Question

Henry Taube, 1983 Nobel Prize winner in chemistry, has studied the mechanisms of the oxidation-reduction reactions of transition metal complexes. In one experiment he and his students studied the following reaction: $$ \begin{aligned} \operatorname{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}^{2+}(a q) &+\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}^{2+}(a q) \\ & \longrightarrow \mathrm{Cr}(\mathrm{III}) \text { complexes }+\mathrm{Co}(\mathrm{II}) \text { complexes } \end{aligned} $$ Chromium(III) and cobalt(III) complexes are substitutionally inert (no exchange of ligands) under conditions of the experiment. Chromium(II) and cobalt(II) complexes can exchange ligands very rapidly. One of the products of the reaction is \(\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{Cl}^{2+} .\) Is this consistent with the reaction proceeding through formation of \(\left(\mathrm{H}_{2} \mathrm{O}\right)_{5} \mathrm{Cr}-\mathrm{Cl}-\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5}\) as an inter- mediate? Explain.

Short answer

The formation of the intermediate \((\mathrm{H}_{2} \mathrm{O})_{5}\mathrm{Cr}\text{-}\mathrm{Cl}\text{-}\mathrm{Co}(\mathrm{NH}_{3})_{5}\) is inconsistent with the experiment's conditions, as both Chromium(III) and Cobalt(III) complexes are inert and don't exchange ligands. Therefore, an alternative reaction mechanism must be involved in the formation of the product \( \mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\mathrm{Cl}^{2+}\).

Step-by-step solution

Understanding the reaction system

Let's first understand the given reaction between the Chromium(II) and Cobalt(III) complexes: \( \operatorname{Cr}(\mathrm{H}_{2}\mathrm{O})_{6}^{2+}(aq) + \mathrm{Co}(\mathrm{NH}_{3})_{5}\mathrm{Cl}^{2+}(aq) \rightarrow \mathrm{Cr}(\mathrm{III})\hspace{2mm} complexes + \mathrm{Co}(\mathrm{II}) \hspace{2mm} complexes\) It's important to note that Chromium(III) and Cobalt(III) complexes are substitutionally inert, meaning they don't exchange ligands under experimental conditions. Conversely, Chromium(II) and Cobalt(II) complexes can exchange ligands rapidly.

Identify the product and intermediate

We are given that one of the products of the reaction is \( \mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\mathrm{Cl}^{2+} \) and we need to verify if this is consistent with the reaction proceeding through formation of the intermediate \( (\mathrm{H}_{2} \mathrm{O})_{5}\mathrm{Cr}\text{-}\mathrm{Cl}\text{-}\mathrm{Co}(\mathrm{NH}_{3})_{5}\).

Analyze the formation of intermediate

For the intermediate \( (\mathrm{H}_{2} \mathrm{O})_{5}\mathrm{Cr}\text{-}\mathrm{Cl}\text{-}\mathrm{Co}(\mathrm{NH}_{3})_{5}\) to form, it would require the exchange of one Cl ligand from the Cobalt(III) complex with one H2O ligand from the Chromium(II) complex. However, both Chromium(III) and Cobalt(III) complexes are inert and don't exchange ligands under experimental conditions. Consequently, it's not feasible to form the given intermediate in this reaction.

Conclusion

In conclusion, the formation of the intermediate \((\mathrm{H}_{2} \mathrm{O})_{5}\mathrm{Cr}\text{-}\mathrm{Cl}\text{-}\mathrm{Co}(\mathrm{NH}_{3})_{5}\) is inconsistent with the experiment's conditions, as both Chromium(III) and Cobalt(III) complexes are inert and don't exchange ligands. Therefore, an alternative reaction mechanism must be involved in the formation of the product \( \mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{5}\mathrm{Cl}^{2+}\).

Key concepts

Transition Metal Complexes

Transition metal complexes are fascinating molecular entities that are fundamental to numerous processes in chemistry and biochemistry. They consist of a central metal atom or ion bonded to surrounding molecules or ions known as ligands. These ligands can donate pairs of electrons to create coordination bonds with the metal.
The versatility of transition metals stems from their ability to adopt multiple oxidation states and coordinate with a variety of ligands. This characteristic enables transition metals to partake in a wide range of chemical reactions, including redox reactions, where the oxidation state of the metal can change. In the exercise involving Henry Taube's experiment, we are dealing with complexes of chromium(II) and cobalt(III), which show different reactivity based on their oxidation states and coordination environment.

Ligand Exchange

Ligand exchange is a key process in coordination chemistry that involves the replacement of one ligand in a metal complex with another. It is a type of substitution reaction and can be rapid or slow, depending on the nature of the metal, its oxidation state, and the ligands involved.
In the context of our exercise, the question arises whether a ligand exchange is taking place to form the observed product. We've learned that while chromium(II) and cobalt(II) complexes can rapidly exchange ligands, chromium(III) and cobalt(III) complexes are substitutionally inert. This property is vital for understanding why certain intermediates cannot be formed as the ligands are not readily swapped in inert complexes.

Reaction Mechanisms

A reaction mechanism is a step-by-step description of the pathway from reactants to products, offering an insight into which bonds break and form during the transition. The knowledge of a reaction mechanism allows chemists to predict the behavior of chemical species, as well as the rate and outcome of a reaction.
In the exercise we've examined, by identifying the products and hypothesized intermediates, we assess the feasibility of the proposed mechanism. Given the substitutional inertness of chromium(III) and cobalt(III) complexes, the proposed ligand exchange intermediate is improbable, suggesting that the actual mechanism does not involve this step. Understanding the appropriate mechanism is crucial not only for comprehension but also for predicting and controlling chemical reactions.

Coordination Chemistry

Coordination chemistry is the field of chemistry that focuses on compounds formed between metal ions and ligands. This field is rich with implications for both theoretical understanding and practical applications such as catalysis, material science, and medicine.
This area of chemistry involves principles such as coordination number, which denotes the number of attachments a central metal ion has to ligands. It also covers the geometries of metal complexes, the nature of the metal-ligand bond, and the dynamics of ligand-substitution reactions. The reaction presented in the exercise illustrates the relevance of coordination chemistry concepts, such as the reactivity of different oxidation states and ligand substitution behavior, in elucidating the mechanisms of complex chemical reactions.