Crystal Structure, Bonding Features, Spectroscopic Characterization, and Thermal Decomposition of a New Hydrated Sodium Nickel Hydrogen Selenite–Selenite

The double salt NaNi2(HSeO3)(SeO3)2·2H2O was synthesized for the first time via a hydrothermal method and comprehensively characterized by single-crystal X-ray diffraction, IR spectroscopy, UV–Vis absorption spectroscopy, and thermal analysis. The crystal structure features a three-dimensional framework composed of isolated trigonal–pyramidal selenite ( $${\text{SeO}}_{3}^{{2 - }}$$ ) and hydrogen selenite ( $${\text{HSeO}}_{3}^{ - }$$ ) groups coordinated to octahedral Ni(II) centers, with sodium cations and crystallization water molecules stabilizing the lattice through an extended hydrogen-bonding network. Spectroscopic data confirm the coexistence of selenite and hydrogen selenite anions, as well as lattice water molecules, alongside intense ligand-to-metal charge-transfer transitions and characteristic d–d bands typical of octahedral Ni(II) complexes. Thermal analysis reveals a multistep decomposition process involving initial dehydration, followed by progressive destabilization of the hydrogen selenite and selenite groups, oxidative transformation to selenate species at elevated temperatures, and eventual collapse of the oxoselenite framework, yielding thermally stable nickel oxide as the final solid residue. This study provides detailed insight into the crystal architecture, bonding characteristics, and thermal stability of this mixed hydrogen selenite–selenite hydrate, contributing to the broader understanding of transition-metal selenite materials and their potential physicochemical applications.