page_title: Metal aquo complex
section_title: Stoichiometry and structure
hierarchical_section_title: Metal aquo complex / Stoichiometry and structure
caption_attribution_description: English: structure of [M(water)6]n+
context_page_description: 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 [Mₙ]ᶻ⁺. Their behavior underpins many aspects of environmental, biological, and industrial chemistry.
context_section_description: The majority of aquo-complexes have the general formula [M(H₂O)₆]ⁿ⁺, with n= 2 or 3; they have an octahedral structure. Some examples are listed in the following table. 
  Tutton's salts, are crystalline compounds with the generic formula (NH₄)₂M(SO₄)₂(H₂O)₆ (M=V²⁺, Cr²⁺, Mn²⁺, Co²⁺, Ni²⁺ and Cu²⁺). Alums, MM'(SO₄)₂(H₂O)₁₂, are also double salts. Both sets of salts contain hexa-aquo metal cations. 
  Silver(I) forms [Ag(H₂O)₄]⁺, a rare example of a tetrahedral aquo complex. Palladium(II) and platinum(II) form square planar aquo complexes with the stoichiometry [M(H₂O)₄]²⁺. Aquo complexes of lanthanide(III) ions are eight- and nine-coordinate, reflecting the large size of the metal centres. Complexes such as [Mo₂(H₂O)₈]⁴⁺ and [Rh₂(H₂O)₁₀]⁴⁺ contain metal-metal bonds.
  Monomeric aquo complexes of Zr, Hf, Nb, Ta, Mo, W, Mn, Tc, Re and Os in oxidation states 4-7 cannot be detected.  The ion [M(H₂O)ₙ]ᵐ⁺ is subject to extensive hydrolysis. For example, [Ti(H₂O)₆]⁴⁺ is unknown: the hydrolyzed species [Ti(OH)₂(H₂O)ₙ]²⁺is the principal species in dilute solutions.  With the higher oxidation states the effective electrical charge on the cation is further reduced by the formation of oxo-complexes. For example vanadium(V) forms vanadyl complexes. The hypothetical reaction
  [V(H₂O)₆]⁵⁺ → [VO(H₂O)₅]³⁺ + 2H⁺
  goes to completion and the hexa-aquo ion cannot be detected in solutions of vanadium(V) compounds. With chromium(VI) and manganese (VII) only oxyanions are known.
caption: Structure of an octahedral metal aquo complex.
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