Coordination Compounds — Nomenclature & Isomerism

Reach for systematic naming whenever you face a metal complex written with square brackets, such as $[Co(NH_3)_6]Cl_3$, and reach for isomerism analysis whenever you suspect one formula could describe more than one compound. These are the two procedures students actually need from coordination chemistry: produce the correct name, and decide whether the same formula hides distinct arrangements.

When to Use This Method

Apply the bracket-first naming routine any time a formula mixes a coordination sphere with counter-ions. In $[Co(NH_3)_6]Cl_3$, the part inside the brackets is the coordination sphere: six ammonia ligands bonded directly to cobalt. The three chlorides outside are counter-ions, not ligands in that written formula. That inside/outside split drives everything downstream, so it is always the first move.

The Naming Procedure, Step by Step

Work through the steps below in order. The accompanying step list mirrors this routine.

1. Find the coordination sphere. Separate the bracketed complex ion from any counter-ions outside the brackets.

2. Name the ligands first. Place ligand names before the metal. Use the special forms where required: ammine for $NH_3$, aqua for $H_2O$, and chlorido, cyanido, or hydroxido for common anionic ligands in modern IUPAC naming. Use prefixes di-, tri-, tetra- to show how many of each ligand are present.

3. Alphabetize by ligand name, not prefix. Ammine sorts under a, not under d for "di."

4. Compute the metal oxidation state. Combine the overall complex charge with the ligand charges, then write the result in Roman numerals.

5. Finish with the metal name. Use the ordinary metal name for cationic or neutral complexes, and the -ate form for anionic complexes. Because the -ate forms are not always obvious by inspection, check the standard name rather than guess. For the full compound, name the cation before the anion: if the complex is the cation, its counter-anion follows; if the complex is the anion, the positive ion comes first.

Worked Example: Naming $[Pt(NH_3)_2Cl_2]$ Start to Finish

This neutral complex runs the whole procedure and exposes isomerism at the end.

Compute the oxidation state of platinum. Ammonia is neutral; each coordinated chloride contributes $-1$:

so

Name the ligands: two ammine and two chlorido. Alphabetically ammine precedes chlorido, giving the base name diamminedichloridoplatinum(II).

But this square-planar formula can be arranged two ways:

• cis: the two chlorido ligands are adjacent
• trans: the two chlorido ligands are opposite

So the isomers are cis-diamminedichloridoplatinum(II) and trans-diamminedichloridoplatinum(II) — same formula, same connectivity, different spatial arrangement. This is geometrical isomerism, the same idea behind the common names cisplatin and transplatin.

Where Each Step Trips People Up, and How to Check

Step 1 — the wrong chloride

A chloride outside the brackets is named chloride; a coordinated chloride inside is named chlorido. Self-check: before naming anything, circle the brackets and label each chloride inside or outside.

Step 3 — sorting by the prefix

Alphabetical order uses the ligand name, so ammine is filed under a. Self-check: cover the di-, tri-, tetra- prefixes and sort what remains.

Step 4 — charging a neutral ligand

$NH_3$ and $H_2O$ are neutral. Treat them as charged and the oxidation number comes out wrong. Self-check: list each ligand's charge in a column before summing.

Isomer step — assuming cis/trans always exists

Geometrical isomers only appear when the geometry and ligand pattern actually allow distinct positions; the formula alone is not enough. Optical isomerism needs a chiral complex (no mirror plane), and structural isomerism — linkage or ionization types — changes what is bonded or what sits inside versus outside the sphere. Self-check: confirm the geometry permits distinct arrangements before writing any cis/trans, fac/mer, or optical label.

Where This Procedure Applies

These rules recur throughout coordination chemistry, transition-metal chemistry, qualitative analysis, catalysis, and bioinorganic chemistry. Even outside those areas, the habit pays off: do not read a formula as a flat list of atoms. Position, charge, and bonding pattern all carry meaning.

Run the Full Procedure Yourself

Name $[Co(NH_3)_5Cl]Cl_2$ end to end. Separate the coordination sphere from the counter-ions, compute cobalt's oxidation state, and identify which chloride is a ligand and which two are outside the brackets. Working that single example through every step is usually what makes the bracket notation finally click.