pentaammine

Pentaammine is a term used in coordination chemistry to describe a complex in which five ammine (NH3) ligands are bound to a central metal ion. The remaining coordination site is occupied by another ligand, most commonly a halide or water, resulting in a six-coordinate, typically octahedral complex of the form [M(NH3)5L]n+. The ammine ligands are neutral, so the overall charge depends on the metal’s oxidation state and the identity of L.

A well-known example is pentaamminechlorocobalt(III) chloride, with the complex cation [Co(NH3)5Cl]2+ and two chloride counterions, giving the salt [Co(NH3)5Cl]Cl2. In this complex, five ammonia ligands and one chloride ligand complete the octahedral coordination sphere around cobalt(III). Similar pentaammine variants exist for other metals by substituting the sixth ligand (L) with different anions or neutral ligands, producing salts of the form [M(NH3)5L]n+.

Preparation commonly involves ligand substitution of a hexammine complex, replacing one NH3 ligand with another ligand such as Cl−, Br−, or H2O to yield [M(NH3)5L]n+. These complexes illustrate the general principle that five ammonia ligands can coordinate alongside a sixth ligand to form stable, six-coordinate species.

In name and usage, pentaammine highlights the five NH3 ligands; the overall properties depend on the metal, its oxidation state, and the identity of the sixth ligand.