Question

Which of the following complex ions has electrons that are symmetrically filled in both \(t_{2g}\) and \(e_g\) orbitals ?

• A. \(\mathrm{[CoF_6]^{3-}}\)
• B. \(\mathrm{[Co(NH_3)_6]^{2+}}\)
• C. \(\mathrm{[Mn(CN)_6]^{4-}}\)
• D. \(\mathrm{[FeF_6]^{3-}}\)

Hint:

Symmetrical filling means each of the five d orbitals is half-filled (t2g3 eg2), which is the high-spin d5 case.

Answer 1

The Correct Option is D

Symmetrical filling in both sets means \(t_{2g}^3 e_g^2\) (every d orbital singly occupied), i.e. high-spin \(\mathrm{d^5}\).
(1) \(\mathrm{Co^{3+}}\) is \(\mathrm{d^6}\): \(t_{2g}^4 e_g^2\) (not symmetric).
(2) \(\mathrm{Co^{2+}}\) is \(\mathrm{d^7}\): \(t_{2g}^5 e_g^2\) (not symmetric).
(3) \(\mathrm{Mn^{2+}}\) is \(\mathrm{d^5}\) but with strong-field \(\mathrm{CN^-}\) it is low spin: \(t_{2g}^5 e_g^0\).
(4) \(\mathrm{Fe^{3+}}\) is \(\mathrm{d^5}\) and \(\mathrm{F^-}\) is a weak-field ligand: high spin \(t_{2g}^3 e_g^2\), symmetric.
Answer: (4) \(\mathrm{[FeF_6]^{3-}}\)