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.
Which of the following statements is incorrect?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Halide complexes are generally low spin complexes
The passage states that with weak ligands such as F−, Cl− and H2O the d-electrons are not rearranged (postulate vi), and that complexes using outer d-orbitals are outer orbital or high spin complexes (postulate vii).
Option (1) Halide complexes are generally low spin complexes — Incorrect about these complexes. Halide ions are weak ligands. The d electrons stay unpaired, no inner d orbitals are freed, and the metal uses outer d orbitals (sp3d2), so halide complexes are generally high spin.
Option (2) Metal carbonyls are low spin complexes — Correct about these complexes. CO is a strong ligand; it pairs up the d electrons, so the carbonyls are low spin.
Option (3) Octahedral complexes are formed when the central metal atom/ion is involved in d2sp3 or sp3d2 hybridization — Correct about these complexes. Both sets give six hybrid orbitals pointing to the corners of an octahedron, one for each of six ligands.
Option (4) Octahedral complexes with d2sp3 hybridization are low spin complexes while with sp3d2 hybridization are high spin complexes — Correct about these complexes. d2sp3 uses the inner (n−1)d orbitals (inner orbital, low spin); sp3d2 uses the outer nd orbitals (outer orbital, high spin).