Q. The number of isoelectronic species among Sc³⁺, Cr²⁺, Mn³⁺, Co³⁺ and Fe³⁺ is ‘n’. If ‘n’ moles of AgCl is formed during the reaction of complex with formula CoCl₃(en)₂NH₃ with excess of AgNO₃ solution, then the number of electrons present in the t_2g orbital of the complex is ________.

Correct Answer: 6

Explanation

First, we find the number of isoelectronic species.

Sc (Z = 21), Sc³⁺ → 18 electrons Cr (Z = 24), Cr²⁺ → 22 electrons Mn (Z = 25), Mn³⁺ → 22 electrons Co (Z = 27), Co³⁺ → 24 electrons Fe (Z = 26), Fe³⁺ → 23 electrons

Cr²⁺ and Mn³⁺ have the same number of electrons (22), hence they are isoelectronic.

Therefore, the number of isoelectronic species is n = 2

Now, the complex is CoCl₃(en)₂NH₃.

On reaction with excess AgNO₃, all ionisable chloride ions form AgCl. Number of moles of AgCl formed = n = 2

This shows that 2 chloride ions are outside the coordination sphere.

Oxidation state of cobalt:

Let oxidation state of Co = x x + (−1 × 1) = +3 x = +3

So, Co is in +3 oxidation state.

Electronic configuration of Co³⁺ : d⁶

The complex contains strong field ligands (en and NH₃), so it is a low spin octahedral complex.

In low spin d⁶ octahedral complex, all six electrons pair up in t_2g orbitals.

Hence, number of electrons present in t_2g orbitals = 6.

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