Question

The correct ascending order of ligand field strength of the given ligand is:
(A) ${{F}^{-}} < {{I}^{-}} < C{{N}^{-}} < {{H}_{2}}O < CO$
(B) ${{I}^{-}} < {{F}^{-}} < {{H}_{2}}O < CO < C{{N}^{-}}$
(C) ${{I}^{-}} < {{F}^{-}} < {{H}_{2}}O < C{{N}^{-}} < CO$
(D) ${{F}^{-}} < {{I}^{-}} < CO < {{H}_{2}}O < C{{N}^{-}}$

Answer 1

To determine the correct order of the strength of a ligand, we first need to know what is a ligand. The molecule which forms a coordination complex when bound to the central atom is known as a ligand.

Complete answer:
We know that ligands act as a Lewis base and donate electrons whereas metals act as a Lewis acid and accept electrons.
Now for bond formation, the energy of the highest occupied molecular orbital of a ligand should be able to overlap with the lowest unoccupied molecular orbital energy of the metal.
Upon bond formation, new molecular orbitals are formed which have a new highest occupied molecular orbital and lowest unoccupied molecular orbital (define reactivity and property reactivity of molecule) and the 5-d orbitals.
These 5-d orbitals split into a set of 3 orbitals of low energy and a set of two orbitals of high energy. The magnitude of the splitting parameter (${{\Delta }_{{\mathrm O}}}$) or the energy difference between the two sets of d-orbitals is found from the ligand's field strength.
The series which sorts the magnitude of ${{\Delta }_{{\mathrm O}}}$ of the ligand is known as the spectrochemical series.
Spectrochemical series in increasing order of ${{\Delta }_{{\mathrm O}}}$ of some ligands is as follows

& {{I}^{-}}~ < B{{r}^{-}} < {{S}^{2-}} < SC{{N}^{-}}\left( Sbonded \right) < C{{l}^{-~}} < {{N}_{3}}^{-} < {{F}^{-}} < NC{{O}^{-}} < O{{H}^{-}} < {{C}_{2}}{{O}_{4}}^{2-} < {{O}^{2-}} \\\ & < {{H}_{2}}O < acetylacetonate < NC{{S}^{-}}\left( Nbonded \right) < C{{H}_{3}}CN < Glycine < pyridine < N{{H}_{3}} \\\ & < ethylenedia\min e < 2,2'-bipyridine < 1,10-phenanthroline < N{{O}_{2}}^{-}\left( Nbonded \right) \\\ & < PP{{h}_{3}} < C{{N}^{-}} < CO \\\ \end{aligned}$$ **From the stereochemical series, we can see that the correct ascending order of ligand field strength of the given ligand is option (C) $${{I}^{-}} < {{F}^{-}} < {{H}_{2}}O < C{{N}^{-}} < CO$$.** **Note:** It should be noted that the ligands towards the beginning of the series are considered to be weak ligands or high spin ligands as they can't forcibly cause electron pairing at 3d level. Whereas ligands towards the end of the series are considered to be strong ligands or low spin ligands as they forcibly cause electron pairing at 3d level and form an octahedral inner orbital complex.