Is the [Co(H2O)6]3+ complex ion expected to be high or low spin?
SOLUTION
What do we know?
The complex has an octahedral shape
The aqua ligand (H2O) is typically regarded as weak-field ligand
The d electron configuration for Co is d6
The splitting energy is small
Therefore, the complex is expected to be high spin.
Strong-field ligands, like CN- and NO2-, increase Δ which results in low spin.
d6
Octahedral high-spin: 4 unpaired electrons, paramagnetic, substitutionally labile. Includes Fe2+ ionic radius 78 pm, Co3+ ionic radius 61 pm.
Octahedral low-spin: no unpaired electrons, diamagnetic, substitutionally inert. Includes Fe2+ ionic radius 62 pm, Co3+ ionic radius 54.5 pm, Ni4+ ionic radius 48 pm.