Cluster-continuum DFT assessment of temperature and isotope effects on the stability of Fe, Ni, Zn, and Cr Aquacomplexes

Transition-metal aquacomplexes are central to aqueous chemistry, but their computed thermodynamic stability strongly depends on electronic-structure and solvation models. Here, model formation/hydration free energies of Zn2+, Fe2+/Fe3+, Ni2+/Ni3+, and Cr2+/Cr3+ aquacomplexes were evaluated using a cluster-continuum framework combining explicit inner-sphere water ligands with continuum solvation. Hartree-Fock, hybrid and meta-GGA functionals, D3BJ dispersion, and CPCM were compared consistently. Temperature effects from 298 to 500 K and H2O/D2O/T2O isotope substitution were analyzed within RRHO thermochemistry. Spin-state checks supported quintet Cr2+, whereas doublet Ni3+ was lower than the quartet production surface, making Ni3+ trends spin-surface-specific. A three-point redox comparison reproduced the Cr3+/Cr2+ < Fe3+/Fe2+ < Ni3+/Ni2+ sequence and was used only qualitatively. Absolute free energies showed strong method dependence, whereas temperature, charge-state, and isotope trends were more consistent. COSMO-RS calculations further indicated that temperature-dependent water solvation strongly controls aquacomplex thermodynamics.