Valence Bond Theory
Important features of Valence Bond Theory (VBT) are:
(1) The central metal ion provides as many hybrid orbitals as are required by the ligands, that is, equal to the coordination number of the complex.
(2) Each ligand has at least one lone pair of electrons.
(3) The lone pairs of electrons of the ligands overlap with the empty hybrid orbitals of the metal ion.
(4) If d orbitals are involved in hybridisation, they may be either the inner (n−1)d orbitals or the outer nd orbitals. The complexes formed in these two ways are known as inner orbital and outer orbital complexes respectively. Generally, inner orbital complexes are low spin and outer orbital complexes are high spin.
(5) If the complex contains one or more unpaired electrons, it is paramagnetic; if it contains no unpaired electron, it is diamagnetic.
What is the spin-only magnetic moment of the complex ion [FeF6]4−? (Atomic number of Fe = 26)
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(\sqrt{24}\) BM
The spin-only magnetic moment depends only on the number of unpaired electrons, n. Working for \( \ce{[FeF6]^4-} \):
Oxidation state of Fe: x + 6(−1) = −4, so x = +2.
Configuration: Fe (Z = 26) is [Ar]3d64s2; losing the two 4s electrons gives \( \ce{Fe^2+} \): 3d6.
Type of ligand: \( \ce{F^-} \) lies near the weak end of the spectrochemical series, so it is a weak field ligand (Δo < P) and the complex is high spin.
Filling the d orbitals: t2g4eg2: one t2g orbital holds a pair, the other four d orbitals hold one electron each.
Unpaired electrons: n = 4.
\( \displaystyle \mu=\sqrt{n(n+2)}\ \mathrm{BM} \)
\( \displaystyle \mu=\sqrt{4(4+2)}\ \mathrm{BM} \)
\( \displaystyle \mu=\sqrt{24}\ \mathrm{BM}\approx 4.9\ \mathrm{BM} \)
NCERT (2026-27 edition):
5.5.4 Crystal Field Theory – “… In general, ligands can be arranged in a series in the order of increasing field strength as given below: I– < Br– < SCN– < Cl– < S2– < F– < OH– < C2O42– < H2O < NCS– < edta4– < NH3 < en < CN– < CO Such a series is termed as spectrochemical series.”
“(i) If Δo < P, the fourth electron enters one of the eg orbitals giving the configuration t2g3eg1. Ligands for which Δo < P are known as weak field ligands and form high spin complexes.”