Which of the following is/are limitations of the valence bond theory?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Both (1) and (2)
Valence bond theory explains the formation, structures and magnetic behaviour of complexes through hybridisation, but it cannot handle energy-based properties such as colour and stability.
Option (1) It does not give a quantitative interpretation of the thermodynamic stabilities of coordination compounds. — A limitation. The theory gives no numbers for the stability of complexes.
Option (2) It does not explain the colour exhibited by coordination compounds. — A limitation. It does not consider the splitting of the d orbitals, so it cannot account for the absorption of visible light.
Option (3) It cannot explain the octahedral geometry of [Co(NH3)6]3+. — Not a limitation.
Valence bond theory does explain it: \( \ce{Co^3+} \) (3d6) pairs its electrons.
Two 3d, one 4s and three 4p orbitals form six d2sp3 hybrid orbitals that hold the six \( \ce{NH3} \) electron pairs, giving an octahedral, diamagnetic complex.
NCERT (2026-27 edition):
5.5.3 Limitations of Valence Bond Theory – “While the VB theory, to a larger extent, explains the formation, structures and magnetic behaviour of coordination compounds, it suffers from the following shortcomings: … (iii) It does not explain the colour exhibited by coordination compounds. (iv) It does not give a quantitative interpretation of the thermodynamic or kinetic stabilities of coordination compounds.”
5.5.1 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.”