Question

The paramagnetic complex ion which has no unpaired electrons in \( t_{2g} \) orbitals is

• A. \( [Fe(CN)_6]^{4-} \)
• B. \( [Fe(CN)_6]^{3-} \)
• C. \( [Zn(NH_3)_6]^{2+} \)
• D. \( [Ni(NH_3)_6]^{2+} \)

Hint:

When analyzing paramagnetic behavior, check the oxidation state of the central metal and the ligand strength. Strong field ligands cause electron pairing, while weak field ligands often lead to unpaired electrons in higher energy orbitals.

Answer 1

The Correct Option is D

The problem asks us to identify a paramagnetic complex ion with no unpaired electrons in the \( t_{2g} \) orbitals. We will analyze each option:

1. \( [Fe(CN)_6]^{4-} \)
Fe in this complex is in the +2 oxidation state, with an electron configuration of \( [Ar] 3d^6 \). CN⁻ is a strong field ligand and causes pairing of electrons. Thus, all six 3d electrons pair, resulting in no unpaired electrons in the \( t_{2g} \) orbitals.

2. \( [Fe(CN)_6]^{3-} \)
Fe in this complex is in the +3 oxidation state, with an electron configuration of \( [Ar] 3d^5 \). Again, CN⁻ is a strong field ligand, causing most electrons to pair. However, one electron remains unpaired in the \( t_{2g} \) orbitals, resulting in a paramagnetic complex.

3. \( [Zn(NH_3)_6]^{2+} \)
Zn in this complex is in the +2 oxidation state, with an electron configuration of \( [Ar] 3d^{10} \). All d-orbitals are completely filled, so there are no unpaired electrons at all.

4. \( [Ni(NH_3)_6]^{2+} \)
Ni in this complex is in the +2 oxidation state, with an electron configuration of \( [Ar] 3d^8 \). NH₃ is a weak field ligand, and does not cause pairing. Electrons fill the \( t_{2g} \) orbitals first, all pairing within them, leaving no unpaired electrons in these orbitals, though there are still unpaired electrons in total.

Thus, while all options may have different numbers of unpaired electrons in their orbitals, the question specifically asks for no unpaired electrons in the \( t_{2g} \) orbitals, which is possible even in a paramagnetic complex like \( [Ni(NH_3)_6]^{2+} \). Consequently, the correct choice is this complex.

Answer 2

Step 1: Identifying oxidation states and electronic configurations
 
- [Fe(CN)₆]⁴⁻
  • Fe is in the +2 oxidation state (3d⁶).
  • CN⁻ is a strong field ligand, causing pairing of electrons.
  • The complex is low spin but still contains unpaired electrons in the t_2g orbitals.

- [Fe(CN)₆]³⁻
  • Fe is in the +3 oxidation state (3d⁵).
  • CN⁻ is a strong field ligand.
  • The complex is low spin and has one unpaired electron in the t_2g orbitals.

- [Zn(NH₃)₆]²⁺
  • Zn is in the +2 oxidation state (3d¹⁰).
  • All orbitals are completely filled, so it is diamagnetic.
  • However, it is not a candidate where paramagnetic behavior is being evaluated.

- [Ni(NH₃)₆]²⁺
  • Ni is in the +2 oxidation state (3d⁸).
  • NH₃ is a weak field ligand, leading to a high spin configuration.
  • The t_2g orbitals are completely filled with paired electrons, making it the correct choice.

Step 2: Conclusion

Since the question asks for the paramagnetic complex with no unpaired electrons in t_2g orbitals, the correct answer is [Ni(NH₃)₆]²⁺.