![Q20 Among the following statements about cobalt complexes, which are CORRECT? Given: Atomic number of Co is 27 (A) [Co(NH₃)₄]²⁺ exhibits square planar geometry (B) [Co(en)₂]³⁺ does not show optical isomerism (en = ethylenediamine) (C) [Co(H₂O)₆]²⁺ is paramagnetic in nature (D) [Co(NH₃)₆]³⁺ shows ligand-to-metal charge transfer](data:image/svg+xml;base64,PHN2ZyB2aWV3Qm94PSIwIDAgMTkyMCAxMDgwIiB3aWR0aD0iMTkyMCIgaGVpZ2h0PSIxMDgwIiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjwvc3ZnPg==)
Q20 Among the following statements about cobalt complexes, which are CORRECT?
Option Analysis
Option A: [Co(NH3)6]3+ exhibits square planar geometry
Incorrect. The complex has six NH3 ligands, giving a coordination number of 6, which results in octahedral geometry due to d2sp3 hybridization. Square planar geometry typically occurs for coordination number 4, common in d8 metals like Ni2+ or Pt2+.
Option B: [Co(en)3]3+ does not show optical isomerism (en = ethylenediamine)
Incorrect. Ethylenediamine (en) is a bidentate ligand forming a chelate ring. [Co(en)3]3+ has octahedral geometry with no plane of symmetry, leading to non-superimposable mirror images (Δ and Λ enantiomers), thus exhibiting optical isomerism.
Option C: [Co(H2O)6]3+ is paramagnetic in nature
Incorrect. Co3+ (d6) in [Co(H2O)6]3+ uses d2sp3 hybridization (inner orbital). H2O acts as a strong field ligand for Co3+, pairing all electrons (low spin, t2g6), making it diamagnetic with no unpaired electrons.
Option D: [Co(NH3)6]3+ shows ligand-to-metal charge transfer
Correct. NH3, a strong field ligand, promotes LMCT transitions where electrons move from ligand orbitals to metal t2g orbitals, observed as intense charge transfer bands in the UV-visible spectrum.
Correct option: D
SEO Optimized Content for CSIR NET Preparation
Cobalt complexes play a key role in CSIR NET Life Sciences preparation, especially understanding [Co(NH3)6]3+ geometry, optical isomerism in [Co(en)3]3+, and magnetic properties of [Co(H2O)6]3+. These topics test coordination chemistry fundamentals like hybridization, ligand field strength, and electron configuration (Co atomic number 27, Co3+ is d6).
Geometry of Cobalt Complexes
[Co(NH3)6]3+ features six monodentate NH3 ligands, confirming octahedral geometry via d2sp3 hybridization. Strong field NH3 causes low-spin pairing, ruling out square planar (for CN=4).
Optical Isomerism Explained
[Co(en)3]3+ with bidentate en ligands forms chiral octahedral structure, showing optical isomerism as enantiomers lack symmetry. This contrasts with symmetric [Co(NH3)6]3+.
Magnetic Properties
[Co(H2O)6]3+ is diamagnetic (all electrons paired by strong field H2O), unlike weak field cases like [CoF6]3- (paramagnetic). [Co(NH3)6]3+ follows similar low-spin diamagnetic behavior.
Charge Transfer in Complexes
[Co(NH3)6]3+ exhibits LMCT due to NH3-to-Co3+ electron transfer, producing intense absorption bands vital for spectroscopic analysis.
