Question

Classify weak and strong ligands out of:
(a) $O{H}^{-}$
(b) $F^{-}$
(c) $I^{-}$
(d) $N{H}_3$
(e) $C{H}_{3}CO{O}^{-}$
(f) en
(g) $C{N}^{-}$
(h) $C{l}^{-}$
(i) $H_{2}O$
$F^{-},I^{-}C{l}^{-}\text{ and }{\text{H}}_{2}O$ are weak ligands, other are strong ligands.
If true enter 1, else enter 0.

Answer 1

Hint: In order to answer this question you need to recall the concept of Spectrochemical series. Strong ligands can donate the electron pair to the acceptor more efficiently.


Complete step by step answer:
- A spectrochemical series is nothing but a list of ligands ordered on ligand strength (ability) and a list of metal ions based on oxidation number, group and its identity.
- In crystal field theory, ligands modify the difference in energy between the d orbital / triangle, called the ligand-field splitting parameter for ligands or the crystal-field splitting parameter, which is mainly observed in differences in colour of similar metal-ligand complexes.
Weak ligands: $I^{-},B{r}^{-},SC{N}^{-},{N_3}^{-},F^{-},H_{2}NCON{H}_2,o{x}^{2-},O^{2-},H_{2}O,NC{S}^{-},py,N{H}_3,$en, bpy
Strong ligands: $N{O_2}^{-},C{H_3}^{-},C_6{H_5}^{-},C{N}^{-},CO$

Let us understand the basic concept of crystal field theory.
In simple words, it describes the breaking of orbital degeneracy in transition metal complexes due to the presence of ligands. This theory describes the strength of the metal ligand bonds, because the strength of the system is altered based on the metal ligand bond.
Let us see the difference between weak ligand and strong ligand.

Weak ligand	Strong field ligand
- A weak ligand or a weak field ligand is a ligand which can result in lower crystal field theory. - The complexes formed with these ligands are also known as high spin complexes. - They are mostly paramagnetic in nature.	- A strong ligand or a strong field ligand is any ligand which has high crystal field theory.- The complexes formed in this are called low spin complexes.- They are mostly diamagnetic or less paramagnetic than weak fields.

- $F^{-},I^{-},C{l}^{-},H_{2}O$ are weak ligands, others are strong ligands. This statement is false as en is also a weak ligand. en stands for ethylene diamine ligand. Therefore, enter 0.



Note: You should know that sometimes there are exceptional cases where weak field ligand acts as strong field ligand. For example; $[PtC{l}_4{]}^{2-}$ and $Co{F_6}^{3-}$.