Question

[co(h2O)6]+2 ion is more stable than [co(h2O)6]+3 ion justify it?

Answer 1

Answer on Question #85704 – Chemistry – Inorganic Chemistry

$\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ ion is more stable than $\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$ ion justify it?

Solution:

$\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ is more stable than $\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$ because $\mathrm{Co}^{2+}$ is more stabilised by weak field ligand, $\mathrm{H}_{2} \mathrm{O}$ than is $\mathrm{Co}^{3+}$ .

With weak-field ligands, the half-filled shell with all spins parallel has extra stability. It's the same reason that Cr has electron configuration [Ar] $4s^{1}3d^{5}$ rather than [Ar] $4s^{2}3d^{4}$ .

$\mathrm{Co}^{2+}(3d^7) = (t2g)5(eg)2$ has higher value of crystal field stabilization energy than $\mathrm{Co}^{3+}(3d^6) = (t2g)4(eg)2$ in the weak octahedral field leading to greater stability of $\mathrm{Co}^{2+}(aq)$ than $\mathrm{Co}^{3+}(aq)$ .

On the other hand, $\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{3+}$ is more stable than $\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}$ because $\mathrm{Co}^{3+}$ is more stabilised by strong field ligand $\mathrm{NH}_{3}$ than is $\mathrm{Co}^{2+}$ . This is due to higher value of crystal field stabilization energy for $\mathrm{Co}^{3+}(3 \mathrm{~d}^{6}) = (\mathrm{t} 2 \mathrm{~g}) 6(\mathrm{eg}) 0$ than $\mathrm{Co}^{2+}(3 \mathrm{~d}^{7}) = (\mathrm{t} 2 \mathrm{~g}) 6(\mathrm{eg}) 1$ in the strong octahedral field ligand $\mathrm{NH}_{3}$ leading to greater stability in former. The crystal field diagram is then:

Note that the full orbital level also has extra stability.

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