• # question_answer Which of the following complexes has the least wavelength of light in visible region? A) ${{[Co{{(CN)}_{6}}]}^{3\,-}}$           B) ${{[Co\,{{({{H}_{2}}O)}_{6}}]}^{3+}}$ C) ${{[Co\,{{(N{{H}_{3}})}_{6}}]}^{3\,-}}$ D) ${{[Co{{F}_{6}}]}^{3\,-}}$

 As we know that, strong field ligand split the five degenerate energy levels with more energy separation than weak field ligand, i.e. as strength of ligand increases crystal field splitting energy increases. Also, $\Delta E=\frac{hc}{\lambda }$    $\Rightarrow \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\Delta E\,\,\propto \,\,\,\frac{1}{\lambda }$            $\Rightarrow \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\lambda \propto \frac{1}{\Delta E}$ As energy separation increases, the wavelength decreases.
 Thus, the correct order of increasing CFSE and decreasing wavelength is \begin{align} & {{[Co{{F}_{6}}]}^{3-}}>{{[Co{{\left( {{H}_{2}}O \right)}_{6}}]}^{3+}}>{{[Co{{\left( N{{H}_{3}} \right)}_{6}}]}^{3+}} \\ & >{{[Co{{\left( CN \right)}_{6}}]}^{3-}} \\ \end{align} Here, strength of ligand increases, $\Delta E$ increases, CFSE increases and $\lambda$absorbed decreases. Hence, correct option is [a].