Question

The complex ion that has the minimum wavelength of absorption in the visible region is:

• A. ${\left[Co\right(N{H}_3{)}_6]}^{3+}$
• B. ${\left[CoCl\right(N{H}_3{)}_5]}^{2+}$
• C. $\text{cis}-{\left[CoC{l}_2\right(N{H}_3{)}_4]}^{+}$
• D. $\text{trans}-{\left[CoC{l}_2\right(N{H}_3{)}_4]}^{+}$

Answer 1

The correct option is A ${\left[Co\right(N{H}_3{)}_6]}^{3+}$

$N{H}_3$ is a strong field ligand. This means that it will increase the energy gap between the $t_{2g}and{e}_g$ orbitals. In other words, one could say that it has a big CFSE. If six strong field ligands are present, the energy gap created is the largest.
We know that energy is inversely proportional to wavelength, hence we can say that ${\left[Co\right(N{H}_3{)}_6]}^{3+}$ will have maximum energy gap and hence, minimum wavelength of absorption.