Problem 116 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following are paramagnetic tetrahedral species? (a) \(\left[\mathrm{FeCl}_{4}\right]^{-}\) (b) \(\left[\mathrm{Ni}(\mathrm{CN})_{4}\right]^{2-}\) (c) \(\left[\mathrm{NiCl}_{4}\right]^{2-}\) (d) \(\left[\mathrm{Zn}\left(\mathrm{NH}_{3}\right)_{4}\right]^{2+}\)

Step-by-Step Solution

Answer

(a) \([\mathrm{FeCl_{4}}]^{-}\) and (c) \([\mathrm{NiCl_{4}}]^{2-}\) are paramagnetic.

Step 1: Determine the oxidation states

First, determine the oxidation states of the central metal atoms in each species. For (a) \([\mathrm{FeCl_{4}}]^{-}\), chlorine has a charge of -1. Therefore, \(\mathrm{Fe} + 4(-1) = -1\), giving \(\mathrm{Fe}\) an oxidation state of +3.For (b) \([\mathrm{Ni(CN)_{4}}]^{2-}\), each \(\mathrm{CN}^{-}\) is -1, so \(\mathrm{Ni} + 4(-1) = -2\), giving \(\mathrm{Ni}\) an oxidation state of +2.For (c) \([\mathrm{NiCl}_{4}]^{2-}\), each chloride \(\mathrm{Cl}^{-}\) is -1, so \(\mathrm{Ni} + 4(-1) = -2\), giving \(\mathrm{Ni}\) an oxidation state of +2.For (d) \([\mathrm{Zn(NH_{3})_{4}}]^{2+}\), ammonia is neutral, so \(\mathrm{Zn} = +2\).

Step 2: Identify the electron configuration

Identify the electron configuration of the metal ions at their given oxidation states:(a) \(\mathrm{Fe}^{3+}\): Atomic number is 26; the electron configuration is [Ar] 3d\(^5\) due to the loss of three electrons.(b) \(\mathrm{Ni}^{2+}\): Atomic number is 28; the electron configuration is [Ar] 3d\(^8\) after losing two electrons.(c) \(\mathrm{Ni}^{2+}\): As in (b), the configuration remains [Ar] 3d\(^8\).(d) \(\mathrm{Zn}^{2+}\): Atomic number is 30; loses two electrons creating a full d-orbital with configuration [Ar] 3d\(^{10}\).

Step 3: Determine d-electron count

Calculate the number of d-electrons in each metal ion:(a) \(\mathrm{Fe}^{3+}\) has 5 d-electrons.(b) \(\mathrm{Ni}^{2+}\) has 8 d-electrons.(c) \(\mathrm{Ni}^{2+}\) has 8 d-electrons as in (b).(d) \(\mathrm{Zn}^{2+}\) has 10 d-electrons.

Step 4: Analyze magnetic properties

Identify whether the species is paramagnetic or diamagnetic based on unpaired electrons:(a) \(\mathrm{Fe}^{3+}\) in \([\mathrm{FeCl_{4}}]^{-}\) has 5 unpaired electrons and is therefore paramagnetic.(b) \(\mathrm{Ni}^{2+}\) in \([\mathrm{Ni(CN)_{4}}]^{2-}\) has paired electrons, making it diamagnetic.(c) \(\mathrm{Ni}^{2+}\) in \([\mathrm{NiCl}_{4}]^{2-}\) has unpaired electrons, making it paramagnetic.(d) \(\mathrm{Zn}^{2+}\) in \([\mathrm{Zn(NH_{3})_{4}}]^{2+}\), all d-electrons are paired, making it diamagnetic.

Key Concepts

Oxidation States Determination

To identify the oxidation state of a metal in a complex, consider the overall charge and the charges of accompanying ligands. For example, if a complex is negatively charged, like \([\left[\mathrm{FeCl}_{4}\right]^{-}\), you must account for the charges each ligand introduces. Chlorine, typically \(-1\), contributes a total charge of \(-4\) from four chlorines. Solving the equation \(\mathrm{Fe} + 4(-1) = -1\), we find iron in \([\mathrm{FeCl}_{4}]^{-}\) to have an oxidation state of +3. In general, follow these steps to determine oxidation states:

• Identify the charge of the entire complex.
• Subtract the known charges of the ligands.
• The remaining value represents the oxidation state of the central metal ion.

Electron Configuration

Once you know the oxidation state, the electron configuration of a central metal can be determined by using its atomic number and subtracting electrons equivalent to the oxidation state. For instance, with \(\mathrm{Fe}^{3+}\), start from iron’s initial configuration, \([\mathrm{Ar}] 4s^2 3d^6\), with an atomic number of 26. Then remove electrons according to the oxidation state (+3), reducing it to \([\mathrm{Ar}] 3d^5\). Here's a simple approach:

• Start with the neutral atom’s electron configuration.
• Subtract electrons based on the oxidation state from the outermost orbitals, usually \(4s\) or \(3d\).
• The resulting configuration reveals the arrangement of electrons in the metal ion.

d-Electron Counting

D-electron counting focuses on identifying the number of d-electrons in a metal ion, crucial for predicting its magnetic properties. After establishing an electron configuration, count the electrons in the d-subshell.For example, \(\mathrm{Ni}^{2+}\) stems from nickel’s neutral \([\mathrm{Ar}] 4s^2 3d^8\) atom. Removing two electrons, the resulting configuration is \([\mathrm{Ar}] 3d^8\). Thus, \(\mathrm{Ni}^{2+}\) has 8 d-electrons. To achieve this:

• Determine the electron configuration.
• Focus on the d-subshell count post oxidation state adjustment.
• The d-electron count impacts magnetic properties, such as paramagnetism.