Question

The complex ion that will lose its crystal field stabilization energy upon oxidation of its metal to +3 state is :
(Phen =

and Ignore pairing energy)

• A. \(\mathrm{[Fe(phen)_3]^{2+}}\)
• B. \(\mathrm{[Zn(phen)_3]^{2+}}\)
• C. \(\mathrm{[Co(phen)_3]^{2+}}\)
• D. \(\mathrm{[Ni(phen)_3]^{2+}}\)

Hint:

Phen is a strong-field ligand, so these octahedral complexes are low spin. Compute CFSE (in \(\Delta_o\)) for the +2 ion and for the +3 ion and see where it decreases.

Answer 1

The Correct Option is A

Phen is a strong-field ligand: low-spin octahedral, pairing energy ignored.
Fe: \(\mathrm{Fe^{2+}}\) = d⁶ = \(t_{2g}^6\): CFSE = 6(-0.4) = -2.4\(\Delta_o\). \(\mathrm{Fe^{3+}}\) = d⁵ = \(t_{2g}^5\): CFSE = 5(-0.4) = -2.0\(\Delta_o\). CFSE decreases (some stabilisation is lost).
Zn: \(\mathrm{Zn^{2+}}\) = d¹⁰: CFSE = 0 (nothing to lose; \(\mathrm{Zn^{3+}}\) is not formed).
Co: \(\mathrm{Co^{2+}}\) = d⁷ = \(t_{2g}^6e_g^1\): CFSE = -2.4 + 0.6 = -1.8\(\Delta_o\). \(\mathrm{Co^{3+}}\) = d⁶ = \(t_{2g}^6\): -2.4\(\Delta_o\). CFSE increases in magnitude.
Ni: \(\mathrm{Ni^{2+}}\) = d⁸ = \(t_{2g}^6e_g^2\): -1.2\(\Delta_o\). \(\mathrm{Ni^{3+}}\) = d⁷ (low spin) = \(t_{2g}^6e_g^1\): -1.8\(\Delta_o\). CFSE increases in magnitude.
Only the Fe complex loses CFSE on oxidation.
Answer: (1) \(\mathrm{[Fe(phen)_3]^{2+}}\)