HexaaquanickelII

Hexaaquanickel(II) refers to the hexaaqua nickel(II) complex, [Ni(H2O)6]2+, the octahedrally coordinated aquo complex of Ni2+. In aqueous solution nickel(II) ions are predominantly present as this solvated cation, although hydrolysis at higher pH yields hydroxo species. The six water ligands establish an octahedral coordination environment around the nickel center. Nickel(II) in this complex is d8 and exhibits two unpaired electrons, making the ion paramagnetic. The complex is typically blue in color in solution, reflecting d–d transitions characteristic of nickel(II) aqua complexes. The hexaaquanickel(II) ion is relatively labile, with water ligands readily substituting by stronger donor ligands such as ammonia, amines, cyanide, or ethylenediamine, giving a range of nickel(II) complexes (for example [Ni(NH3)6]2+). In solid salts, the cation [Ni(H2O)6]2+ is balanced by various counterions, as in nickel(II) sulfate or nickel(II) chloride hydrates. This aquo complex serves as a common starting material in nickel coordination chemistry and electrochemistry. It can participate in ligand substitution reactions, hydrolysis, and complex formation with polydentate ligands to yield chelates. Safety considerations: nickel compounds can be hazardous; exposure should be minimised and handled according to safety data sheets.