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

(b) The coordination number of Hg in [Hg(en)_2]^{2+} is 4.

Step 1: Analyze Option (a)

Examine the complex ion \ \(\mathrm{K}_2[\mathrm{PtCl}_4]\) \ and determine the charge on the complex. The ion consists of potassium ions \((\mathrm{K}^+)\) and the complex \([\mathrm{PtCl}_4]^{2-}\). Since potassium has a charge of +1, and there are two of them, their total charge is +2, meaning the charge on \([\mathrm{PtCl}_4]\) must be -2 to balance it out. Hence, option (a) is incorrect because the charge is \(-2\), not \(+2\).

Step 2: Analyze Option (b)

Examine the complex ion \ \(\left[\mathrm{Hg}(\mathrm{en})_2\right]^{2+}\)\. Ethylenediamine (en) is a bidentate ligand, meaning it can bind through two sites. Therefore, since there are two \(\mathrm{en}\) ligands, the coordination number of \(\mathrm{Hg}\) is 2 x 2 = 4, making option (b) correct.

Step 3: Analyze Option (c)

Examine the complex ion \ \([\mathrm{Fe}(\mathrm{CN})_6]^{4-}\). \ The charge on the complex is \(-4\), and each cyanide ligand \((\mathrm{CN}^-)\) has a charge of \(-1\). Thus, the total negative charge contributed by the ligands is \(-6\). To find the oxidation state of iron, let it be \(x\): \[ x + (-6) = -4 \\ x = +2 \]Thus, the oxidation number of iron is \(+2\), not \(+6\), making option (c) incorrect.

Step 4: Analyze Option (d)

In transition metal complexes, ligands act as Lewis bases, not Lewis acids. Ligands donate electron pairs to the metal atoms. Thus, option (d) is incorrect because it states the opposite.

Step 5: Conclusion

After examining all the options, option (b) is the correct statement: "The coordination number of \(\mathrm{Hg}\) in \([\mathrm{Hg}(\mathrm{en})_2]^{2+}\) is 4." This is determined by the two bidentate ligands (ethylenediamine) coordinating to \(\mathrm{Hg}\).

Key Concepts

Complex Ions

Complex ions are one of the fascinating features of coordination chemistry. They consist of a central metal ion or atom surrounded by molecules or anions called ligands.
This entire assembly maintains an overall charge, thus forming an ion.Consider the example \(\mathrm{K}_2[\mathrm{PtCl}_4]\). This complex ion comprises a platinum metal center surrounded by four chloride ligands. Potassium ions balance the negative charge of the complex ion. Each chloride ion presents a \(-1\) charge, summing up to \(-4\). The platinum center neutralizes most of this charge, leading the whole complex \([\mathrm{PtCl}_4]^{2-}\) to have a \(-2\) charge.
- Central metal ions typically derive from transition metals.- Ligands can be neutral molecules or negatively charged ions.- The net charge is determined by the charges of the metal and surrounding ligands.Understanding the structure and formation of complex ions is crucial in areas like bioinorganic chemistry, where they play a pivotal role in metalloproteins and catalytic cycles.

Oxidation Number

The concept of the oxidation number is vital for tracking electron transfer in chemical reactions.
In coordination complexes, the oxidation number indicates the hypothetical charge on the central metal if all ligands were removed.Take the example of \([\mathrm{Fe}(\mathrm{CN})_6]^{4-}\). Each cyanide ion carries a \(-1\) charge, cumulatively providing \(-6\) charge from six \(\mathrm{CN}^-\) ligands. For the complex to bear a \(-4\) charge overall, the oxidation state of iron must be \(+2\).
This is calculated as:\[x + (-6) = -4 \implies x = +2\]Some key points to remember:- The sum of the oxidation numbers equals the total charge of the complex ion.- Understanding oxidation numbers helps predict reactivity and coordination chemistry stability.- Transition metal complexes often entail varying oxidation states, contributing to their richness.Mastering how to determine and apply oxidation numbers unlocks deeper insights into both simple and complex reactions.

Transition Metal Complexes

Transition metal complexes stand out due to their unique properties and wide applications.
Their unique ability to form coordination complexes stems from the presence of vacant d-orbitals that can accept electron pairs from ligands.A complex like \([\mathrm{Hg}(\mathrm{en})_2]^{2+}\) demonstrates this beautifully. Here, the mercury center is bound by two bidentate ethylenediamine ligands, resulting in a coordination number of 4. This distinctive characteristic allows for a diverse range of geometries and chemical behaviors.Key attributes of transition metal complexes include:- Variable oxidation states: Many metals can exist in different oxidation states, affecting their color and magnetic properties.- Formation with various ligands: From small molecules to larger entities, ligands profoundly influence the complex's properties.- Role in catalysis and biochemical processes: Transition metal complexes are crucial in industrial catalysis and biological systems.Understanding these complexes' diverse roles broadens one's appreciation of both their practical applications and their underlying chemical principles.