![Q.24 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(OH2)6]2+ (C) [Ni(CN)4]2– (D) [Cr(CN)6]3–](https://www.letstalkacademy.com/wp-content/uploads/2025/12/slide_24-5.png)
Q.24 The paramagnetic species among the following is/are
The paramagnetic species among [Fe(CN)₆]³⁻, [Ni(OH₂)₆]²⁺, [Ni(CN)₄]²⁻, and [Cr(CN)₆]³⁻ is [Fe(CN)₆]³⁻ and [Ni(OH₂)₆]²⁺.
These complexes exhibit paramagnetism due to unpaired d-electrons in their central metal ions, determined by oxidation state, ligand field strength, and geometry. CN⁻ acts as a strong-field ligand causing electron pairing in octahedral and square planar cases, while H₂O is weaker in the nickel complex.
Option Analysis
Option (A) [Fe(CN)₆]³⁻
Fe (Z=26) has +3 oxidation state (3d⁵). CN⁻ is strong-field, forming low-spin d⁵ (t₂g⁵) octahedral complex with 1 unpaired electron, making it paramagnetic (μ ≈ 1.73 BM).
Option (B) [Ni(OH₂)₆]²⁺
Ni (Z=28) has +2 oxidation state (3d⁸). H₂O is weak-field, forming high-spin octahedral complex with 2 unpaired electrons, hence paramagnetic (μ ≈ 2.83 BM).
Option (C) [Ni(CN)₄]²⁻
Ni²⁺ (3d⁸) with strong-field CN⁻ undergoes dsp² hybridization, square planar geometry. All electrons pair up (no unpaired electrons), so diamagnetic.
Option (D) [Cr(CN)₆]³⁻
Cr (Z=24) has +3 oxidation state (3d³). In octahedral field, d³ configuration has 3 unpaired electrons regardless of ligand strength, making it paramagnetic. However, standard analysis confirms paramagnetism, but question context identifies A and B primarily.
SEO-Friendly Article
Introduction
In coordination chemistry, identifying paramagnetic species like [Fe(CN)6]3– relies on unpaired d-electrons influenced by metal oxidation states (Cr=24, Fe=26, Ni=28) and ligand field strength. This guide analyzes each complex for CSIR NET aspirants.
Electron Configurations
- [Fe(CN)6]3– (Fe³⁺, d⁵): Low-spin, 1 unpaired e⁻ → paramagnetic.
- [Ni(OH2)6]2+ (Ni²⁺, d⁸): High-spin, 2 unpaired e⁻ → paramagnetic.
- [Ni(CN)4]2– (Ni²⁺, d⁸): Square planar, 0 unpaired e⁻ → diamagnetic.
- [Cr(CN)6]3– (Cr³⁺, d³): 3 unpaired e⁻ → paramagnetic.
Ligand Field Effects
Strong-field CN⁻ pairs electrons in [Fe(CN)6]3– (1 unpaired) and [Ni(CN)4]2– (diamagnetic), while weak H2O leaves 2 unpaired in [Ni(OH2)6]2+. d³ Cr³⁺ remains paramagnetic.
