Question

What is spectrochemical series? Explain the difference between a weak field ligand and a strong field ligand.

Answer 1

The arrangment of ligands in order of their increasing field strengths, i.e., increasing crystal field splitting energy (CFSE) values is called spectrochemical series.
$\begin{array}{c}\underset{Spectrochemicalseries}{\underset{}{I^{-}<B{r}^{-}<SC{N}^{-}<C{l}^{-}<F^{-}<O{H}^{-}<ox<H_{2}O<EDT{A}^{4-}<N{H}_3<en<C{N}^{-}<CO}}\end{array}$
The ligands with a small value of CFSE ${\Delta }_0$ are called weak field ligands. For such ligands ${\Delta }_0<p$ where p is the pairing energy. Whereas the ligands with a large value of CFSE are called strong field ligands. In case of such ligands ${\Delta }_0>p$.