Question

Splitting energy $\left({\Delta }_o\right)$ can usually be measured from the absorption spectra of the complex ions. In simple cases when light is absorbed by a complex ion, an electron in one of the lower energy orbitals is excited to one of the higher energy orbital. The energy corresponding to the frequency of absorbed light is equal to ${\Delta }_o$. If value of ${\Delta }_o$ for the complex is in visible region, the complex is coloured and the value of ${\Delta }_o$ lies in ultraviolet or infrared region, the complex is colourless. For octahedral complexes the crystal field stabilization energy is given by CFSE $=\left[-0.4t_{2g}N+0.6e_g{N}^{\prime }\right]{\Delta }_o$, where $N$ and $N^{\prime }$ are number of electrons in $t_{2g}$ and $e_g$ orbitals respectively
Among the following pairs of complexes, in which case the ${\Delta }_0$ value is higher for the first one?

• A. ${\left[Co{F}_6\right]}^{3-}$ and ${\left[Co{\left(N{H}_3\right)}_6\right]}^{3+}$
• B. ${\left[Co{\left(H_{2}O\right)}_6\right]}^{2+}$ and ${\left[Co{\left(H_{2}O\right)}_6\right]}^{3+}$
• C. ${\left[Rh{\left(H_{2}O\right)}_6\right]}^{3+}$ and ${\left[Co{\left(H_{2}O\right)}_6\right]}^{3+}$
• D. ${\left[Co{\left(N{H}_3\right)}_6\right]}^{3+}$ and ${\left[Co(CN{)}_6\right]}^{3-}$

Answer 1

Answer: (C)

In complexes ${\left[Rh{\left(H_{2}O\right)}_6\right]}^{3+}$ and ${\left[Co{\left(H_{2}O\right)}_6\right]}^{3+}$,central metal cations have same oxidation state as well as same ligands and they fall in same group, but ${\Delta }_0$ of ${\left[Rh{\left(H_{2}O\right)}_6\right]}^{3+}>{\Delta }_0$ of ${\left[Co{\left(H_{2}O\right)}_6\right]}^{3+}$ because $R{h}^{3+}$ has high $Z_{\text{eff }}$ value then $C{o}^{3+}$