a substitution reaction?

MLx + L' ML(x-1)L' + L


In this reaction, a Lewis base (such as Cl-, HO-, NH3, CO) substitutes for another Lewis base at the metal. The coordination number at the metal remains the same.

There are several ways in which a substitution reaction can take place:
  1. Dissociative substitution involves the reversible dissociation of one ligand to form an intermediate complex with one fewer ligand followed by the addition another ligand to the metal.
      (a) The rate is first order in MLx and zero order in L'.
      (b) The rate doesn't depend on the nucleophilicity of L'.
      (c) This is common for octahedral complexes and other complexes with an 18 electron count.



  2. Associative substitution occurs when the incoming ligand first adds to the complex to form an intermediate complex with one lignad more than the starting complex followed by a loss of one ligand.
      (a) The rate is first order in MLx and first order in L'.
      (b) The rate is directly proportional to the nucleophilicity of L'.
      (c) This is common for square planar complexes and other complexes with less than an 18 electron count.



  3. Interchange substitution is intermediate between the two. There is no intermediate complex formed. Instead bond making to the new ligand occurs along with bond breaking to the original ligand. This is very similar to SN2 substitution in organic chemistry.
      (a) The rate is first order in MLx and zero order in L'.
      (b) The rate doesn't depend on the nucleophilicity of L'.
      (c) The rate is inversely proportional to the strength of the M-L (leaving group) bond.



    Labile and Inert Complexes Octahedral Complexes

    Labile: Reactions occur during time of mixing.
    • At least one vacant d orbital

    • Examples:
        low spin complexes of Sc(III), Y(III), Ti(IV), Zr(IV), Hf(IV), Nb(V), Ta(V), Mo(VI), W(VI)

        Ti(III), V(IV), Mo(V), W(V), Re(VI)

        Ti(II), V(III), Nb(III), Ta(III), Mo(IV), W(IV), Re(V), Ru(VI)


    Inert: Reactions are very slow.
    • All d orbitals have at least 1 electron.

    • Examples:
        high spin complexes of V(II), Cr(III), Mo(III), W(III), Mn(IV), Re(IV)

        [Cr(CN)6]4-, [Cr(bpy)3]2+, Mn(CN)6]3-, Re(III), Ru(IV), Os(IV)

        [Cr(bpy)3]+, [Mn(CN)6]4-, Re(II), [Fe(CN)6]3-, [Fe(bpy)3]3+, Ru(III), Os(III), Ir(IV)

        [Mn(CN)6]5-, [Fe(CN)6]4-, [Fe(bpy)3]2+, Ru(II), Os(II), Co(III), Rh(III), Ir(III), Ni(IV), Pd(IV), Pt(IV)


    Trans Effect


    The ability of a group to direct substitution trans to itself in a metal complex is called the trans effect. This is most important in square planar complexes but also applies to octahedral complexes.

    Trans effect order
      H2O < OH- < NH3 < RNH2 < pyridine < Cl- < Br- < SCN-, I-, NO2-, SO3H-, PR3, R2S, SC(NH2)2 < NO, CO, C2H4, CN-

      Cl- < Ph- < CH3- < H-



    Other Effects on the Rate

    1. Protonation of a ligand

    2. Basic hydrolysis of a ligand

    3. Other ligand modifications

    4. Oxidation/Reduction