Question #44287

This question tests the student's ability to recognize a critical exception in Coordination Chemistry: while $H_2O$ is generally a weak field ligand (as seen with $Fe^{3+}$), the increased effective nuclear charge and oxidation state of $Co^{3+}$ increase the crystal field splitting energy ($\Delta_o$) such that $\Delta_o > P$.

This forces $[Co(H_2O)_6]^{3+}$ into a low-spin $t_{2g}^6 e_g^0$ configuration.

Students who blindly apply the spectrochemical series will assume a high-spin $t_{2g}^4 e_g^2$ configuration, leading to incorrect values for unpaired electrons ($x=4$), CFSE magnitude ($y=0.4$), and $e_g$ occupancy change ($z=0$), trapping them with the distractor 4.4.

Furthermore, it connects this electronic anomaly directly to the physical phenomenon of ionic radius contraction.