Hexammine cobalt ion. Molecular model of the structure of the cobalt (III) hexammine ion. This is a central cobalt ion (Co 3+) surrounded by six ammonia groups (NH3). Here, the nitrogen atoms are blue, the hydrogen atoms are white, and the central cobalt ion is dark green. This is a classic example of a transition metal ion forming a co-ordination complex with octahedral geometry. Transition metal complexes that have octahedral geometry are known as Werner complexes. Cobalt, unlike other transition metal ions, such as nickel, binds very tightly to the ammonia groups, making this an inert metal complex.