Metal aquo complex

In chemistry, 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 . 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 aquo complexes are mono-nuclear, with the general formula , with n = 2 or 3; they have an octahedral structure. The water molecules function as Lewis bases, donating a pair of electrons to the metal ion and forming a dative covalent bond with it. Typical examples are listed in the following table. {| class=wikitable style=text-align:center !Complex!!colour!!electron config.!! distance (Å)||water exchangerate (s−1, 25 °C)!!M2+/3+ self-exchangerate (M−1s−1, 25 °C) |- | | violet | (t2g)1 |2.025 |1.8e5 | |- | | violet | (t2g)3 |2.12 |8.7e1 |fast |- | | green | (t2g)2 |1.991 |5.0e2 |fast |- | | blue | (t2g)3(eg)1 |2.06 and 2.33 |1.2e8 |slow |- | | violet | (t2g)3 |1.961 |2.4e-6 |slow |- | | pale pink | (t2g)3(eg)2 |2.177 |2.1e7 | |- | | pale blue-green | (t2g)4(eg)2 |2.095 |4.4e6 |fast |- | | pale violet | (t2g)3(eg)2 |1.990 |1.6e2 |fast |- | | pink | (t2g)5(eg)2 |2.08 |3.2e6 | |- | | green | (t2g)6(eg)2 |2.05 |3.2e4 | |- | | blue | (t2g)6(eg)3 |1.97 and 2.30 |5.7e9 | |- | | colorless | (t2g)6(eg)4 | 2.03-2.10 | fast | |- |} Tutton's salts are crystalline compounds with the generic formula (where M = , , , , , or ). Alums, , are also double salts. Both sets of salts contain hexa-aquo metal cations. Silver(I) forms , a rare example of a tetrahedral aquo complex. Palladium(II) and platinum(II) were once thought to form square planar aquo complexes. Aquo complexes of lanthanide(III) ions are eight- and nine-coordinate, reflecting the large size of the metal centres.