A critical study of the magnetic data of coordination compounds of metals of the first transition series reveals some complications. For metal ions with up to three electrons in the d orbitals, like Ti3+(d1); V3+(d2); Cr3+(d3); two vacant d orbitals are available for octahedral hybridization with 4s and 4p orbitals. The magnetic behaviour of these free ions and their coordination entities is similar. When more than three 3d electrons are present, the required pair of 3d orbitals for octahedral hybridization is not directly available (as a consequence of Hund’s rule). 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. The magnetic data agree with maximum spin pairing in many cases, especially with coordination compounds containing d6 ions. However, with species containing d4 and d5 ions there are complications.
Consider the following statements about the magnetic properties of coordination compounds.
(A) The magnetic moment of coordination compounds can be measured by magnetic susceptibility experiments.
(B) \( \ce{[MnCl6]^3-} \) is an inner orbital complex involving d2sp3 hybridisation.
(C) \( \ce{[Fe(CN)6]^3-} \) has a magnetic moment of a single unpaired electron.
(D) The valence bond theory does not give a quantitative interpretation of magnetic data.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (C) and (D) only
The measured magnetic moment tells us the number of unpaired electrons in a complex; the valence bond theory then explains it as an inner orbital (d2sp3) or an outer orbital (sp3d2) complex.
(A) The magnetic moment of coordination compounds can be measured by magnetic susceptibility experiments. — True. These experiments give the magnetic moment, and so the number of unpaired electrons.
(B) \( \ce{[MnCl6]^3-} \) is an inner orbital complex involving d2sp3 hybridisation. — False. Weak \( \ce{Cl^-} \) leaves the four 3d electrons of Mn3+ unpaired, so the metal uses outer 4d orbitals: it is an outer orbital complex (sp3d2) with four unpaired electrons.
(C) \( \ce{[Fe(CN)6]^3-} \) has a magnetic moment of a single unpaired electron. — True. Strong \( \ce{CN^-} \) pairs the five 3d electrons of Fe3+ into three orbitals, leaving one unpaired electron (d2sp3).
(D) The valence bond theory does not give a quantitative interpretation of magnetic data. — True. This is one of the limitations of the theory.
NCERT (2026-27 edition):
5.5.2 Magnetic Properties of Coordination Compounds – “The magnetic moment of coordination compounds can be measured by the magnetic susceptibility experiments. The results can be used to obtain information about the number of unpaired electrons and hence structures adopted by metal complexes.”
“…[Fe(CN)6]3– has magnetic moment of a single unpaired electron while [FeF6]3– has a paramagnetic moment of five unpaired electrons.” … “On the other hand, [MnCl6]3–, [FeF6]3– and [CoF6]3– are outer orbital complexes involving sp3d2 hybridisation and are paramagnetic corresponding to four, five and four unpaired electrons.”
5.5.3 Limitations of Valence Bond Theory – “(ii) It does not give quantitative interpretation of magnetic data.”