Consider the following statements about the magnetic properties of coordination compounds and the valence bond theory.
(A) The valence bond theory explains the magnetic data of some complexes in terms of the formation of inner orbital and outer orbital coordination entities.
(B) The spin only magnetic moment of [MnBr4]2− is 5.9 BM, which shows that the complex ion is square planar rather than tetrahedral.
(C) For metal ions with up to three electrons in the d orbitals, like Cr3+, the magnetic behaviour of the free ions and their coordination entities is similar.
(D) For d4 ions such as Cr2+ and Mn3+, a vacant pair of 3d orbitals for octahedral hybridisation results by pairing of the four 3d electrons in the remaining three 3d orbitals, which leaves no unpaired electron.
(E) The valence bond theory does not distinguish between weak and strong ligands.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B) and (D) only
The magnetic moment of a complex tells how many unpaired electrons it has. In the valence bond theory this number shows which d orbitals are free for hybridisation, and so the likely shape.
(A) The valence bond theory explains the magnetic data of some complexes in terms of the formation of inner orbital and outer orbital coordination entities. — Correct statement. Complexes of the same metal ion with different ligands can have different numbers of unpaired electrons; the theory explains this by the use of inner (3d) or outer (4d) orbitals in hybridisation.
(B) The spin only magnetic moment of [MnBr4]2− is 5.9 BM, which shows that the complex ion is square planar rather than tetrahedral. — Incorrect statement. 5.9 BM means five unpaired electrons in Mn2+ (3d5), so no 3d orbital is free for dsp2. The ion is tetrahedral (sp3), not square planar.
(C) For metal ions with up to three electrons in the d orbitals, like Cr3+, the magnetic behaviour of the free ions and their coordination entities is similar. — Correct statement. With d1 to d3, two 3d orbitals are already empty for octahedral hybridisation, so no pairing is needed.
(D) For d4 ions such as Cr2+ and Mn3+, a vacant pair of 3d orbitals for octahedral hybridisation results by pairing of the four 3d electrons in the remaining three 3d orbitals, which leaves no unpaired electron. — Incorrect statement. Four electrons spread over three 3d orbitals give one pair and two unpaired electrons. No unpaired electron is left only in the d6 case (three pairs).
(E) The valence bond theory does not distinguish between weak and strong ligands. — Correct statement. This is one of the limitations of the valence bond theory.
NCERT (2026-27 edition):
5.5.2 Magnetic Properties of Coordination Compounds – “For metal ions with upto three electrons in the d orbitals, like Ti3+ (d1); V3+ (d2); Cr3+ (d3); two vacant d orbitals are available for octahedral hybridisation with 4s and 4p orbitals. The magnetic behaviour of these free ions and their coordination entities is similar. … Thus, for d4 (Cr2+, Mn3+), d5 (Mn2+, Fe3+), d6 (Fe2+, Co3+) cases, a vacant pair of d orbitals results only by pairing of 3d electrons which leaves two, one and zero unpaired electrons, respectively.”
Example 5.7 – “The spin only magnetic moment of [MnBr4]2– is 5.9 BM. … But the fact that the magnetic moment of the complex ion is 5.9 BM, it should be tetrahedral in shape rather than square planar because of the presence of five unpaired electrons in the d orbitals.”
5.5.2 Magnetic Properties of Coordination Compounds – “However, with species containing d4 and d5 ions there are complications. … This apparent anomaly is explained by valence bond theory in terms of formation of inner orbital and outer orbital coordination entities.”
5.5.3 Limitations of Valence Bond Theory – “…(vi) It does not distinguish between weak and strong ligands.”