Question

The paramagnetic species among the following is/are (Given: Atomic numbers of Cr = 24; Fe = 26; Ni = 28):

• A. ([Fe(CN)_6]^{3-})
• B. ([Ni(OH_2)_6]^{2+})
• C. ([Ni(CN)_4]^{2-})
• D. ([Cr(CN)_6]^{3-})

Hint:

To determine paramagnetism, first calculate the oxidation state of the central metal ion. Then, use the ligand field strength (strong or weak) to decide the spin state of the complex and identify unpaired electrons.

Answer 1

The Correct Option is A

Step 1: Understanding paramagnetism.

Paramagnetism arises due to the presence of unpaired electrons in a species. To determine whether a complex is paramagnetic, we evaluate the electronic configuration of the central metal ion and the effect of the ligands on its d-orbitals.

Step 2: Analyzing the given options.

• Option (A): \([\text{Fe}(\text{CN})_6]^{3-}\)

  Iron in this complex is in the +3 oxidation state (\(\text{Fe}^{3+}\), \(3d^5\)). Cyanide (\(\text{CN}^-\)) is a strong field ligand, leading to a low-spin complex with one unpaired electron. Therefore, this species is paramagnetic.

• Option (B): \([\text{Ni}(\text{OH}_2)_6]^{2+}\)

  Nickel in this complex is in the +2 oxidation state (\(\text{Ni}^{2+}\), \(3d^8\)). Water (\(\text{H}_2\text{O}\)) is a weak field ligand, leading to a high-spin complex with two unpaired electrons. Therefore, this species is paramagnetic.

• Option (C): \([\text{Ni}(\text{CN})_4]^{2-}\)

  Nickel in this complex is in the +2 oxidation state (\(\text{Ni}^{2+}\), \(3d^8\)). Cyanide (\(\text{CN}^-\)) is a strong field ligand, leading to a low-spin complex with all electrons paired. Therefore, this species is diamagnetic.

• Option (D): \([\text{Cr}(\text{CN})_6]^{3-}\)

  Chromium in this complex is in the +3 oxidation state (\(\text{Cr}^{3+}\), \(3d^3\)). Cyanide (\(\text{CN}^-\)) is a strong field ligand, leading to a low-spin complex with three unpaired electrons. Therefore, this species is paramagnetic.

Step 3: Conclusion.

The paramagnetic species are \([\text{Fe}(\text{CN})_6]^{3-}\), \([\text{Ni}(\text{OH}_2)_6]^{2+}\), and \([\text{Cr}(\text{CN})_6]^{3-}\), corresponding to options (A), (B), and (D).