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.
How many unpaired electrons are present in [Fe(H2O)6]2+?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 4
Oxidation state of Fe: in \( \ce{[Fe(H2O)6]^2+} \) water is neutral, so x + 6(0) = +2 and x = +2.
Configuration: Fe (Z = 26) is [Ar]3d64s2; losing the two 4s electrons gives \( \ce{Fe^2+} \): 3d6.
Type of ligand: towards \( \ce{Fe^2+} \), water acts as a weak field ligand (Δo < P), so the 3d electrons are not forced to pair and the complex is high spin. The full spin pairing of d6 ions described in the passage needs strong field ligands.
Filling the d orbitals: t2g4eg2.
Paired orbitals: one t2g orbital holds a pair.
Unpaired electrons: two t2g and both eg orbitals hold single electrons, so there are 4 unpaired electrons.
Check: Table 4.7 lists 4 unpaired electrons for \( \ce{Fe^2+} \); its data are mainly for hydrated ions.
NCERT (2026-27 edition):
5.5.4 Crystal Field Theory – “(i) If Δo < P, the fourth electron enters one of the eg orbitals giving the configuration t2g3eg1. Ligands for which Δo < P are known as weak field ligands and form high spin complexes.”
4.3.9 Magnetic Properties – “… The magnetic moments calculated from the ‘spin-only’ formula and those derived experimentally for some ions of the first row transition elements are given in Table 4.7. The experimental data are mainly for hydrated ions in solution or in the solid state.”
Table 4.7: Calculated and Observed Magnetic Moments (BM)
