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

Explanation

The arrangelllent of ligands in. order of their increasing field strengths. i.e., increasing crystal field splitting energy (CFSE) values is called spectrochemical series.
`underset("Spectrochemical series")(underbrace(I^(-)ltBr^(-)ltSCN^(-)ltCl^(-)ltF^(-)ltOH^(-)lt"ox"ltH_(2)Olt"EDTA"^(4-)ltNH_(3)lt"en"ltCN^(-)ltCO))`
The ligands with a small value of CFSE `(Delta_(0))` are called weak field ligands, For such ligands `Delta_(0) lt 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) gt P`.
When ligands approach a transition metal ion, the d-orbitals split into two sets `(t_(2g) and e_(g))`, one with lower energy and the other with higher energy. The difference of energy between the two sets of orbitals is called crystal field splitting energy, `Delta_(0)` for octahedral field (and `Delta_(t)` for tetrahedral field)
If `Delta_(0) lt P` (pairing energy), the `4^(th)` electron enters one of the `e_(g)` orbitals giving the configuration `t_(2g)^(3)e_(g)^(1)` thereby forming high spin complex. Such ligands for which `Delta_(0) lt P` are known as weak field ligands.
If `Delta_(0) gt P`, the `4^(th)` electron pairs up in one of the `t_(2g)` orbitals giving the configuration `t_(2g)^(4)e_(g)^(0)` thus forming low spin complexes. Such ligands for which `Delta_(0) gt P` are called strong field ligands.