Question

Which of the following orders is correct in the spectrochemical series of the ligands?
A. \[C{l^ - } < {F^ - } < {C_2}O_4^{2 - } < NO_2^ - < C{N^ - }\]
B. \[NO_2^ - < {C_2}O_4^{2 - } < C{l^ - } < {F^ - } < C{N^ - }\]
C. \[{C_2}O_4^{2 - } < {F^ - } < C{l^ - } < NO_2^ - < C{N^ - }\]
D. \[{F^ - } < C{l^ - } < NO_2^ - < {C_2}O_4^{2 - }\]

Answer 1

Hint: We know that, the ligand has the capacity to fluctuate the energy of the d-orbitals in the metal complex as per the charge which ligand carry, when it approaches the metal, the orbitals get distorted which is dependent on the nature of the ligand. The level of distortion caused by the ligand to the metal ion d-orbitals the ligands are arranged in the spectrochemical series.

Complete step by step answer
Ligand are the species that are bound with the metal ion to form the complex compound. It shares the electron with metal ions, electrons will occupy the d-orbital and form coordinate covalent bonds.
Increasing order of strengths for Ligands:
\[
{I^ - }\; < {\rm{ }}B{r^ - }\; < {\rm{ }}{S^{2 - }}\; < \;SC{N^ - }\;\left( {S-bonded} \right){\rm{ }} < {\rm{ }}C{l^{ - \;}} < {\rm{ }}{N_3}^ - \; < {\rm{ }}{F^ - } < {\rm{ }}NC{O^ - }\; < {\rm{ }}O{H^ - }\; < \;{C_2}{O_4}^{2 - }\; < {\rm{ }}{O^{2 - }}\\
 < {\rm{ }}{H_2}O{\rm{ }} < {\rm{ }}aca{c^ - }\;\left( {acetylacetonate} \right){\rm{ }} < \;NC{S^ - }\;\left( {N-bonded} \right){\rm{ }} < {\rm{ }}C{H_3}CN{\rm{ }} < {\rm{ }}gly{\rm{ }}(glycine){\rm{ }} < {\rm{ }}py{\rm{ }}(pyridine){\rm{ }} < \\
{\rm{ }}N{H_3}\; < {\rm{ }}en{\rm{ }}(ethylenediamine){\rm{ }} < {\rm{ }}bipy{\rm{ }}(2,2' - bipyridine){\rm{ }} < {\rm{ }}phen{\rm{ }}(1,10 - phenanthroline){\rm{ }} < {\rm{ }}N{O_2}^ - \;\left( {N-bonded} \right){\rm{ }}\\
 < {\rm{ }}PP{h_3}\; < {\rm{ }}C{N^ - }\; < {\rm{ }}CO
\]
Weak field ligand: \[{H_2}O,{\rm{ }}{F^ - },{\rm{ }}C{l^ - },{\rm{ }}O{H^ - }\] (d-levels get least affected).
Strong field ligand: \[CO,{\rm{ }}C{N^ - },{\rm{ }}N{H_3},{\rm{ }}PP{h_3}\] (d-levels get strongly affected).
There can be two factors which make the difference between the weakest and the strongest ligands. First, the electronegativity of the atom presents in the ligand, which has a strong ability to attract the electrons present in the metal’s d-orbitals., and second is the electrons of the ligands which will take part in back bonding. This explains why coordination complexes of the same metal with the same oxidation state but different ligands vary so much in color. There are also variations in the octahedral splitting energy with changing oxidation state of the metal and changing period number. This explains why when the oxidation number is changed the color of the complex changes.

Hence, the final answer is option (A) \[C{l^ - } < {F^ - } < {C_2}O_4^{2 - } < NO_2^ - < C{N^ - }\]

Note:
On the basis of these series the colour of the complex compound can be known. These series will allow us to know the strength of the ligand and the extent to e=which the shape of the complex will get distorted.