The plot of the magnitude of hydration enthalpy ( | _ hyd H| ) versus atomic number for the divalent ( M²⁺ ) ions of the 3d transition series exhibits a general upward trend but features a distinct local minimum (a dip in magnitude) near the middle of the series, marked as point 'X'. Identify the metal ion corresponding to point 'X' and the primary reason for this anomalous dip in the magnitude of hydration enthalpy.

Step-by-step solution

The magnitude of hydration enthalpy generally increases across the 3d series due to the decrease in ionic size. However, the actual values show a 'double-hump' curve because of the additional thermodynamic stabilization provided by the Crystal Field Stabilization Energy (CFSE) in the octahedral aqueous field ( [M(H₂O)₆]²⁺ ). For the d^5 high-spin configuration of Mn ²⁺ , the CFSE is zero ( t_ 2g ^3 e_g^2 ). Because it lacks this extra stabilization, its hydration enthalpy magnitude is lower than expected from the g