Problem 118 - A Complete Resource Book in Chemistry for JEE Main

Question

The proposed complex with the nomenclature chlorodiaquatriammine cobalt (III) chloride can be represented wrongly as (a) \(\left[\mathrm{CoCl}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}\right] \mathrm{Cl}_{2}\) (b) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{H}_{2} \mathrm{O}\right) \mathrm{Cl}_{3}\right]\) (c) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2} \mathrm{Cl}_{2}\right] \mathrm{Cl}\) (d) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{H}_{2} \mathrm{O}\right)_{3}\right] \mathrm{Cl}_{3}\)

Step-by-Step Solution

Answer

The wrong representation is (c).

Step 1: Understanding the Naming Convention

The complex is named chlorodiaquatriammine cobalt (III) chloride. This indicates the presence of: one chlorine (chloro) ligand, two water molecules (diaqua), three ammonia molecules (triammine), a cobalt ion in the +3 oxidation state (indicated by (III)), and it is chloride because it has excess chloride ions outside the coordination sphere.

Step 2: Constructing the Correct Formula

Based on the complex's name, the correct coordination complex should be: \[\left[\mathrm{Co} \left(\mathrm{NH}_{3}\right)_{3} \left(\mathrm{H}_{2}\mathrm{O}\right)_{2} \mathrm{Cl}\right] \] This involves the cobalt ion bound to three ammonia ligands, two water ligands, and one chloride ligand. Since the complex is stated to involve cobalt (III), its overall oxidation state should be +3.

Step 3: Balancing the Charge

The charge of the inner coordination sphere must balance with the charge of the anions outside the sphere. Here, the coordination sphere has an overall charge of +1 (cobalt is +3, minus two for the internal water and ammonia, netting +1 when considering all ligand charges summing to zero). To balance the overall charge to make it a neutral compound, one chloride ion outside the coordination complex sphere is necessary.

Step 4: Matching with the Given Options

We compare the correct composition and charge balance to the given options:- Option (a): \(\left[\mathrm{CoCl}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{H}_{2}\mathrm{O}\right)_{2}\right] \mathrm{Cl}_{2}\) is correct. It contains one internal Cl, balancing +3 charge with two external Cl ions.- Option (b) and (d) do not have the internal one Cl needed.- Option (c), \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{H}_{2}\mathrm{O}\right)_{2} \mathrm{Cl}_{2}\right] \mathrm{Cl}\), is incorrect because it has extra internal chlorine, leading to incorrect charge distribution.

Key Concepts

Naming Coordination Compounds

Naming coordination compounds can seem tricky, but it follows set conventions that make it systematic and predictable. When you encounter a name like "chlorodiaquatriammine cobalt (III) chloride," each part relays specific information:

• **Prefix**: "chloro" indicates the presence of a chloride ion within the complex.
• **Ligands**: "diaqua" signals two water molecules in the complex, and "triammine" means three ammonia molecules.
• **Metal and Oxidation State**: "cobalt (III)" specifies the metal involved and its oxidation state, which is +3 in this case.
• **Anions**: The term "chloride" at the end refers to chloride ions that may be present outside the coordination sphere.

Remember, the order starts with the ligands in alphabetical order, followed by the metal ion and its oxidation state. This systematic approach allows chemists to easily identify the components and structure of the compound from its name.

Oxidation States

Understanding oxidation states in coordination chemistry involves recognizing the charge distribution across the compound. Here, the "cobalt (III)" component is a vital clue. It informs us that cobalt has an oxidation state of +3. When constructing or analyzing a complex, consider how the charges balance:

• **Cobalt (III)**: Carries a charge of +3.
• **Ligands**: Water and ammonia are neutral ligands, meaning they add no additional charge to the complex. Chloride, however, is an anion with a -1 charge.
• **Total Charge**: Inside the coordination sphere, neutral ligands and one chloride contribute to an overall positive charge of +2. This results from the cobalt (+3) minus the chloride (-1).

Correctly balancing the internal and external ions ensures accurate representation of the compound's overall neutrality or charge.

Ligand Bonding

Ligand bonding in coordination chemistry refers to the way ligands attach to the central metal atom. They donate electron pairs to form coordinate bonds with the metal, stabilizing the complex.Types of ligands:

• **Neutral Ligands**: Such as ammonia \((\text{NH}_3)\) and water \((\text{H}_2\text{O})\) are neutral, contributing zero to the overall charge because they don't carry a charge themselves.
• **Anionic Ligands**: Like chloride \((\text{Cl}^-)\), which has a negative charge, impacting the compound’s net charge.

The way these ligands bind affects the geometry and properties of the coordination complex. The number of bonds typically reflects the complex's coordination number, where ligands provide the necessary electrons to satisfy the metal's electron deficiency.

Coordination Complex Formulas

Constructing coordination complex formulas requires a systematic approach to capturing both the composition and charge balance of the compound. Components of the formula:

• **Central metal ion**: Initiates the formula, noted here as Co for cobalt.
• **Ligands**: Follow the central ion, arranged in alphabetical order according to their names in the compound name.
• **Oxidation state representation**: Embedded within the compound's name, though not in the formula, guides the charge balance understanding.
• **Charge balancing**: Adjusts between the internal complex and external anions to achieve a neutral or specified charge. For example, one additional chloride outside balances the complex's internal +2 charge to create an overall neutral compound.

This careful composition ensures that chemists can derive both the structure and charge of the complex directly from its formula, mirroring its systematic name.