Choose the Incorrect option regarding valence bond theory of coordination compound.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → It does not distinguish between outer orbital and inner orbital complexes.
Valence bond theory (VBT) treats a complex as the metal ion using a set of hybrid orbitals (sp3, dsp2, d2sp3, sp3d2) to accept electron pairs from the ligands. The type of hybridisation gives the geometry, and the number of unpaired electrons left on the metal gives the magnetic behaviour. VBT explains formation, structure and magnetism only in a qualitative way, and this is where its limitations lie.
Option (1) It does not give a quantitative interpretation of the thermodynamic stabilities of coordination compounds. — A correct statement. VBT gives no energy terms for the metal–ligand bonds, so it cannot calculate or compare stabilities of complexes in numbers. This is a real limitation.
Option (2) It does not explain the colour exhibited by coordination compounds. — A correct statement. Colour comes from d–d transitions between split d orbitals. VBT does not split the d orbitals, so it cannot explain colour; crystal field theory does.
Option (3) It does not give quantitative interpretation of magnetic data. — A correct statement. VBT only counts unpaired electrons to say whether a complex is paramagnetic or diamagnetic. It cannot account for the measured values of magnetic moments.
Option (4) It does not distinguish between outer orbital and inner orbital complexes. — The incorrect statement. Making this distinction is in fact a strength of VBT. When the inner 3d orbitals are used (d2sp3), the complex is an inner orbital (low spin) complex, e.g. [Co(NH3)6]3+. When the outer 4d orbitals are used (sp3d2), it is an outer orbital (high spin) complex, e.g. [CoF6]3−. What VBT cannot do is distinguish between weak and strong ligands, that is, say in advance which ligands will give which type.
NCERT (2026-27 edition):
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:
(i) It involves a number of assumptions.
(ii) It does not give quantitative interpretation of magnetic data.
(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.
(v) It does not make exact predictions regarding the tetrahedral and square planar structures of 4-coordinate complexes.
(vi) It does not distinguish between weak and strong ligands.”
Valence Bond Theory – “… In the formation of this complex, 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.”