Counter Ion
An ion outside the coordination sphere that balances the charge of the complex ion.
Unit
Dimensionless
In depth
Counter ions are held by ordinary electrostatic attraction and dissociate completely in solution, so they give all their usual analytical reactions. Measuring the electrical conductivity of a solution reveals how many ions the compound produces and hence how the formula is divided.
Examples in the real world
The sulfate in [Cu(NH₃)₄]SO₄; the potassium ions in K₃[Fe(CN)₆].