Problem 55 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following is true? (a) The charge on the complex ion in \(\mathrm{K}_{2}\left[\mathrm{PtCl}_{4}\right]\) is \(+2\) (b) The coordination number of \(\mathrm{Hg}\) in \(\left[\mathrm{Hg}(\mathrm{en})_{2}\right]^{2+}\) is 4 (here en is ethylenediamine) (c) The oxidation number of iron in \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4}\) is \(+6\). (d) In transition metal complexes, ligands act as lewis acid

Step-by-Step Solution

Answer

Option (b) is true.

Step 1: Analyze Option (a)

First, let's examine the complex ion in \( \mathrm{K}_{2}\left[\mathrm{PtCl}_{4}\right] \). Here, the counterion is potassium \(\mathrm{K}^+\), which has a charge of \(+1\). Since there are two potassium ions, they contribute a total charge of \(+2\). To balance this, the charge on the complex ion \([\mathrm{PtCl}_{4}]\) must be \(-2\). Thus, Option (a) "The charge on the complex ion in \(\mathrm{K}_{2}\left[\mathrm{PtCl}_{4}\right]\) is \(+2\)" is incorrect.

Step 2: Analyze Option (b)

In the complex ion \(\left[\mathrm{Hg}(\mathrm{en})_{2}\right]^{2+} \), ethylenediamine (\(\mathrm{en}\)) is a bidentate ligand, meaning each en can coordinate with two sites on the metal. As there are two en ligands, the coordination number of \(\mathrm{Hg}\) is \(2 \times 2 = 4\). Thus, Option (b) "The coordination number of \(\mathrm{Hg}\) in \(\left[\mathrm{Hg}(\mathrm{en})_{2}\right]^{2+}\) is 4" is correct.

Step 3: Analyze Option (c)

Consider the complex \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-} \). Cyanide \((\mathrm{CN}^-)\) is a monodentate ligand with a charge of \(-1\). Six cyanides contribute \(-6\) charge in total. Therefore, to achieve the overall charge of \(-4\), the oxidation state of iron must be \(+2\) (since \(+2 - 6 = -4\)). Thus, Option (c) "The oxidation number of iron in \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}\) is \(+6\)" is incorrect.

Step 4: Analyze Option (d)

In transition metal complexes, ligands donate electron pairs to the metal center and hence act as Lewis bases (not acids). Therefore, Option (d) "In transition metal complexes, ligands act as Lewis acid" is incorrect.

Key Concepts

Complex Ion Charge Calculation

The calculation of the charge on a complex ion involves understanding the charges of the atoms or molecules that form the complex. Let's take the example of the complex \[\text{K}_{2}[\text{PtCl}_{4}]\] to illustrate this process. In this complex, potassium \((\text{K}^+)\) serves as the counterion. Each potassium ion has a charge of \(+1\). Since there are two potassium ions connected to the complex,

• The combined charge contributed by potassium is \(+2\).
• The charge on the complex ion itself \([\text{PtCl}_{4}]\) must balance the entire structure.
• Thus, the complex ion must have a charge of \(-2\) to counteract the \(+2\) from the potassium ions.

This balancing act is crucial to understanding how complex ions acquire their charges, helping you deduce the nature and interaction of the constituents within the complex.

Coordination Number

The coordination number in a coordination complex refers to the number of ligand donor atoms that are bonded directly to the central metal ion. In the complex \([\text{Hg}(\text{en})_{2}]^{2+}\), ethylenediamine \((\text{en})\) is the ligand involved here. Ethylenediamine is known as a bidentate ligand. This means each \(\text{en}\) can attach itself to two coordination sites on the mercury.To determine the coordination number of \(\text{Hg}\) in this scenario:

• Consider that each bidentate ligand uses two coordination sites.
• There are two ethylenediamine ligands.
• This results in a coordination number of \(2 \times 2 = 4\) for \(\text{Hg}\).

The coordination number plays an essential role in dictating the geometry and stability of the complex, influencing things like reactivity and how the compound is used in various applications.

Oxidation State Determination

Determining the oxidation state in a coordination complex is a vital skill that helps predict the behavior and properties of the complex. Consider \([\text{Fe}(\text{CN})_{6}]^{4-}\) as an example. Here, cyanide \((\text{CN}^-)\) is a monodentate ligand, contributing a charge of \(-1\) per cyanide ion.To calculate the oxidation state of iron \(\text{Fe}\):

• Recognize that six cyanide ions contribute a total charge of \(-6\) (\(6 \times -1 = -6\)).
• The overall charge of the complex is \(-4\).
• For the complex to have an overall charge of \(-4\), iron must have an oxidation state of \(+2\) to balance the charges: \(+2 - 6 = -4\).

Understanding oxidation states is essential for interpreting chemical reactions and electron transfer processes, providing insight into the reactivity and potential applications of coordination complexes.