Cluster-Continuum Calculations of Coordination Structures and Formation Constants for Aqueous Cadmium Halide Complexes

This
study investigates the coordination structures and formation
constants of cadmium halide complexes (CdXn(H2O)(CN–n)(2–n)+, X = F, Cl, Br, and I; n = 1,
2, 3, and 4; and CN = 3, 4, 5, and 6) using a quantum-chemical cluster-continuum
model. By comparing the relative energies of 24 different coordination
structures for each category of CdXn(H2O)(CN–n)(2–n)+ (X = F, Cl, Br, and I), it is found that all four
types of complexes prefer low-coordination structures, primarily existing
in tri- or tetra-coordination. In calculating the solvation energy
of halide ions using different thermodynamic cycle methods, it is
found that the cluster cycle yields results closer to experimental
values than the monomer cycle. Halide ions with high electronegativity
and small radii require more water molecules to form stable solvation
structures. In complex formation constant (log K)
calculations, the log K values from monomer cycles
are generally larger than those from the cluster cycles. The accuracy
of log K estimation is jointly affected by the thermodynamic
cycle type, the solvation characteristics of the halide ion, and the
entropy change of the reaction. Accurate log K determination
for cadmium halide complexes is enabled by the “cluster-size-matching”
approach. This work aids in predicting the coordination structures
of heavy metal–halide complexes, especially offering a feasibility
reference for thermodynamic cycle methods in calculating their thermodynamic
parameters.