Looking for classes? Ksquare Career Institute, Bengaluru →
In which of the following co-ordination entities the magnitude of $\Delta_0$ (CFSE in octahedral field) will be maximum ?
(Atomic number of Co = 27)
(Atomic number of Co = 27)
A
$[Co(CN)_6]^{3-}$
B
$[Co(C_2O_4)_3]^{3-}$
C
$[Co(H_2O)_6]^{3+}$
D
$[Co(NH_3)_6]^{3+}$
Detailed Solution
In all four complexes cobalt is in the +3 oxidation state ($3d^6$) in an octahedral field, so the metal ion and the geometry are the same.
The CFSE in an octahedral field then depends only upon the nature of the ligands: stronger the ligand, larger will be the value of $\Delta_{oct}$.
Spectrochemical series (increasing field strength): $I^- \lt Br^- \lt Cl^- \lt F^- \lt OH^- \lt C_2O_4^{2-} \lt H_2O \lt NH_3 \lt en \lt CN^- \lt CO$
Among the given ligands the order of field strength is: $C_2O_4^{2-} \lt H_2O \lt NH_3 \lt CN^-$
$CN^-$ is the strongest field ligand here, so it causes the largest splitting.
Hence $\Delta_0$ is maximum for $[Co(CN)_6]^{3-}$.
The CFSE in an octahedral field then depends only upon the nature of the ligands: stronger the ligand, larger will be the value of $\Delta_{oct}$.
Spectrochemical series (increasing field strength): $I^- \lt Br^- \lt Cl^- \lt F^- \lt OH^- \lt C_2O_4^{2-} \lt H_2O \lt NH_3 \lt en \lt CN^- \lt CO$
Among the given ligands the order of field strength is: $C_2O_4^{2-} \lt H_2O \lt NH_3 \lt CN^-$
$CN^-$ is the strongest field ligand here, so it causes the largest splitting.
Hence $\Delta_0$ is maximum for $[Co(CN)_6]^{3-}$.