Which of the following inner orbital octahedral complex ions are diamagnetic?
All the ligands are strong and would thus force electron pairing given a chance! But from the given, both Co3+ and Fe+2 both have the d6 configuration, which means there just won't be any unpaired electrons. Three of the d-orbitals are completely filled - paired because of strong field ligands. To accommodate six ligands, with two unfilled d-orbitals, the hybridization then becomes d2sp3. Hence, these two are diamagnetic!
Let us look at the electronic configuration for the Co3+ complex:
For the ferrocyanide anion, the lower t2g gets completely filled (low-spin configuration) because of the strong field ligand and high Δoct: