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

Question

The IUPAC name for the complex \(\left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)\left(\mathrm{NH}_{3}\right)_{3}\right]\) \(\mathrm{Cl}_{2}\) is (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 (a) nitrite-N-pentaamminecobalt(III) chloride.

Step 1: Identify the complex ion and ligands

The complex is \(\left[\mathrm{Co}\left(\mathrm{NO}_{2}\right)\left(\mathrm{NH}_{3}\right)_{3}\right]\). It contains cobalt (Co) as the central metal ion, \(\mathrm{NO}_{2}^-\) as the nitrite ion, and three \(\mathrm{NH}_{3}\) (ammonia) molecules as ligands.

Step 2: Determine the charge of the complex

Cobalt is the central metal ion. Given that nitrite (\(\mathrm{NO}_{2}^-\)) has a charge of -1, and ammonia (\(\mathrm{NH}_{3}\)) has no charge, the complex must have a charge of +3 to balance the two negative chloride ions (\(\mathrm{Cl}_{2}\)). Therefore, cobalt must have an oxidation state of +3.

Step 3: Determine the naming order of ligands

According to IUPAC naming, ligands are listed alphabetically. Here, ammonia (\(\mathrm{NH}_{3}\)) is named 'ammine', and nitrite (\(\mathrm{NO}_{2}^-\)) is specified by the prefix 'nitrito-N' to indicate its bonding through the nitrogen atom.

Step 4: Name the entire complex

List the ligands alphabetically, followed by the metal. The structure is named nitrito-N-triaminecobalt(III) to indicate the oxidation state of cobalt (+3), finishing with 'chloride' for the counter ion \(\mathrm{Cl}_{2}\).

Step 5: Match with the given options

Comparing with the given options, choice (a) 'nitrite-N-pentaamminecobalt(III) chloride' matches the structuring of Step 4, as it correctly denotes each ligand, the metal's oxidation state, and the chloride counter ion.

Key Concepts

Coordination Complexes

When studying coordination complexes, it's essential to understand that these structures consist of a central metal atom or ion, surrounded by surrounding molecules or ions known as ligands. Ligands are capable of donating at least one pair of electrons to the metal, forming what is called a coordination bond.
For instance, in the complex \(\left[ \mathrm{Co} \left( \mathrm{NO}_{2} \right) \left( \mathrm{NH}_{3} \right)_{3} \right]\), cobalt (Co) stands as the central metal ion, while \(\mathrm{NO}_{2}^{-}\) and \(\mathrm{NH}_{3}\) serve as ligands, surrounding the central ion.

• The metal ion acts as a Lewis acid (electron pair acceptor).
• Ligands are Lewis bases (electron pair donors).

Such complexes bear significance in various chemical reactions and industrial applications, including catalysis and material synthesis.
As these complexes are neutral due to surrounding counter ions like chlorides, they can participate in reactions effectively.

Oxidation States

Understanding oxidation states within coordination complexes is crucial for naming and understanding their chemical behavior. The oxidation state indicates the charge of the central metal atom, which influences the complex's overall charge.
In our example, the cobalt ion has an oxidation state of +3, balanced by surrounding ligands and ions. Given the negative charge on the nitrite ion \(\mathrm{NO}_{2}^{-}\) and neutral charge from ammonia \(\mathrm{NH}_{3}\), the cobalt's +3 state neutralizes the complex with the chloride ions in \(\mathrm{Cl}_{2}\), leaving it neutral.

• Oxidation state helps Determines the electron configuration of the metal ion.
• It influences properties such as color and magnetism of the complex.

Hence, correctly determining the oxidation state is imperative in the accurate description and naming of coordination complexes.

Naming Ligands

Naming ligands correctly is a cornerstone of IUPAC nomenclature for coordination complexes. It involves identifying and listing ligands alphabetically before the central metal in the name, and often using specific nomenclature rules for certain ligands.
For example, in the complex \(\left[ \mathrm{Co} \left( \mathrm{NO}_{2} \right) \left( \mathrm{NH}_{3} \right)_{3} \right]\), ligands include ammonia \(\mathrm{NH}_{3}\), which is termed 'ammine', and nitrite \(\mathrm{NO}_{2}^{-}\), referred to by 'nitrito-N', denoting bonding through the nitrogen.

• Ligands such as NH₃ are named as 'ammine'.
• Nitrite’s notation varies depending on its point of attachment (oxygen or nitrogen), specified as 'nitrito-N'.

Proper ligand naming conveys how atoms within the ligand are bonded to the central metal, ensuring clarity and consistency in chemical communication.

Chemical Bonding in Complexes

Chemical bonding in complexes involves specialized interactions where ligands donate their lone pairs to the metal centre, forming coordinate covalent bonds. This interaction leads to a stable formation with characteristic geometries and chemical properties.
In coordination complexes like \(\left[ \mathrm{Co} \left( \mathrm{NO}_{2} \right) \left( \mathrm{NH}_{3} \right)_{3} \right]\), the metal-ligand interaction resembles a dative bond, where both electrons in the bond originate from the ligand.

• Ligands attach via dative bonds, providing pairing electrons to the empty orbitals of the metal.
• Complex stability arises from this sharing and redistribution of electrons.

These interactions underpin the reactivity and utility of coordination complexes in various fields such as catalysis, medicinal chemistry, and material sciences, by altering electron density and the chemical environment around the metal.