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

(a) is correct; (b), (c), and (d) are incorrect representations.

Step 1: Identify the name components

The name 'chlorodiaquatriammine cobalt (III) chloride' implies a complex ion structure where 'chloro' represents one Cl ligand, 'di-aqua' means two water molecules (\(\text{H}_2\text{O}\)), and 'tri-ammine' represents three ammonia molecules (\(\text{NH}_3\)). 'Cobalt (III)' indicates that cobalt is in the +3 oxidation state. 'Chloride' at the end suggests that the complex has one or more Cl ions outside of the coordination sphere.

Step 2: Assemble the correct formula

Based on the nomenclature, the complex should have cobalt coordinated with one Cl, two water molecules, and three ammonia molecules, written as: \[ [\text{CoCl}(\text{NH}_3)_3(\text{H}_2\text{O})_2] \]. The presence of 'chloride' at the end of the name implies one chloride ion outside the brackets, so the complete compound should be: \[ [\text{CoCl}(\text{NH}_3)_3(\text{H}_2\text{O})_2] \text{Cl}_2 \].

Step 3: Compare options with the correct formula

Now, compare each given option to the correct formula:- (a) \([\text{CoCl}(\text{NH}_3)_3(\text{H}_2\text{O})_2]\text{Cl}_2\) matches perfectly.- (b) \([\text{Co}(\text{NH}_3)_3(\text{H}_2\text{O})\text{Cl}_3]\) does not have the correct number or distribution of ligands or external chloride ions.- (c) \([\text{Co}(\text{NH}_3)_3(\text{H}_2\text{O})_2 \text{Cl}_2]\text{Cl}\) has two chloride ions inside the brackets which does not match the nomenclature.- (d) \([\text{Co}(\text{NH}_3)_3(\text{H}_2\text{O})_3]\text{Cl}_3\) has the wrong number of water molecules.Option (b), (c), and (d) are incorrect.

Key Concepts

Nomenclature of Coordination Compounds

Understanding the method for naming coordination compounds is crucial for deciphering their structure and composition. In coordination chemistry, a systematic approach is used to name compounds based on specific rules outlined by IUPAC (International Union of Pure and Applied Chemistry).

Names are constructed by describing the ligands, then the metal, and finally the oxidation state of the metal in Roman numerals. Ligands are named in alphabetical order without considering prefixes like di-, tri-, which denote their numbers. For example, in the name "chlorodiaquatriammine cobalt (III) chloride":

• Chloro indicates a single chlorine ligand bound to the cobalt center.
• Di-aqua shows two water molecules are bonded.
• Tri-ammine indicates the presence of three ammonia molecules.
• Cobalt (III) implies that cobalt is at an oxidation state of +3.
• Chloride outside the brackets indicates a counterion to balance the charge of the complex.

These naming conventions allow chemists to recreate the structure of a given coordination compound just from its name.

Coordination Chemistry

Coordination chemistry involves the study of compounds that have a central metal atom or ion surrounded by molecules or anions called ligands. These ligands "coordinate" with the metal, forming coordination complexes through coordinate covalent bonds.

The metal's role is critical as it is typically a transition metal, which can accommodate numerous ligands depending on its electron configuration and oxidation state. Ligands could be water, ammonia, chloride ions, etc. They modify the properties and reactivity of the central metal ion.

Applications of coordination compounds are extensive:

• They play a significant role in biological systems; for example, hemoglobin in blood is a coordination compound.
• Coordination complexes are applied in catalysis and industrial processes as catalysts.
• They are used in analytical chemistry for detecting metal ions.

Understanding coordination chemistry allows one to grasp the behavior of these complexes in different chemical environments and applications.

Oxidation States

Oxidation states, also known as oxidation numbers, indicate the degree of oxidation of an atom within a compound. They help in understanding the electron distribution between atoms in a complex.

For coordination compounds, oxidation states are pivotal in deducing the charge of the complex, predicting reactivity, and constructing these compounds correctly. Marked in Roman numerals next to the metal name, they reveal the charge the metal ion carries.

In our example, 'Cobalt (III)' specifies an oxidation state of +3 for cobalt. This means cobalt can donate or share electrons with its ligands to achieve a stable electron configuration. The charge on the complex can be calculated by:

• Summing up the charges of the central metal ion and all ligands within the coordination sphere.
• The total charge aids in determining how many counterions like chloride are required to balance the compound to a neutral state.

Understanding oxidation states provides essential insights into the redox behavior and stability of coordination compounds.