Question

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

Answer 1

Hint: The crystal field theory is based on some assumptions which are: ligands are considered as a point charge and the interaction between the orbitals of ligands and metal atoms is negligible. It assumes that the d-orbitals at the initial state will have the same energy but when a complex is formed, the degeneracy of these orbitals is destroyed and led to splitting in two energy levels.

Complete answer:
Spectrochemical series is an experimentally determined series in which ligands have been arranged in an increasing order of their strength of splitting of d-orbitals. A pattern of increasing order of sigma donation is as follows:
Halide donors $<$ oxygen donors $<$ nitrogen donors $<$ carbon donors
The spectrochemical series is represented as follows:
$I^{-}<B{r}^{-}<S^{2-}<C{l}^{-}<N{O}_3^{-}<F^{-}<O{H}^{-}<EtOH<oxalate<H_{2}O<EDTA<N{H}_3<$ pyridine $<$ ethylenediamine $<$ dipyridyl $<$ o-phenanthroline $<N{O}_2^{-}<C{N}^{-}<CO$.
Difference between weak field ligand and strong field ligand is as follows:

Strong field ligands	Weak field ligands
These are the ligands used in octahedral complexes in which the crystal field stabilization energy ${\mathrm{\Delta }}_o$ is greater than pairing energy (p).	These are the ligands used in octahedral complexes in which the crystal field stabilization energy ${\mathrm{\Delta }}_o$ is less than pairing energy (p) in a single orbital.
Contains C, N and P as donor sites.	Contains X, O and S as donor atoms
Complexes formed by these ligands are also known as low spin complexes.	Complexes formed by these ligands are also known as high spin complexes.
The complexes formed are mostly diamagnetic or comparatively less paramagnetic in nature.	The complexes formed are generally paramagnetic in nature.
Example: $C{N}^{-}$, $NC{S}^{-}$, etc.	Example: $F^{-}$, $SC{N}^{-}$, etc.

Note:
It is important to remember that the crystal field splitting produced by strong field ligands is double to that of weak field ligands. This is because of pi bonding in which the metal atoms donate electrons from a filled $t_{2g}$ orbital to the vacant orbital of ligand known as back bonding. Thus, strong field ligands act as sigma donors and pi acceptors.