Which of the following is not a limitation of valence bond theory (VBT)?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → It does not explain the concept of the high spin and low spin complex.
The valence bond theory explains the formation, shape and magnetic behaviour of complexes through hybridisation of the metal orbitals. It explains high spin and low spin complexes as outer orbital and inner orbital complexes. In \( \ce{[Co(NH3)6]^3+} \), the 3d electrons of \( \ce{Co^3+} \) pair up, inner d orbitals are used (d2sp3), and the complex is low spin and diamagnetic. In \( \ce{[CoF6]^3-} \), the outer 4d orbitals are used (sp3d2), four electrons stay unpaired, and the complex is high spin.
Option (1) It does not distinguish between weak and strong ligands. — A limitation of VBT. The theory cannot say in advance which ligands force electron pairing; that has to be read from the measured magnetic moment.
Option (2) It does not explain the color exhibited by the coordination complex. — A limitation of VBT. Colour arises from d–d transitions between split d levels, which the valence bond theory does not consider; crystal field theory explains it.
Option (3) It does not explain the concept of the high spin and low spin complex. — Not a limitation of VBT. The theory explains high spin and low spin complexes as outer orbital and inner orbital complexes.
Option (4) It does not give predictions regarding the tetrahedral and square plane complex in coordination number 4. — A limitation of VBT. For a four-coordinate complex the theory cannot predict with certainty whether the shape will be tetrahedral (sp3) or square planar (dsp2).
NCERT (2026-27 edition):
Limitations of Valence Bond Theory – …
Valence Bond Theory – “… 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. The paramagnetic octahedral complex, [CoF6]3– uses outer orbital (4d) in hybridisation (sp3d2). It is thus called outer orbital or high spin or spin free complex.”