Question

Which order is correct in the spectrochemical series of ligands?
A. \[\;C{l^ - } < {F^ - } < {C_2}{O_4}{^{2 - }} < N{O_2}^ - < C{N^ - }\]
B.\[C{N^ - } < {C_2}{O_4}{^{2 - }} < C{l^ - } < N{O_2}^ - < {F^ - }\]
C.\[{C_2}{O_4}{^{2 - }} < {F^ - } < C{l^ - } < N{O_2}^ - < C{N^ - }\]
D.\[{F^ - } < C{l^ - } < N{O_2}^ - < C{N^ - } < {C_2}{O_4}{^{2 - }}\]

Answer 1

Hint: A spectrochemical series is a list of ligands which are ordered on ligand strength and a list of metal ions based on the oxidation number, group and its identity. The ligands are ranked in the spectrochemical series according to the energy difference ${\Delta _0}$ between the \[{t_{2g}}\] and \[{e_g}\;\] orbitals in their octahedral complexes and this energy difference is measured in the spectral transition between these levels, which often lies in the visible part of the spectrum and it is responsible for the colors of the complexes with partially filled d-orbitals.

Complete answer:
The order of the spectrochemical series can be derived from the understanding that ligands are frequently classified by their donor or acceptor abilities.
The correct order for the spectrochemical series of ligands is
\[C{l^ - } < {F^ - } < {C_2}{O_4}{^{2 - }} < N{O_2}^ - < C{N^ - }\]
And, this order is based on the increasing field strength and is determined from experimental measurements.
The Ligands that are arranged on the left end of this spectrochemical series are normally regarded as the weaker ligands and they cannot cause forcible pairing of electrons within the 3d level, and thus they form outer orbital octahedral complexes that are high spin. Whereas, the ligands that are at the right end are stronger ligands and they form inner orbital octahedral complexes after forcible pairing of electrons within 3d level and hence they are called low spin ligands.

Therefore, the correct answer is option (A).

Note: The spectrochemical series was first proposed in 1938 based on the results of absorption spectra of cobalt complexes. Ligands that produce a large splitting are called strong field ligands, whereas those that produce a small splitting are known as weak field ligands.