To explain bonding in complexes, the valence bond theory was proposed by Linus Pauling. The main postulates of VB theory are as follows:
(v) In the presence of strong ligands such as CN−, NO, CO, the d-electrons are rearranged vacating some d-orbitals (only when there are more than three d-electrons) which can then take part in hybridisation.
(vi) In the presence of weak ligands such as F−, Cl−, H2O, etc., the d-electrons are not rearranged.
(vii) The d-orbitals involved in hybridisation may be either (n−1)d orbitals or outer d-orbitals.
The complexes formed by the involvement of (n−1)d orbitals in hybridisation are called inner orbital complexes or low spin complexes. The complexes formed by the involvement of outer d-orbitals are called outer orbital complexes or high spin complexes.
(viii) Each ligand contains a lone pair of electrons. A dative bond is formed by the overlap of a vacant hybrid orbital of metal ion and a filled orbital of ligand.
(ix) The complex will be paramagnetic, if unpaired electrons are present; otherwise it is diamagnetic.
What is the spin-only magnetic moment of the tetrahedral complex ion \( \ce{[NiCl4]^2-} \)? (Atomic number of Ni = 28)
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 2.83 BM
The spin-only magnetic moment depends only on the number of unpaired electrons (n): \( \mu = \sqrt{n(n+2)} \) BM. In a tetrahedral complex the metal uses one 4s and three 4p orbitals, so its 3d electrons are not disturbed.
Oxidation state of Ni: x + 4(−1) = −2, so x = +2.
Ni (Z = 28): [Ar] 3d8 4s2; Ni2+: [Ar] 3d8.
\( \ce{Cl^-} \) is a weak ligand; the hybridisation is sp3 and the eight 3d electrons stay as three pairs and two single electrons, n = 2.
\( \mu = \sqrt{2(2+2)} = \sqrt{8} = 2.83 \) BM.
NCERT (2026-27 edition):
5.5.1 Valence Bond Theory – “In tetrahedral complexes one s and three p orbitals are hybridised to form four equivalent orbitals oriented tetrahedrally. This is illustrated … for [NiCl4]2–. Here nickel is in +2 oxidation state and the ion has the electronic configuration 3d8.” … “Each Cl– ion donates a pair of electrons. The compound is paramagnetic since it contains two unpaired electrons.”
4.3.9 Magnetic Properties – “…For these, the magnetic moment is determined by the number of unpaired electrons and is calculated by using the ‘spin-only’ formula, i.e., μ = \( \sqrt{n(n+2)} \) where n is the number of unpaired electrons and μ is the magnetic moment in units of Bohr magneton (BM).”