Problem 4 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The oxidation states of \(\mathrm{Cr}\) in \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{Cl}_{3}, \quad\left[\mathrm{Cr}\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)_{2}\right]\), and \(\mathrm{K}_{2}\left[\mathrm{Cr}(\mathrm{CN})_{2}(\mathrm{O})_{2}\left(\mathrm{O}_{2}\right)\left(\mathrm{NH}_{3}\right)\right]\) respectively are : [Main 2018] (a) \(+3,+4\), and \(+6\) (b) \(+3\), \(+2\), and \(+4\) (c) \(+3,0\), and \(+6\) (d) \(+3,0\), and \(+4\)

Step-by-Step Solution

Answer

The oxidation states of \(\mathrm{Cr}\) are \(+3, 0,\) and \(+6\).

Step 1: Identify the Oxidation State in \(\left[\mathrm{Cr}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]\mathrm{Cl}_{3}\)

In this compound, the counter ions are 3 \(\mathrm{Cl}^-\) ions. To balance the charge of \(3\times (-1) = -3\) from \(\mathrm{Cl}^+\), \(\mathrm{Cr}\) must have a charge of \(+3\). Thus, the oxidation state of \(\mathrm{Cr}\) is \(+3\).

Step 2: Identify the Oxidation State in \(\left[\mathrm{Cr}\left(\mathrm{C}_{6} \mathrm{H}_{6}\right)_{2}\right]\)

\(\mathrm{C}_{6} \mathrm{H}_{6}\) (benzene) is a neutral ligand. Since there are no counter ions, the complex is neutral, which implies the oxidation state of \(\mathrm{Cr}\) is \(0\).

Step 3: Identify the Oxidation State in \(\mathrm{K}_{2}\left[\mathrm{Cr}(\mathrm{CN})_{2}(\mathrm{O})_{2}\left(\mathrm{O}_{2}\right)\left(\mathrm{NH}_{3}\right)\right]\)

Potassium (\(\mathrm{K}^+\)) contributes \(+2\) to the compound charge. Consider each ligand: \(\mathrm{CN}^-\) has \(-1\), \(\mathrm{O}^{2-}\) has \(-2\), and \(\mathrm{NH}_3\) is neutral. For \(\mathrm{O}_2\) (superoxo), assume \(-1\). The oxidation state of \(\mathrm{Cr}\) must balance the overall charge. Thus, \(x + 2(-1) + 2(-2) + (-1) + 0 = -2\), giving \(x = +6\).

Key Concepts

Transition Metals

Transition metals, like chromium ( Cr ), possess unique properties that make them fascinating in the study of chemistry. These metals are part of the d-block of the periodic table, which gives rise to their characteristic properties. One key feature of transition metals is their ability to exhibit a range of oxidation states. This versatile behavior is due to their electron configuration, which allows electrons to be lost from both the (s) and (d) orbitals.

• They often form colored compounds, thanks to the d-d transitions of their electrons.
• Transition metals are known to form complex ions with various ligands, leading to the creation of coordination compounds.
• They often exhibit catalytic properties, aiding numerous industrial chemical reactions.

Chromium, in particular, can exist in multiple oxidation states such as +2, +3, +6 among others, which contribute to its distinct chemical behavior and interactions in different compounds. Understanding these properties provides insight into how transition metals influence numerous chemical processes.

Coordination Compounds

Coordination compounds consist of a central metal atom or ion bonded to a surrounding array of molecules or ions, known as ligands. Chromium's ability to form such compounds is a testament to the transition metals' prowess in chemistry. The ligands donate electron pairs to the metal, forming coordinate covalent bonds. This interaction leads to distinct geometries and properties of the resulting complex.

• The central metal ion and its ligands create a coordination sphere.
• Ligands can be neutral or charged; they determine the overall charge of the complex.
• Coordination numbers, which indicate the number of ligand atoms directly bonded to the metal, play a crucial role in defining the structure and reactivity of these compounds.

The study of coordination chemistry not only reveals the intricacies of metal-ligand interactions but also enhances our understanding of catalysis and biological systems where such compounds are pivotal.

Oxidation State Determination

Determining the oxidation state of a metal in a compound is crucial for understanding its electronic structure and chemical behavior. This analysis involves evaluating the charge balance within coordination compounds. Let's delve into the determination process as exemplified by the given chromium compounds:
1. **\([\mathrm{Cr}(H_2O)_6]Cl_3\) Complex:**
Water (H2O) is neutral, while chloride (Cl^-) is negatively charged. With three chloride ions, the total negative charge is −3. To balance this, chromium must be +3, making the complex neutral.
2. **\([\mathrm{Cr}(C_6H_6)_2]\) Compound:**
Benzene (C6H6) is also neutral. Since there are no counter ions, the entire complex is neutral, giving chromium an oxidation state of 0.
3. **\(K_2[\mathrm{Cr}(CN)_2(O)_2(O_2)(NH_3)]\) Compound:**
Potassium (K^+) adds a +2 charge. The sum of negative charges from CN^- and O^{2-} is −6. A superoxo group (O2), typically recognized as −1, leads to a total of −7. Balancing the equation, chromium must have an oxidation state of +6 to neutralize charges.
By applying these principles, oxidation states are methodically concluded. This informs insights into metal behavior in complexes, facilitating predictions on reactivity and functionality.