Question

Which of the following complex is octahedral, diamagnetic and the most stable?

• A. K₃[CO(CN)₆]
• B. [Ni(NH₃)₆]Cl₂
• C. The correct option is(A): Na₃[COCl₆]
• D. [Co(H₂O)₆]Cl₂

Answer 1

The Correct Option is A

To determine which complex is octahedral, diamagnetic, and the most stable, we must examine each option based on their coordination chemistry.

1. K₃[Co(CN)₆]:
   • The coordination compound consists of cobalt (Co) with a coordination number of 6, attached to six cyanide (CN^-) ligands.
   • Cyanide is a strong field ligand, which causes the pairing of electrons in cobalt, reflecting a low-spin configuration.
   • For cobalt in the +3 oxidation state (Co³⁺), the electronic configuration is [Ar] 3d⁶.
   • The presence of a strong field ligand like CN^- leads to the pairing of all 3d electrons, making the complex diamagnetic (no unpaired electrons).
   • The complex is octahedral due to the six-ligand coordination.
   • This makes K₃[Co(CN)₆] stable, octahedral, and diamagnetic.
2. [Ni(NH₃)₆]Cl₂:
   • This is a nickel-based complex where nickel (Ni) has a +2 oxidation state, and ammonia (NH₃) acts as a ligand.
   • Ammonia is a neutral ligand and not as strong a field ligand as cyanide, causing a high-spin configuration.
   • For Ni²⁺, the electronic configuration is [Ar] 3d⁸. It generally has unpaired electrons, leading to paramagnetism.
   • The complex is octahedral, indicated by the six ammonia ligands, but it is not diamagnetic.
3. Na₃[CoCl₆]:
   • This complex features cobalt with six chloride ligands.
   • Chloride ion (Cl^-) is a weak field ligand, typically leading to high-spin complexes.
   • Cobalt here is likely in the +3 oxidation state, with electrons in the 3d⁶ configuration, potentially leading to unpaired electrons, thus paramagnetic.
   • The structure would still be octahedral due to the six-ligand arrangement, but not diamagnetic nor as stable as cyanide complexes.
4. [Co(H₂O)₆]Cl₂:
   • This complex uses the water molecule as a ligand, which is a weaker field ligand.
   • For cobalt in +2 oxidation (Co²⁺), the electronic configuration is [Ar] 3d⁷, leading to unpaired electrons and paramagnetism.
   • Although the structure is octahedral, it is neither diamagnetic nor the most stable.

In conclusion, K₃[Co(CN)₆] is the only complex that fulfills all the criteria of being octahedral, diamagnetic, and the most stable due to the presence of strong field ligands causing electron pairing and stabilization.