Question

The number of isoelectronic species among S2−, C4−, Mn2+, Co3+ and Fe3+ is ‘n’.

If ‘n’ moles of AgCl is formed during the reaction of the complex with formula
CoCl2(en)2NH3
with excess AgNO3 solution, then the number of electrons present in the t2g orbital of the complex is ________.

Hint:

For coordination complexes: - Identify oxidation state of metal. - Check whether ligands are strong or weak field. - Use crystal field theory to distribute electrons in \(t_{2g}\) and \(e_g\).

Answer 1

Correct Answer: 6

To solve this problem, we need to determine the number of isoelectronic species and then calculate the number of electrons in the t_2g orbital of the complex CoCl₂(en)₂NH₃ after reacting with excess AgNO₃.
First, let's find the isoelectronic species among S²⁻, C⁴⁻, Mn²⁺, Co³⁺, and Fe³⁺.

• The electron configuration for these ions are:
  • S²⁻ has 18 electrons (from 16 + 2)
  • C⁴⁻ has 10 electrons (from 6 + 4)
  • Mn²⁺ has 23 electrons (from 25 - 2)
  • Co³⁺ has 24 electrons (from 27 - 3)
  • Fe³⁺ has 23 electrons (from 26 - 3)
• Isoelectronic species have the same number of electrons. Here, Mn²⁺ and Fe³⁺ each have 23 electrons.

The number of isoelectronic species n is 2.
For the complex CoCl₂(en)₂NH₃, determine how many moles of AgCl are formed when reacting with excess AgNO₃:

• AgNO₃ reacts with Cl⁻ ions forming AgCl. Each mole of AgCl corresponds to 1 mole of Cl⁻.
• From the complex, we see CoCl₂, meaning there are 2 Cl⁻ ions available per complex molecule. Thus, 2 moles of AgCl are formed for every mole of CoCl₂(en)₂NH₃.
• Therefore, n = 2 moles of AgCl confirms the number of Cl⁻ ions and ligands.
• Co in CoCl₂(en)₂NH₃ is in the 3+ oxidation state (Co³⁺).

The electron configuration of Co³⁺ is [Ar]3d⁶. Its d-orbital splits into t_2g and e_g in octahedral complexes:

• In low spin complexes (as with strong field ligands like en), the electron configuration for t_2g is 3d_t2g⁶e_g⁰.
• Thus, the number of electrons in the t_2g orbital is 6.

Verifying with the expected range of 6,6: the solution matches the range.
Therefore, the number of electrons in the t_2g orbital is 6.