Problem 163 - A Complete Resource Book in Chemistry for JEE Main

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

Step-by-Step Solution

Answer

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

Step 1: Identify the Ligands and Central Metal

The coordination complex is \( \left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)\left(\mathrm{NH}_{3}\right)_{5}\right] \). Here, the ligands are \( \text{NH}_3 \) (ammine) and \( \text{NO}_2^- \) (nitrite), and the central metal is cobalt (\( \text{Co} \)).

Step 2: Determine Ligand Order

When naming complex ions, ligands are named in alphabetical order using their IUPAC names. Here, 'ammine' (NH3) comes before 'nitrite', so the order is 'ammine' and then 'nitrite'.

Step 3: Identify the Denticity of Ligands

\( \text{NH}_3 \) is a monodentate ligand (binds through one atom), while \( \text{NO}_2^- \) can bind in two ways (as nitrito-N or nitrito-O), but here is specified as nitrito-N since it's named as 'nitrite-N'.

Step 4: Assign Oxidation State to the Metal

The metal cobalt must balance the charges. The compound is neutral overall and consists of \( \left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)\left(\mathrm{NH}_{3}\right)_{5}\right]^{2+} \) and two chloride ions (2Cl^-). Thus, the charge on \( \left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)\left(\mathrm{NH}_{3}\right)_{5}\right] \) is +3, leading to cobalt in the +3 oxidation state.

Step 5: Assemble the Complex Name

Consider the coordination sphere \( \left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)\left(\mathrm{NH}_{3}\right)_{5}\right] \) and use the order of ligands determined. Add the oxidation state using Roman numerals. The full name is 'pentaammine nitrite-N cobalt(III) chloride'.

Step 6: Translate to Standard Options

Match the correctly derived name with the provided options. The correct answer is (d) pentaamminonitrite-N-cobalt(III) chloride.

Key Concepts

Coordination Complex

A coordination complex is a structure consisting of a central metal atom or ion bonded to surrounding molecules or anions, referred to as ligands. These ligands can donate electron pairs to the metal, forming these specific bonds known as coordinate covalent or dative bonds. Coordination complexes are crucial in various chemical processes and often have fascinating colors due to the electronic transitions in the d-orbitals of the central metal. In the exercise, our coordination complex is \( [\text{Co}(\text{NO}_2)(\text{NH}_3)_5] \), with \( \text{Co} \) as the central metal. Exploring coordination complexes helps in understanding diverse applications, from catalysis in industrial processes to biological systems.

Oxidation State

The oxidation state of an element in a compound indicates the degree of oxidation or reduction. It represents the number of electrons lost or gained by an atom of that element in the compound. For coordination complexes, the oxidation state is crucial to determine the electronic configuration and reactivity of the metal center.
In our given exercise, cobalt (\( \text{Co} \)) requires careful analysis regarding its oxidation state. The ligand \( \text{NO}_2^- \) is negatively charged, and \( \text{NH}_3 \) is neutral. The overall charge of the coordination sphere \( [\text{Co}(\text{NO}_2)(\text{NH}_3)_5] \) is \(+2\) due to two \( \text{Cl}^- \) ions balancing the charge. This leads to cobalt having an oxidation state of \(+3\).
Understanding oxidation states directly influences the IUPAC naming and helps in anticipating the behavior of the metal within chemical reactions.

Ligands in Chemistry

Ligands play a significant role in the chemistry of coordination complexes. They are atoms, ions, or molecules that donate a pair of electrons to the metal center to form the complex.

• Monodentate ligands like \( \text{NH}_3 \) attach through a single donor atom.
• Ligands such as \( \text{NO}_2^- \) can be ambidentate, meaning they can bind through different atoms, in this case, either 'N' or 'O'. In our examples, \( \text{NO}_2^- \) binds through the nitrogen, hence the denotation 'nitrito-N.'

Ligand behavior influences the geometry, stability, and reactivity of the complex. Being aware of the ligand types assists in predicting how the coordination complex will react in different environments.

Chemical Naming Conventions

The standardized IUPAC naming conventions play a critical role in ensuring clear communication among chemists worldwide. For coordination complexes, these conventions provide specific guidelines for naming the chemical species involved:

• Ligands are named before the central metal in alphabetical order. For example, in the exercise solution, 'ammine' (\( \text{NH}_3 \)) precedes 'nitrite' (\( \text{NO}_2^- \)).
• Suffixes and prefixes indicate the type and number of ligands ('pentaammine' for five \( \text{NH}_3 \) ligands).
• The oxidation state of the metal is specified in Roman numerals after the metal's name, such as cobalt(III) chloride.

Correct use of these conventions allows scientists from different disciplines to understand the specific composition and structure of complexes efficiently. These rules also help distinguish between very similar chemical entities.