Question

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

Answer 1

Spectrochemical series

• Common ligands are arranged in spectrochemical series in increasing order of their crystal-field splitting energy (CFSE).
• Metals are electron acceptors, and ligands are electron donors.
• There are two different types of ligands: weak field ligands and strong field ligands.
• Strong field ligands result in significant crystal field splitting and have a low spin value, whereas weak field ligands cause low crystal field splitting and have a high value.
• Strong field ligands are found on the right hand side of the series, whereas weak field ligands are found on the left hand side
• Additionally, strong field ligands split the $d$-orbitals more than weak field ligands do.
• Order of the spectrochemical series as follows:

${\mathrm{I}}^{–}<{\mathrm{Br}}^{–}<{\mathrm{Cl}}^{–}<{\mathrm{SCN}}^{–}<{\mathrm{F}}^{–}<{\mathrm{OH}}^{–}<{\mathrm{C}}_2{{\mathrm{O}}_4}^{2-}<{\mathrm{H}}_2\mathrm{O}<{\mathrm{NCS}}^{–}<{\mathrm{EDTA}}^{4-}<{\mathrm{NH}}_3<\mathrm{en}<{\mathrm{CN}}^{–}<\mathrm{CO}$

Difference between Strong field and Weak field ligands

S.No.	Strong field ligands	Weak field ligands
1.	These are the ligands used in octahedral complexes when the pairing energy is larger than the crystal field stabilization energy.	These ligands are used in octahedral complexes where the pairing energy in a single orbital is lower than the crystal field stabilization energy
2.	Low spin complexes are another name for the complexes created by these ligands.	These ligand-based compounds are also referred to as high spin complexes.
3.	The complexes formed are mostly diamagnetic or slightly paramagnetic.	The complexes formed are mostly paramagnetic.
4.	Example: ${\mathrm{OH}}^{-},{\mathrm{CN}}^{-}\mathrm{and}{\mathrm{NH}}_3$	Examples: ${\mathrm{F}}^{-},{\mathrm{I}}^{-}\mathrm{and}{\mathrm{H}}_2\mathrm{O}$