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Why are square brackets used in formulas for complex ions?

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Structural diagram of the vitamin B12 coenzyme molecule, highlighting its organometallic core with five nitrogen-cobalt bonds and one cobalt-carbon bond at the center, surrounded by complex organic groups. — coordination compound Coordination compounds contain a central metal atom surrounded by nonmetal atoms or groups of atoms, called ligands. For example, vitamin B₁₂ is made up of a central metallic cobalt ion bound to multiple nitrogen-containing ligands.

In the study of coordination compounds, square brackets are used in chemical formulas to define the coordination sphere.1

This sphere encompasses the central metal atom or ion and all the ligands directly bonded to it. By enclosing these components in brackets, chemists clearly distinguish the species that are chemically coordinated to the metal from any ions that exist outside this primary sphere.12

Furthermore, the brackets serve to indicate that the enclosed group functions as a single, discrete unit. When the coordination sphere carries an overall electrical charge, the brackets allow the charge to be assigned to the unit as a whole, rather than to individual atoms within the complex. For example, in the formula for the hexaaquanickel(2+) ion, [Ni(H2O)6]2+, the brackets signify that the 2+ charge applies to the entire complex ion.3