Question

When gallium is dissolved in a solution of KOH in liquid \(\mathrm{NH}_{3},\) a salt \(\mathrm{K}[\mathrm{I}]\) is formed which is an amido complex of Ga(III). Heating one equivalent of \(\mathrm{K}[\mathrm{I}]\) at \(570 \mathrm{K}\) under vacuum liberates two equivalents of \(\mathrm{NH}_{3}\) and produces a Ga(III) imido complex K[II]. Partial neutralization of \(\mathrm{K}[\mathrm{I}]\) with \(\mathrm{NH}_{4} \mathrm{Cl}\) yields \(\mathrm{Ga}\left(\mathrm{NH}_{2}\right)_{3}\) Suggest identities for the salts \(\mathrm{K}[\mathrm{I}]\) and \(\mathrm{K}[\mathrm{II}],\) and write equations for the thermal decomposition and partial neutralization reactions of \(\mathrm{K}[\mathrm{I}] .\) Hint: an imido complex formally contains \(\mathrm{NH}^{2-}\)

Short answer

K[I] is \(K_3[\text{Ga(NH}_2\text{)}_6]\); K[II] is \(K[\text{Ga(NH)}_3]\).

Step-by-step solution

Analyze the given information

We know that the salt \(K[I]\) is an amido complex of Ga(III), which suggests it contains amide ions \((\text{NH}_2^-).\) On heating \(K[I]\) at 570 K, two equivalents of \(\text{NH}_3\) are released, forming \(K[II].\) This indicates \(K[II]\) could be an imido complex, containing \(\text{NH}^{2-}.\) The partial neutralization with \(\text{NH}_4\text{Cl}\) yields \(\text{Ga(NH}_2\text{)}_3.\)

Identify salt K[I]

Based on the dissolution of gallium in KOH and liquid ammonia to form an amido complex, the salt \(K[I]\) can be identified as \(K_3[\text{Ga(NH}_2\text{)}_6].\) This fits the description as an amido complex of Ga(III).

Identify salt K[II]

During the decomposition of \(K[I],\) two equivalents of \(\text{NH}_3\) are liberated, indicating a reduction in the number of \(\text{NH}_2^-\) ions, forming \(K[II].\) Therefore, \(K[II]\) should be identified as \(K[\text{Ga(NH)}_3].\)

Write the decomposition reaction

For the thermal decomposition, the reaction can be written as: \[K_3[\text{Ga(NH}_2\text{)}_6] \rightarrow K[\text{Ga(NH)}_3] + 2 \text{NH}_3 + 2 K\text{NH}_2.\]

Write the neutralization reaction

Using the hint that \(K[I]\) can react with \(\text{NH}_4\text{Cl}\), the partial neutralization equation to form \(\text{Ga(NH}_2\text{)}_3\) is: \[K_3[\text{Ga(NH}_2\text{)}_6] + 3\text{NH}_4\text{Cl} \rightarrow \text{Ga(NH}_2\text{)}_3 + 3 K\text{Cl} + 3\text{NH}_3.\]

Key concepts

Gallium Complexes

Gallium, represented by the symbol Ga, is known for its interesting chemistry, particularly in forming complexes with various ligands. A gallium complex is a compound wherein gallium atoms are coordinated with other atoms or groups of atoms, known as ligands. These complexes can show a variety of chemical behaviors, making them important in inorganic chemistry.

In the context of the exercise, we encounter two distinctive gallium complexes: the amido complex and the imido complex. The amido complex, represented as

• \(K_3[ ext{Ga(NH}_2)_6]\)

is formed when gallium is treated in a solution of KOH and liquid ammonia. This complex features the Ga(III) ion coordinated with amide ions, \( ext{NH}_2^-\). This coordination is significant because it determines the chemical properties and reactions of the gallium complex.

When this amido complex undergoes thermal decomposition, it transforms into a new complex, referred to as the imido complex:

• \(K[ ext{Ga(NH)}_3]\)

This transformation highlights a key aspect of gallium chemistry—it often involves changes in the oxidation state and coordination environment, affecting reactivity.

Amido Complexes

Amido complexes are a fascinating class of compounds in inorganic chemistry. They contain metal ions covalently bonded to amide ions, \( ext{NH}_2^-\). These anions provide their lone pair electrons to form a stable coordination bond with the metal, in this case, gallium.

The amido complex \(K_3[ ext{Ga(NH}_2)_6]\) plays a central role in our exercise. It is described as a stable compound formed by dissolving gallium in a mixture of KOH and liquid ammonia. The amide ions here act as ligands, surrounding the gallium ion and stabilizing it in the complex.

When this amido complex is heated, it undergoes a chemical change to form an imido complex, releasing ammonia gas \( ext{NH}_3\) in the process. This reaction underscores a typical property of amido complexes – they can act as precursors to other, often quite different, chemical species through thermal or other forms of decomposition. Thus, understanding these complexes helps us predict and control chemical reactivity in various applications, including synthesis and materials science.

Thermal Decomposition Reactions

Thermal decomposition is a process where a chemical compound breaks down into simpler compounds or elements when heated. This is particularly relevant in the study of gallium complexes, where heat induces significant changes in their structure.

In the case of our gallium amido complex \(K_3[ ext{Ga(NH}_2)_6]\), heating at 570 K leads to a decomposition reaction that liberates two equivalents of \( ext{NH}_3\), resulting in an imido complex \(K[ ext{Ga(NH)}_3]\). This reaction can be written as:

• \[K_3[ ext{Ga(NH}_2)_6] \rightarrow K[ ext{Ga(NH)}_3] + 2 \text{NH}_3 + 2 K\text{NH}_2.\]

The release of ammonia signifies a change in the coordination environment and signifies the completion of the decomposition reaction. Such reactions are crucial in complex chemistry, as they offer a route to different products from a single precursor compound.

Understanding thermal decomposition not only helps comprehend these chemical changes but also enables chemists to employ such transformations in practical applications. This includes the creation of new materials, catalysis, or even recycling of metal complexes through controlled thermal processes.