Question

Correct increasing order for the wavelengths of absorption in the visible region the complexes of $Co^{3+}$ is :

• A. $[Co(H_2O)_6]^{3+},\, [Co(en)_3]^{3+},\, [Co(NH_3)_6]^{3+}$
• B. $[Co(H_2O)_6]^{3+},\, [Co(NH_3)_6]^{3+},\, [Co(en)_3]^{3+}$
• C. $[Co(NH_3)_6]^{3+},\, [Co(en)_3]^{3+} ,\, [Co(H_2O)_6]^{3+}$
• D. $[Co(en)_3]^{3+} ,\, [Co(NH_3)_6]^{3+},\, [Co(H_2O)_6]^{3+}$

Answer 1

Answer: (D)

The order of the ligand in the spectrochemical series

$H _{2} O < NH _{3}< en$

Hence, the wavelength of the light observed will be in the order

$\left[ Co \left( H _{2} O \right)_{6}\right]^{3+}<\left[ Co \left( NH _{3}\right)_{6}\right]^{3+}<\left[ Co ( en )_{3}\right]^{3+}$

Thus, wavelength absorbed will be in the opposite order

i.e., $\left[ Co ( en )_{3}\right]^{3+},\left[ Co \left( NH _{3}\right)_{6}\right]^{3+},\left[ Co \left( H _{2} O \right)_{6}\right]^{3+}$