Aquo complex
Metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorates. They have the general stoichiometry [M(H2O)n]z+. Their behavior underpins many aspects of environmental, biological, and industrial chemistry. This article focuses on complexes where water is the only ligand ("homoleptic aquo complexes"), but of course many complexes are known to consist of a mix of aquo and other ligands.
Most common are the octahedral complexes with the formula [M(H2O)6]2+ and [M(H2O)6]3+. Some members of this series are listed in the table below. A few aquo complexes exist with coordination numbers lower than six. Palladium(II) and platinum(II), for example, form square planar aquo complexes with the stoichiometry [M(H2O)4]2+. Aquo complexes of the lanthanide trications are eight- and nine-coordinate, reflecting the large size of the metal centres.
Aquo complexes of about one third of the transition metals (Zr, Hf, Nb, Ta, Mo, W, Tc, Re, Os and Au) are either unknown or rarely described. Aquo complexes of M4+ centres would be extraordinarily acidic. For example, [Ti(H2O)6]4+ is unknown, but [Ti(H2O)6]3+ is well characterized. This acidification is related to the stoichiometry of the Zr(IV) aquo complex [Zr4(OH)12(H2O)16]8+ (see zirconyl chloride. Similarly, [V(H2O)6]5+ is unknown, but its conjugate base, [VO(H2O)5]2+ is highly stable. Univalent metal centres such as Cu(I) and Rh(I) rarely form isolable complexes with water. Ag(I) form tetrahedral [Ag(H2O)4]+, a rare example of a tetrahedral aquo complex.
Some aquo complexes also contain metal-metal bonds. Two examples are [Mo2(H2O)8]4+ and [Rh2(H2O)10]4+.
Many transition metal salts are aquo complexes, not only in solution but also as solids. A wide range of stoichiometries are known. Many complexes also crystallize with water of hydration, wherein some of the water is not an aquo ligand, but stabilizes the structure by noncovalent interactions.
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