The experimental hydration enthalpies of the 3d series M²⁺ ions deviate from the theoretical values calculated based purely on electrostatics (which assume a smooth decrease in ionic size). This extra thermodynamic stabilization is primarily due to the aqueous ligand field. Among the following ions, for which ion is the magnitude of this deviation (extra stabilization) maximum?

Step-by-step solution

The deviation between the experimental hydration enthalpy and the theoretical electrostatic hydration enthalpy is equal to the Crystal Field Stabilization Energy (CFSE) of the octahedral aqua complex, [M(H₂O)₆]²⁺ . Water is a weak field ligand, so these ions form high-spin complexes (except for d^8 , which has only one configuration). Let us calculate the CFSE for each ion in terms of the octahedral splitting parameter ( _o ): Cr ²⁺ ( 3d^4 high-spin): t_ 2g ^3 e_g^1 CFSE = 3(-0.4 _o) + 1(0.6 _o) = -0.6 _o Mn ²⁺ ( 3