Tetrammine

Tetrammine is a descriptor used in coordination chemistry to refer to a metal complex that contains four ammine ligands, NH3, bound to a central metal ion. In an octahedral complex these four NH3 ligands occupy four coordination sites, while the remaining two sites are typically filled by other ligands such as water, chloride, or another anionic ligand, giving a general formula of [M(NH3)4(L)2]^n+.

Common metal centers for tetrammine complexes include nickel(II), cobalt(III), and copper(II). Well-known examples include diaquatetraammine cobalt(III) complexes, such as [Co(NH3)4(H2O)2]^3+; aquatetramminenickel(II), such as [Ni(NH3)4(H2O)2]^2+; and tetrammine copper(II), such as [Cu(NH3)4(H2O)2]^2+. The designation emphasizes four neutral ammine ligands rather than charged ammine species, with the remaining two coordination sites occupied by other neutral or anionic ligands.

Synthesis of tetrammine complexes typically involves substitution reactions where ammonia replaces labile ligands (often aqua ligands) in preformed metal hydrates or halide complexes in aqueous solution. Ammonia acts as a soft, moderate-field ligand, and the resulting complexes can exhibit characteristic color changes depending on the metal and oxidation state. Stability and reactivity depend on the metal central ion, the other ligands, and the overall charge of the complex.

Tetrammine compounds are commonly discussed in inorganic chemistry as examples of ligand substitution, coordination geometry, and ligand-field effects. Related terms include pentaammine and hexaammine complexes.