Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

The oxidation number of \(\mathrm{Fe}\) in \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}, \mathrm{Cr}\) in \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{NO}_{2}\right)_{3}\right]\) and \(\mathrm{Ni}\) in \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\) are respectively (a) \(0,+3,+2\) (b) \(+3,+3,0\) (c) \(+3,0,+3\) (d) \(+2,+3,0\)

Step-by-Step Solution

Answer

The correct answer is (d) \(+2, +3, 0\).

Step 1: Understanding the oxidation state of Iron in \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}\)

In \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}\), the cyanide ion \(\mathrm{CN}^{-}\) has a charge of \(-1\). There are 6 cyanide ions, contributing a total of \(-6\) charge. The overall charge of the compound is \(-4\). Thus, the charge on \(\mathrm{Fe}\) should be \(+2\) to balance the total charge: \(\mathrm{Fe} + (-6) = -4\), which means \(\mathrm{Fe} = +2\).

Step 2: Assessing the oxidation state of Chromium in \(\left[\mathrm{Cr}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{NO}_{2}\right)_{3}\right]\)

In this complex, ammonia \((\mathrm{NH}_{3})\) is neutral, contributing \(0\) to the oxidation number. The nitrite ion \(\mathrm{NO}_{2}^{-}\) has a charge of \(-1\). There are 3 nitrite ions, contributing \(-3\) charge. The entire complex is neutral, so the oxidation state of \(\mathrm{Cr}\) is \(+3\) to balance the charges: \(\mathrm{Cr} + 0 + (-3) = 0\), which makes \(\mathrm{Cr} = +3\).

Step 3: Determining the oxidation state of Nickel in \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\)

The carbon monoxide \((\mathrm{CO})\) is a neutral ligand, contributing \(0\) charge. With 4 \(\mathrm{CO}\) ligands, the total charge from ligands is \(0\). The entire complex is neutral, so the oxidation state of \(\mathrm{Ni}\) must be \(0\) to keep the complex neutral: \(\mathrm{Ni} + 0 = 0\), thus \(\mathrm{Ni} = 0\).

Step 4: Conclusion: Selecting the Correct Answer

Based on the calculations, the oxidation numbers for \(\mathrm{Fe}\), \(\mathrm{Cr}\), and \(\mathrm{Ni}\) are \(+2, +3, \text{and } 0\) respectively. Hence, the correct answer is option \((d)\) \(+2, +3, 0\).

Key Concepts

Transition Metals

Transition metals are located in the center of the periodic table. These metals include elements such as iron (Fe), chromium (Cr), and nickel (Ni). Understanding transition metals is essential, as they often form colorful compounds when mixed with other elements. They can also have various oxidation states, which refers to the number of electrons they can lose, gain, or share when forming bonds.

One of the defining characteristics of transition metals is their ability to change oxidation states. For instance, iron can exist in oxidation states of +2 or +3, depending on the compound it forms with other elements. This flexibility is responsible for many of the interesting chemical properties and reactions involving these metals.

Coordination Compounds

Coordination compounds consist of a central metal atom or ion surrounded by molecules or ions, known as ligands. These complexes have unique properties and structures that result from the specific interaction between the central metal and the surrounding ligands.

• The central metal atom in a coordination compound can be a transition metal, such as Fe, Cr, or Ni.
• Ligands can be molecules or ions like water, ammonia, or cyanide, that donate electrons to the metal center.
• Coordination compounds often exhibit vibrantly colored solutions, varying based on the metal and ligands.

Coordination compounds are crucial in fields like bioinorganic chemistry, where they mimic the metal centers found in enzymes and help to understand their functionality. The coordination number, which is the number of ligand attachments to the metal, is an essential aspect in defining these compounds.

Oxidation State Determination

Determining the oxidation state of a metal in coordination compounds is key to understanding their chemical nature and reactivity. The oxidation state is an indicator of the degree of oxidation of an atom within a compound.

• For any compound, the sum of the oxidation states of all atoms must equal the total charge of the compound. This allows you to solve for unknown oxidation states.
• The oxidation state of the central metal in a coordination compound can be determined by balancing the charges of the ligands and the overall charge of the complex.
• Neutral ligands such as carbon monoxide (CO) are assigned a charge of 0, affecting only the charges due to the metal itself when in a complex.

Accurate determination of oxidation states aids in predicting how a compound might react under different conditions or how stable it might be.

Ligand Charge

Ligands are the atoms, molecules, or ions that can donate one or more pairs of electrons to a central metal atom to form a coordination compound. The charge of the ligand can greatly influence the oxidation state of the metal within the compound.

• Ligands like cyanide (CN^-) carry a negative charge, which must be considered when calculating the oxidation state of the metal.
• Neutral ligands, such as ammonia (NH₃), do not affect the oxidation state directly because their charge contribution is zero.
• Identifying and understanding the charge of each ligand helps in balancing the overall charge of the compound.

Consideration of ligand charge is crucial in coordination chemistry, as it affects the electron count and overall geometry of a coordination complex.