Mx6H2O

Mx6H2O is a term used in inorganic chemistry to denote a class of hexaaqua metal complexes, in which a central metal atom or ion M is coordinated by six water molecules. In many contexts the core species is written as the hexaaqua cation [M(H2O)6]n+, with six water ligands forming an octahedral ligand shell around the metal center. The exact charge and stoichiometry depend on the metal and its oxidation state; in solid hydrates, six coordinated water molecules may be accompanied by additional waters of crystallization in the lattice.

Structure and bonding within these complexes are characterized by a nearly octahedral arrangement of water ligands around M. The aqua ligands act as neutral ligands through lone-pair donation from the oxygen atoms, while the metal center provides the appropriate d-orbital framework to accommodate ligand field interactions. Because water is a relatively labile ligand, these aqua complexes readily undergo ligand substitution when reacting with stronger field ligands or bases.

Formation and occurrence: in aqueous solution, many metal ions spontaneously form six-coordinate aquated species, and when crystallized with counterions they can appear as hydrates containing the same [M(H2O)6] motif. Dehydration can occur upon heating or under certain chemical conditions, leading to changes in coordination number or to the formation of other hydrated species.

Properties and applications: aqua complexes exhibit color changes governed by the metal, and their electronic structures can be probed by UV–visible spectroscopy. They serve as fundamental models in coordination chemistry for studying ligand substitution kinetics, hydrolysis, and catalytic mechanisms in water. They also provide insight into the behavior of hydrated metal ions in solution and in solid-state materials.

See also: hexaaqua ion, aqua complex, coordination chemistry.