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

Step 1: Check isoelectronic species.
S2− has 18 electrons.
C4− has 10 electrons.
Mn2+ has 23 electrons (25 − 2).
Co3+ has 24 electrons (27 − 3).
Fe3+ has 23 electrons (26 − 3).

Isoelectronic species are Mn2+ and Fe3+, both having 23 electrons.
So, n = 2.

Step 2: Complex formula.
The complex is CoCl2(en)2NH3.
Coordination number is 6 (2 Cl−, 2 en, 1 NH3).
Ethylenediamine and ammonia are neutral ligands, chloride is negative.
Hence, oxidation state of cobalt is +3.

Step 3: Reaction with AgNO3.
Two chloride ions are outside the coordination sphere and react with AgNO3 to form AgCl.
Therefore, 2 moles of AgCl are formed, matching n = 2.

Step 4: Electronic configuration of Co3+.
Cobalt atomic configuration is 3d7 4s2.
Co3+ becomes 3d6.
With strong field ligands like en and NH3, the complex is low spin octahedral.
Electronic configuration becomes t2g6 eg0.

Step 5: Number of electrons in t2g orbital.
t2g6 means there are 6 electrons in the t2g orbitals.

Final Answer:
Number of electrons in t2g orbital = 6