Question

What is the EAN of [Co(NH₃)₆]³⁺?

• A. 36
• B. 38
• C. 35
• D. 54

Hint:

For coordination complexes, calculate EAN by adding the electrons from the metal and the ligands.

Answer 1

The Correct Option is A

The Effective Atomic Number (EAN) of the complex ion \([Co(NH_3)_6]^{3+}\) represents the total number of electrons surrounding the central cobalt ion. It's calculated as follows: \[ \text{EAN} = \text{Number of electrons on the metal ion} + \text{Number of electrons donated by the ligands} \]

1. Determine the Cobalt Oxidation State:
   In \([Co(NH_3)_6]^{3+}\), \(NH_3\) ligands are neutral. Cobalt's oxidation state is determined by the complex's overall charge: \[ \text{Let the oxidation state of Co be } x. \] \[ x + 6 \times (\text{charge of each } NH_3) = +3 \] \[ x + 6 \times 0 = +3 \Rightarrow x = +3 \] Therefore, cobalt's oxidation state is +3.
2. Determine Electrons on the Cobalt Ion:
   - Neutral cobalt (\(Co\)) (atomic number 27) has 27 electrons.
   - \(Co^{3+}\) has lost 3 electrons. \[ \text{Number of electrons on } Co^{3+} = 27 - 3 = 24 \, \text{electrons} \]
3. Determine Electrons Donated by Ligands:
   - Each \(NH_3\) ligand donates one electron pair (2 electrons).
   - Six \(NH_3\) ligands are present. \[ \text{Total electrons donated by ligands} = 6 \times 2 = 12 \, \text{electrons} \]
4. Calculate EAN:
   \[ \text{EAN} = \text{Electrons on } Co^{3+} + \text{Electrons donated by ligands} \] \[ \text{EAN} = 24 + 12 = 36 \, \text{electrons} \]
5. 18-Electron Rule Verification:
   Stable transition metal complexes often follow the 18-electron rule. This corresponds to the electron configuration of the nearest noble gas.
   • The EAN of \(Co^{3+}\) in \([Co(NH_3)_6]^{3+}\) is 36, suggesting it aligns with the 18-electron rule, indicating stability through coordination.

Conclusion:
The Effective Atomic Number (EAN) of \([Co(NH_3)_6]^{3+}\) is 36.