HexaamminecobaltIIIIon

Hexaamminecobalt(III) ion refers to the octahedral coordination complex [Co(NH3)6]3+, in which a cobalt(III) center is bound to six neutral ammonia (ammine) ligands. In many salts this cation is paired with three counterions, for example [Co(NH3)6]Cl3.

The cobalt center in this complex is coordinated by six NH3 donors, giving an 18-electron count (Co(III) d6 plus 12 electrons from the six ligands). The geometry is octahedral, and the complex is typically low-spin, with a t2g^6 eg^0 electronic configuration, resulting in diamagnetism.

Synthesis of the hexammine complex can occur by ligand substitution, where water ligands are replaced by ammonia. A representative reaction is [Co(H2O)6]3+ + 6 NH3 → [Co(NH3)6]3+ + 6 H2O. The complex is relatively inert toward ligand substitution compared with many cobalt(II) species, reflecting the stabilizing effect of cobalt(III).

In coordination chemistry, hexamminecobalt(III) ion is a classic example used to illustrate octahedral, low-spin cobalt(III) chemistry and ligand-field effects. It serves as a model system for studying ligand binding, substitution kinetics, and the electronic structure of d6 metal centers in a strong-field environment.

Safety notes: cobalt compounds can be toxic if ingested or inhaled; appropriate laboratory precautions are required.