Question

Which of the following has all paired electrons in t_2g? 

(1) [Cr(H₂O)₆]³⁺ 

(2) [Co(H₂O)₆]²⁺ 

(3) [Co(H₂O)₆]³⁺ 

(4) [Fe(H₂O)₆]²⁺

Answer 1

Correct answer is: (3)  [Co(H₂O)₆]³⁺

[Cr(H₂O)₆]³⁺ ⇒ Cr⁺³ ⇒ 3d³ ⇒ d²sp³ ⇒  \(t_{2g}^{3}e_g^{0}\)

[Co(H₂O)₆]²⁺ ⇒ Co⁺² ⇒ 3d⁷ ⇒ sp³d² ⇒  \(t_{2g}^{5}e_g^{2}\)

[Co(H₂O)₆]³⁺ ⇒ Co³⁺ ⇒  3d⁶ ⇒ d²sp³ ⇒  \(t_{2g}^{6}e_g^{0}\)

[Fe(H₂O)₆]²⁺ ⇒ Fe²⁺ ⇒ 3d⁶ ⇒ sp³d² ⇒  \(t_{2g}^{4}e_g^{2}\)

In [Co(H₂O)₆]³⁺ all electron are present in t_2g set t_2g set have all paired electrons.