Substitution Processes

Substitution Reaction Mechanisms

When we discuss a substitution reaction in inorganic chemistry we ask ourselves questions about these aspects of the reaction:

• Stoichiometric mechanism -- What are the steps involved? Does a ligand dissociate during the reaction, etc.?
• Intimate mechanism -- What factors specifically affect the rate constant of a particular step? Why is the rate faster for R=Me than for R=Ph etc.?

In particular, we need to ask if a reaction proceeds through an intermediate. This immediately raises the question, "what is an intermediate?" On the potential energy diagram shown below, an intermediate is drawn and connected to the reactants and products by the blue line. But what if the energy barrier to get out of this potential well is so small that we can't detect it? To us, the reaction coordinate might appear not to include an intermediate at all as indicated by the red line:

To this end, we will say that for an intermediate to exist we must have some experimental evidence for its existence. This evidence can be a direct observation (by NMR, IR, UV-VIS etc.) or indirect evidence (kinetic analysis).

Classification of Substitution Mechanisms

We can classify reactions into three groups based on their stoichiometric mechanism:

• A - Associative. Involves an intermediate of increased coordination number.
• D - Dissociative. Involves an intermediate of decreased coordination number.
• I - Interchange. No evidence for an intermediate.

Likewise, we can divide the intimate mechanism into two classes:

• a - Rate is affected by the entering group.
• b - Rate is affected by the leaving group.

Overall this gives us four limiting cases. A classic organic description of each reaction mechanism is given in parentheses:

• D (comparable to the S_N1 limiting case)
• A (comparable to the S_N2 limiting case)
• I_a (comparable to typical S_N2 reactions)
• I_d (comparable to typical S_N1 reactions)

Rate Laws for Substitution Mechanisms

The rate laws for different classes of reactions can be derived rather easily. Here are the rate laws and free energy diagrams associated with the mechanisms that we have discussed:

In the case of the I_a mechanism, we could say that the bond from the incoming ligand starts to form before the leaving one starts to break. In the I_d mechanism, we could say that an existing metal-ligand bond starts to lengthen or weaken before the incoming ligand arrives. A "pure" interchange would have the leaving ligand-metal bond weaken at the same time that the incoming ligand-metal bond forms.

Additional Resources

• Reaction Mechanisms at Chemistry LibreTexts covers some of the main types.
• Some Chemistry Lecture Videos has an Intro to Inorganic Reaction Mechanisms on YouTube as well as many more in the Inorganic realm..
• Dermont O'Hare has several nice lectures on mechanisms of organometallic reaction, courtesy of the Inernet Archive.
• Reaction Mechanisms of Inorganic and Organometallic Systems, 3rd Edition
  Robert B. Jordan / Hardcover / Published 1998 / 384 pages
  Easily understandable advanced undergraduate to graduate-level text dealing with kinetics, ligand substitution, fluxional processes, mechanisms (and skepticism!), electron transfer, photochemistry, experimental methods, solvent exchange, orbital symmetry rules, C-H activation and more. Comes with 900+ references and sample problems.
• Inorganic and Organometallic Reaction Mechanisms, 2nd Edition
  Jim D. Atwood / Hardcover / Published 1997
  A classic text in the area.