Question

The IUPAC name for the complex \(\left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)\left(\mathrm{NH}_{3}\right)_{5}\right]\) \(\mathrm{Cl}_{2}\) is [2006] (a) nitrite-N-pentaamminecobalt(III) chloride (b) nitrite-N-pentaamminecobalt(II) chloride (c) pentaamminonitrite-N-cobalt(II) chloride (d) pentaamminonitrite-N-cobalt(III) chloride

Short answer

The IUPAC name is (d) pentaamminonitrite-N-cobalt(III) chloride.

Step-by-step solution

Identify the Ligands

In the complex \( \left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)\left(\mathrm{NH}_{3}\right)_{5}\right] \), there are two ligands: \( \mathrm{NO}_2^- \), which is named as 'nitrite' when \( N \) is the donor atom, and \( \mathrm{NH}_3 \), which is named as 'ammine'.

Determine the Oxidation State of the Metal

The complex \( \left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)\left(\mathrm{NH}_{3}\right)_{5}\right] \) is neutral. Since \( \mathrm{NH}_3 \) is neutral, we only need to consider \( \mathrm{NO}_2^- \), which has a -1 charge, while \( \mathrm{Cl}_2 \) outside has a -2 charge, making the complex cationic. The oxidation state of cobalt must be +3 to balance these charges and make the entire compound neutral.

Order of Naming

According to the IUPAC nomenclature rules, the ligands in a complex are named in alphabetical order before the metal. Here, 'ammine' comes before 'nitrite'. Therefore, the name starts with 'pentaamm ine-', indicating five ammine groups.

Use the Correct Naming for Ligands and Specify Donor Atom

Ligands such as \( \mathrm{NO}_2^- \) that can bind through different atoms must specify the donor atom, which here is \( N \). Thus, \( \mathrm{NO}_2^- \) is named as 'nitrite-N'.

Name the Central Metal and Counter Ions

The central metal is cobalt with an oxidation state of +3, so it is written as 'cobalt(III)'. The counter ions \( \mathrm{Cl}_2 \) are written last as 'chloride'.

Combine the Names to Formulate the IUPAC Name

Combining all the elements, the IUPAC name becomes 'pentaamminonitrite-N-cobalt(III) chloride', corresponding to option (d).

Key concepts

Coordination Chemistry

Coordination chemistry revolves around the study of complex compounds, which consist of a central metal atom or ion surrounded by molecules or ions called ligands. These ligands form coordinate bonds with the metal, creating a stable structure known as a coordination complex. Understanding coordination chemistry involves dissecting the interactions and bonding between the metal and ligands in these complexes.
Coordination complexes can vary widely in their structure, properties, and reactivity depending on factors like the nature and number of ligands, the oxidation state of the metal, and the overall geometry of the complex.
In the context of the provided exercise, a cobalt ion structured with nitrite and ammine ligands highlights the importance of learning how the coordination sphere is formed and its significance in chemical nomenclature.

Oxidation States

Determining the oxidation state of the central metal in a complex is crucial for naming it correctly. The oxidation state refers to the charge that an atom would have if all bonds were ionic. In practice, it helps to understand electron distribution in a complex compound. When dealing with complex compounds, identifying the oxidation state involves considering the charges of the ligands and any additional anionic or cationic parts surrounding the complex.
In our example, the cobalt complex \( \left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)\left(\mathrm{NH}_{3}\right)_{5}\right] \) is presented with ammine (neutral) and nitrite ligands. Given the negative charge of the nitrite, finding the oxidation state of cobalt requires balancing these charges to ensure the compound remains neutral, which results in an oxidation state of +3 for cobalt. Understanding this process helps in predicting the geometry and reactivity of the complex.

Ligand Nomenclature

The naming of ligands is a core aspect of IUPAC naming rules, reflecting their role in the coordination complex. Ligands can be charged or neutral and they often determine the compound's properties. Each ligand has a specific name, which may change depending on how it binds to the metal and whether it utilizes different donor atoms.
For example, nitrite \( (\mathrm{NO}_2^-) \) can bind through the oxygen or the nitrogen. In this instance, it's binding through nitrogen, so it’s referred to as 'nitrite-N'. Ammine represents \(\mathrm{NH}_3\), a neutral ligand.
When naming complexes, ligands are arranged alphabetically, influencing the sequence in which they appear within the compound name. This alphabetical order combined with proliferation of correct prefixes for number of ligands, like 'penta' for five ammines in the provided example, determines their placement within the full IUPAC name.

Complex Compounds

Complex compounds consist of a central metal atom or ion and surrounding ligands, which can significantly influence the chemical's properties. These compounds, also known as coordination complexes, exhibit unique behaviors and uses in various fields like catalysis, material science, and medicine.
The example provided in the exercise features cobalt as the central metal ion surrounded by nitrite and ammine ligands. Such compounds can form different geometries, like octahedral or tetrahedral, which can affect their stability and reactivities.
Understanding how to properly assemble and name these compounds using IUPAC guidelines, as depicted in the exercise, is key to mastering their complexities and predicting potential applications or reactions they might undergo. Correct IUPAC naming not only helps in clear scientific communication but also in predicting compound behavior.