Hexaaqua

Hexaaqua refers to a coordination motif in which six water molecules act as ligands surrounding a central metal ion, forming a hexaaquometal complex. The usual representation is [M(H2O)6]n+, where M is a metal and n reflects the overall charge of the complex.

In most aqueous solutions the geometry is octahedral, with six M–O bonds to water ligands. These aqua ligands exchange rapidly with surrounding water, so the complex is typically described as labile. The color and spectral properties arise from d-electron transitions of the central metal and vary with both metal identity and oxidation state.

Common examples include the hexaaquairon(III) ion [Fe(H2O)6]3+ and the hexaaquacopper(II) ion [Cu(H2O)6]2+. In solution these ions can undergo hydrolysis, where a proton is removed from a coordinated water ligand to form hydroxo complexes such as [M(H2O)5(OH)]2+ or analogous species, depending on pH and metal. Further deprotonation can yield polynuclear or polymeric hydroxo species at higher pH. The pKa values for aqua ligands vary across metals and influence hydrolysis behavior, solubility, and catalytic activity.

Hexaaqua complexes are foundational in inorganic chemistry, appearing as intermediates in metal hydration, ligand binding, acid–base equilibria, and catalysis. They provide a straightforward model for studying ligand-field effects, hydration energetics, and the interplay between a metal center and solvent.