Formulas of Mononuclear Coordination Entities

A formula is a compact way of stating what a compound is made of. For a mononuclear coordination entity — one built around a single central metal atom or ion — the formula is assembled by following a fixed sequence of rules rather than by convenience or habit.

Order and placement

• The central atom is written first, before any of the ligands.
• The ligands follow in alphabetical order. This ordering is based purely on the ligand's name — it does not matter whether the ligand happens to be neutral, positively charged, or negatively charged; charge plays no role in deciding where a ligand sits in the list.
• Polydentate ligands (those that attach through more than one donor atom) are alphabetised the same way as any other ligand. When a ligand is referred to by an abbreviated form, the alphabetical position is fixed by the first letter of that abbreviation, not by the full chemical name behind it.

The 2004 IUPAC draft goes a step further and recommends that ligands always be sorted alphabetically, with charge ignored completely — the same convention used above.

Brackets and spacing

• The formula for the entire coordination entity — whether it carries a charge or is neutral — is enclosed in square brackets. This bracketed unit is the coordination sphere: central atom plus all attached ligands, treated as one block.
• If a ligand itself is polyatomic (made of more than one atom), its own formula is enclosed in parentheses inside the square brackets, and an abbreviated ligand name is parenthesised in the same way.
• There is no space left between the ligands and the metal anywhere inside the coordination sphere — the whole bracketed unit is written as one continuous block.

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