Inner orbital complex among the following is:
(A) \([Co(NH_3)_6]^{3+}\)
(B) \([CoF_6]^{3-}\)
(C) \([Ni(CN)_4]^{2-}\)
(D) \([MnCl_6]^{3-}\)
(E) \([FeF_6]^{3-}\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A) and (C) only
A complex is an inner orbital (low spin) complex when the inner (n−1)d orbitals are used in hybridisation, i.e. d2sp3 or dsp2. This happens with strong field ligands that pair up the d electrons.
(A) \( \ce{[Co(NH3)6]^3+} \) — Inner orbital complex. Co (Z = 27) is [Ar]3d74s2; with neutral \( \ce{NH3} \), x = +3, so \( \ce{Co^3+} \) is 3d6. \( \ce{NH3} \) pairs the six electrons into three 3d orbitals, leaving two 3d orbitals for d2sp3; diamagnetic.
(B) \( \ce{[CoF6]^3-} \) — Outer orbital complex. x + 6(−1) = −3 gives \( \ce{Co^3+} \), 3d6; weak field \( \ce{F-} \) leaves four electrons unpaired, so 4d orbitals are used: sp3d2.
(C) \( \ce{[Ni(CN)4]^2-} \) — Inner orbital complex. Ni (Z = 28) is [Ar]3d84s2; x + 4(−1) = −2 gives \( \ce{Ni^2+} \), 3d8. \( \ce{CN-} \) pairs the electrons into four 3d orbitals, freeing one 3d orbital for dsp2 (square planar); diamagnetic.
(D) \( \ce{[MnCl6]^3-} \) — Outer orbital complex. Mn (Z = 25) is [Ar]3d54s2; x + 6(−1) = −3 gives \( \ce{Mn^3+} \), 3d4; weak field \( \ce{Cl-} \) leaves four electrons unpaired: sp3d2.
(E) \( \ce{[FeF6]^3-} \) — Outer orbital complex. Fe (Z = 26) is [Ar]3d64s2; x + 6(−1) = −3 gives \( \ce{Fe^3+} \), 3d5; weak field \( \ce{F-} \) leaves five electrons unpaired: sp3d2.
NCERT (2026-27 edition):
Valence Bond Theory – “… In the diamagnetic octahedral complex, [Co(NH3)6]3+, the cobalt ion is in +3 oxidation state and has the electronic configuration 3d6. Six pairs of electrons, one from each NH3 molecule, occupy the six hybrid orbitals. Thus, the complex has octahedral geometry and is diamagnetic because of the absence of unpaired electron. In the formation of this complex, since the inner d orbital (3d) is used in hybridisation, the complex, [Co(NH3)6]3+ is called an inner orbital or low spin or spin paired complex.”