Problem 6 - A Complete Resource Book in Chemistry for JEE Main

Question

In which of the following metal complex, does the central metal atom have zero Oxidation state? (a) \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{SO}_{4}\) (b) \(\mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]\) (c) \(\left[\mathrm{Ni}(\mathrm{CO})_{4}\right]\) (d) \(\left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right]\)

Step-by-Step Solution

Answer

The central metal atom has zero oxidation state in complex (c) \( \left[\mathrm{Ni}(\mathrm{CO})_{4}\right] \).

Step 1: Understand the problem

We need to determine in which metal complex the central metal atom has an oxidation state of zero. Oxidation state provides insight into the electron distribution in a chemical species.

Step 2: Analyze each complex

We will determine the oxidation state of the central metal in each complex as follows.

Step 3: Calculate oxidation state for (a)

In the complex \( \left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{SO}_{4} \), ammonia (\(\mathrm{NH}_{3}\)) is neutral with oxidation state 0, and sulfate (\(\mathrm{SO}_{4}^{2-}\)) has a -2 charge. Setting the sum of oxidation states to zero, copper oxidation state = +2.

Step 4: Calculate oxidation state for (b)

In the complex \( \mathrm{K}_{3}\left[\mathrm{Fe}(\mathrm{CN})_{6}\right] \), potassium (\(\mathrm{K}^+\)) has a +1 charge, and cyanide ion (\(\mathrm{CN}^-\)) has a -1 charge. The sum of charges outside is 3*+1 = +3. Thus, the oxidation state of iron (Fe) = +3.

Step 5: Calculate oxidation state for (c)

In the complex \( \left[\mathrm{Ni}(\mathrm{CO})_{4}\right] \), carbon monoxide (\(\mathrm{CO}\)) is neutral with an oxidation state of 0. Since there are four CO ligands and all are neutral, nickel must have an oxidation state of 0 to balance the charge to zero.

Step 6: Calculate oxidation state for (d)

In the complex \( \left[\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\right] \), ammonia is neutral at 0, and chloride (\(\mathrm{Cl}^-\)) has a -1 charge. The total negative charge from chlorine is -2, meaning the oxidation state of platinum (Pt) = +2.

Step 7: Conclusion

After calculating the oxidation states: (a) Cu is +2, (b) Fe is +3, (c) Ni is 0, and (d) Pt is +2. Therefore, the only complex with the metal in oxidation state 0 is (c) \( \left[\mathrm{Ni}(\mathrm{CO})_{4}\right] \).

Key Concepts

metal complexes

Metal complexes often consist of a central metal atom or ion surrounded by molecules or ions called ligands. These metal-ligand assemblies influence the properties and reactivity of a compound. The metal center can coordinate with ligands like water, ammonia, or other ions, forming a specific geometric structure.
The geometry of metal complexes depends primarily on the number and spatial arrangement of the ligands attached to the central metal. For instance:

• Linear, such as in \([\text{Ag}\text{(NH}_3\text{)}_2]\text{⁺}\)
• Octahedral, as observed in \([\text{CoF}_6\text{³⁻}]\)
• Square planar, typically seen in \([\text{PtCl}_4]^{2-}\)

These structural arrangements significantly affect a metal complex’s chemical activities and interaction with other compounds.

electron distribution

In metal complexes, the concept of electron distribution refers to how electrons are shared or distributed across the central metal atom and its ligands. This distribution is crucial as it affects the complex’s color, magnetism, and overall stability.
Electrons may reside in the d-orbitals of the central metal. The specific arrangement of these electrons can influence how ligands interact with the metal. Ligands can either be "electron-pair donors," contributing electrons to the empty orbitals of the metal, or alter the energy levels of these orbitals through "field effects."
Understanding electron distribution requires knowledge of the oxidation state, which hints at how electrons are arranged and helps predict the reactivity and properties of the complex.
The extent of electron sharing between metal and ligands is described by terms like "covalent" or "ionic," with covalent implying more shared and ionic indicating more transferred electrons.

ligand charge analysis

Ligand charge analysis involves evaluating the charges of the ligands attached to the central metal in a complex. This analysis helps to determine the oxidation state of the metal by calculating the total charge that results from the combination of metal and ligands.
In ligand charge analysis, each ligand's charge is carefully assigned, as not all ligands are neutral. For example, ammonia (NH₃) is neutral, while cyanide (CN⁻) carries a negative charge. Similarly:

• Sulfate (SO₄²⁻) can add a −2 charge to a complex.
• Carbon monoxide (CO) is neutral and does not affect the net charge.
• Chloride (Cl⁻) contributes a −1 charge from each atom within a complex.

Summing these charges alongside the overall charge of the complex allows chemists to calculate the metal's oxidation state. Accurate ligand charge analysis is vital to predict a complex’s behavior in chemical reactions, assisting students in understanding these fascinating compounds clearly and comprehensibly.