Coordination Sphere and Counter Ions

The complex ion of a coordination compound -- the central metal atom/ion together with all the ligands directly attached to it -- is collectively called the coordination sphere, and it is conventionally enclosed in square brackets carrying its net charge. Any other, ionisable ions written outside that bracket are called counter ions. For example, in K₄[Fe(CN)₆] the coordination sphere is the complex ion [Fe(CN) …