Question

Identify the complex which exhibits all 3 characteristics: paramagnetic; high spin configuration; octahedral geometry.

• A. $[\text{Ni}(\text{H}_2\text{O})_2(\text{C}_2\text{O}_4)_2]^{2-}$
• B. $[\text{Co}(\text{NH}_3)(\text{Cl})(\text{en})_2]^{2+}$
• C. $[\text{Co}(\text{NH}_3)_5\text{Cl}]\text{Cl}_2$
• D. $[\text{Ni}(\text{CO})_4]$

Hint:

$\text{Ni}^{2+}$ is a $\text{d}^8$ system. In an octahedral field, it always contains exactly $2$ unpaired electrons regardless of ligand field strength, making it permanently paramagnetic. Since Option (A) is the only 6-coordinate octahedral nickel complex, it is the correct choice.

Answer 1

The Correct Option is A

Concept: Crystal Field Theory (CFT) dictates the spin state, geometry, and magnetic properties of coordination complexes based on the metal ion's d-electron count and ligand field strength:
• Octahedral complexes have a coordination number of 6.
• High-spin configurations occur when weak-field ligands produce a small crystal field splitting energy ($\Delta_o < P$), allowing electrons to occupy higher-energy orbitals before pairing.
• Paramagnetism requires the presence of one or more unpaired electrons.

Step 1: Evaluate the geometry and configuration of Option (A).
In $[\text{Ni}(\text{H}_2\text{O})_2(\text{C}_2\text{O}_4)_2]^{2-}$:
• Oxalate ($\text{ox}$) is a bidentate ligand and water ($\text{H}_2\text{O}$) is monodentate. The total coordination number is $(2 \times 2) + 2 = 6$, confirming an octahedral geometry.
• $\text{Ni}$ has an oxidation state of $+2$, which corresponds to a $\text{d}^8$ electron configuration.
• Both $\text{H}_2\text{O}$ and $\text{ox}^{2-}$ are weak-field ligands, resulting in a high-spin configuration.
• For an octahedral $\text{d}^8$ system, the electrons fill the $t_{2g}$ and $e_g$ subshells as $t_{2g}^6 e_g^2$. This leaves two unpaired electrons in the $e_g$ orbitals, making the complex paramagnetic. This complex satisfies all three criteria.

Step 2: Verify why the other options are excluded.

• Complexes (B) and (C) contain $\text{Co}^{3+}$ ($\text{d}^6$) bonded to strong-field ligands like $\text{NH}_3$ and ethylenediamine ($\text{en}$). This causes all electrons to pair up in the lower $t_{2g}$ level ($t_{2g}^6 e_g^0$), forming diamagnetic, low-spin complexes.
• Complex (D), $[\text{Ni}(\text{CO})_4]$, is a four-coordinate tetrahedral complex, not octahedral.