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

Step 1:

The common ligands can be arranged in ascending order of crystal field splitting Δ. The order remains practically constant for different metals, and this series is called the spectrochemical series.

The ligands present on the R.H.S of the series are strong field ligands while that on the L.H.S are weak field ligands. 

The spectrochemical series is an experimentally determined series. The series is as follows:

I - < Br- < S2- < SCN- < CI- < N3 < F-< OH- < C2O42- ~ 
H2O< NCS- ~ H- < CN- < NH3< en ~ SO32- < NO2- < phen 
< CO

Step 2:

Ligands that cause only a small degree of crystal field splitting are termed weak field ligands. Ligands that cause a large splitting are called strong field ligands.