21.2 Naming Complex Ions and Coordination Compounds
PRE-HEALTH PREP
Naming Complex Ions & Coordination Compounds
Cations vs. Anions
General Chemistry 21.2
Lesson Overview
Coordination compounds follow a systematic IUPAC naming convention in which ligands are listed alphabetically before the central metal, with numerical prefixes indicating the count of each ligand type. Complex cations retain the English metal name, while complex anions require an -ate suffix—using Latin roots such as ferrate, cuprate, and aurate for metals whose symbols derive from Latin. The naming system also distinguishes between monodentate and polydentate ligands through separate prefix sets (di/tri/tetra vs. bis/tris/tetrakis) and provides a reversible method for converting between compound names and chemical formulas.
Table of Contents
- Naming Complex Cations
- 6 Numerical Prefixes for Ligands
- Worked Example: Complex Cation
- Naming Complex Anions: The -ate Ending
- 6 Latin Metal Roots for Complex Anions
- Worked Example: Tetrachloroaurate(III)
- Polydentate Ligands and Alternative Prefixes
- Worked Example: Tris(ethylenediamine)iron(II)
- Naming Complete Coordination Compounds
- Going from Name to Formula
- 7 Key Takeaways
| RULES FOR NAMING COMPLEX IONS AND COORDINATION COMPOUNDS |
|---|
| 1. Name the cation as the first word, then the anion as the second word in a coordination compound. Neutral complexes are named using a single word. |
| Specific to Complex Ions 2. Name the ligands first in alphabetical order. Use the appropriate Latin numerical prefix (di, tri, tetra, penta, hexa). Use the Greek prefixes (bis, tris, tetrakis, pentakis, hexakis) for polydentate ligands and for ligands that already begin with a latin prefix as part of their name (ex. diethylenetriamine). Hint: negatively charged ligands are all named ending with –o. |
| 3. Name the metal. If the complex ion is a cation or neutral use the English name of the metal. If the complex ion is an anion, add –ate to the ending. Use the Latin root in anions for the following: ferrate for Fe, cuprate for Cu, argentate for Ag, aurate for Au, plumbate for Pb, stannate for Sn. |
| 4. Give the oxidation state of the central metal ion in parenthesis. |
Naming Complex Cations
3 Core Rules: Ligands First, Metal Last
Naming coordination compounds involves one immediately counterintuitive convention: the order of elements in the formula is the reverse of the order in the name. In the chemical formula, the metal is written first and the ligands follow. In the name, however, the ligands come first and the metal is placed at the end. This applies to both complex cations and neutral complexes.
Ligands are named in alphabetical order based on their ligand names — not the names of the parent molecules. The 3 common ligand names you need to know are:
- Aqua (H₂O) — water as a ligand is called aqua
- Ammine (NH₃) — ammonia as a ligand is called ammine (note the double m)
- Chloro (Cl⁻) — chloride as a ligand is called chloro
Alphabetical ordering uses the ligand name itself, so: ammine (a) comes before aqua (aq), which comes before chloro (ch).
6 Numerical Prefixes for Ligands
A numerical prefix immediately before each ligand name indicates how many of that ligand are present in the complex. These 6 standard prefixes cover all coordination numbers typically encountered in general chemistry:
| Number of Ligands | Prefix |
|---|---|
| 1 | (none — omit the prefix entirely) |
| 2 | di- |
| 3 | tri- |
| 4 | tetra- |
| 5 | penta- |
| 6 | hexa- |
Important: When there is only one of a particular ligand, no prefix is used at all. Do not write "mono" — simply omit the prefix.
Worked Example: A Complex Cation
Example: Name the complex ion [Cr(NH₃)₃(H₂O)₂Cl]²⁺
Step 1 — Name the ligands in alphabetical order with prefixes.
- Ammine (from NH₃): there are 3 → triammine
- Aqua (from H₂O): there are 2 → diaqua
- Chloro (from Cl⁻): there is 1 → chloro (no prefix)
Step 2 — Determine the metal's oxidation state.
The overall complex ion has a charge of +2. Ammine and aqua are both neutral (charge = 0). Chloro carries a charge of −1. Setting up the equation: oxidation state of Cr + (0 × 3) + (0 × 2) + (−1 × 1) = +2, which gives Cr = +3.
Step 3 — Name the metal with its Roman numeral oxidation state.
Chromium in the +3 state is written as chromium(III).
Full name: triamminediaquachlorochromium(III) ion
Because this is a standalone cation rather than part of a neutral compound, the word "ion" is appended at the end — just as Na⁺ alone is called the "sodium ion."
Spelling tip: In organic chemistry, nitrogen-containing compounds are called amines (one m). In coordination chemistry, the ligand is spelled ammine with two m's. Do not conflate the two.
Naming Complex Anions: The -ate Ending
The Key Difference Between Cations and Anions
The naming procedure for complex anions follows the same ligand-first, metal-last pattern — with one critical distinction. When the entire complex ion carries an overall negative charge, the metal name receives an -ate suffix instead of the plain English name.
A common point of confusion: the "overall negative charge" refers to the complex as a whole, not to the metal's oxidation state. The metal itself always carries a positive (or occasionally zero) oxidation state. It is the sum of all components — metal plus ligands — that determines the overall charge of the complex ion.
Rule: Overall complex charge is negative → metal name + -ate ending. Overall complex charge is positive or zero → plain English metal name.
6 Latin Metal Roots for Complex Anions
Several transition metals have chemical symbols derived from their Latin names rather than their English names. For complex anions only, these 6 metals use their Latin root combined with the -ate suffix. If the same metal appears in a complex cation, the ordinary English name is used.
| Metal (English) | Symbol | Latin Root | Anion Name (-ate) |
|---|---|---|---|
| Iron | Fe | ferr- (from ferrum) | ferrate |
| Copper | Cu | cupr- | cuprate |
| Silver | Ag | argent- | argentate |
| Gold | Au | aur- | aurate |
| Lead | Pb | plumb- | plumbate |
| Tin | Sn | stann- | stannate |
Memory aid: If you have studied French, you may recognize argent as the French word for "money" or "silver" — a useful hook for remembering the silver symbol Ag and the anion name argentate.
Worked Example: Tetrachloroaurate(III) Ion
Example: Name the complex ion [AuCl₄]⁻
Step 1 — Name the ligands.
Chloro (from Cl⁻): there are 4 → tetrachloro.
Step 2 — Name the metal with the -ate suffix (complex anion).
Gold is Au; its Latin root is aur-. Because this complex has an overall negative charge, use the Latin root with -ate: aurate.
Step 3 — Determine the oxidation state of gold.
Each chloro ligand carries −1; four chloro ligands total −4. Overall complex charge = −1. Therefore: Au + (−4) = −1, giving Au = +3.
Full name: tetrachloroaurate(III) ion
Polydentate Ligands and 3 Alternative Numerical Prefixes
Bis, Tris, and Tetrakis
Polydentate ligands — those that bind through more than one donor atom — require a different set of numerical prefixes. The standard di-, tri-, tetra- prefixes are reserved for simple monodentate ligands. When counting polydentate ligands, use these 3 alternative prefixes instead:
| Number | Polydentate Prefix |
|---|---|
| 2 | bis- |
| 3 | tris- |
| 4 | tetrakis- |
In addition, the full systematic name of a polydentate ligand (not an abbreviation) is enclosed in parentheses when it appears in the compound name. Common polydentate ligands to know include:
- Ethylenediamine (abbreviated en) — bidentate, neutral
- o-Phenanthroline (abbreviated o-phen) — bidentate, neutral
- Oxalato (abbreviated ox) — bidentate, charge of −2
- Diethylenetriamine (abbreviated dien) — tridentate, neutral
Common mistake: Do not write "triethylenediamine" when you mean three ethylenediamine ligands. The correct prefix for three polydentate ligands is tris, not tri. The full name goes in parentheses: tris(ethylenediamine).
Worked Example: Tris(ethylenediamine)iron(II) Ion
Example: Name the complex ion [Fe(en)₃]²⁺
Step 1 — Name the polydentate ligand with the appropriate prefix.
Ethylenediamine (en) is present three times. Using the polydentate prefix: tris(ethylenediamine). Note that the full name "ethylenediamine" contains "diamine" with one m — unlike the ligand "ammine," which has two.
Step 2 — Name the metal.
The overall complex charge is +2, so this is a complex cation. Use the English name: iron (not ferrate).
Step 3 — Determine the oxidation state.
Ethylenediamine is neutral. The iron alone accounts for the entire +2 charge of the complex.
Full name: tris(ethylenediamine)iron(II) ion
Naming Complete Coordination Compounds
Cation First, Anion Second
When a coordination compound consists of both a cation and an anion — at least one of which is a complex ion — it is named exactly as any other ionic compound: cation name first, anion name second, no "ion" appended at the end.
The number of counter-ions needed to achieve electrical neutrality is implied by the charges, just as in simple ionic nomenclature. For example, MgCl₂ is "magnesium chloride," not "magnesium dichloride." The same principle applies regardless of how many counter-ions are present in the formula.
Example A: Complex cation + simple anion
The complex cation [Cr(NH₃)₃(H₂O)₂Cl]²⁺ paired with two Cl⁻ counter-ions gives a neutral compound. Dropping the word "ion" from the cation name and appending the anion name yields:
triamminediaquachlorochromium(III) chloride
The prefix "di-" does not appear before "chloride" because simple anion counts are never prefixed in ionic compound names.
Example B: Simple cation + complex anion (Sodium tetrachloroaurate(III))
The complex anion [AuCl₄]⁻ paired with one Na⁺ counter-ion gives a neutral compound. Cation is named first, anion second:
sodium tetrachloroaurate(III)
As always, the number of counter-ions needed is implied by the charges of the ions and is not reflected in the name.
Going from Name to Formula
Working Backward: Both Ions Are Complex
Converting a coordination compound name into a formula reverses the process you have been practicing. Parse the name to extract the metal identity, ligand identities, ligand counts, and metal oxidation state for each ion, then determine how many formula units of each ion are needed for a neutral compound.
Example: Write the formula for hexaamminenickel(II) hexacyanoferrate(II).
Cation — hexaamminenickel(II):
Metal: Ni (nickel). Ligand: ammine (NH₃), 6 of them (hexa-). Ni is +2; ammine is neutral. Overall cation charge = +2.
Formula of cation: [Ni(NH₃)₆]²⁺
Anion — hexacyanoferrate(II):
Metal: Fe (ferr- → iron). Ligand: cyano (CN⁻), 6 of them (hexa-). Each cyano carries −1; Fe is +2. Overall charge: +2 + 6(−1) = −4.
Formula of anion: [Fe(CN)₆]⁴⁻
Balancing the formula:
Cation charge = +2; anion charge = −4. To reach neutrality: 2 cations × (+2) = +4; 1 anion × (−4) = −4. Place a subscript 2 on the cation.
Full formula: [Ni(NH₃)₆]₂[Fe(CN)₆]
Pattern to remember: All negatively charged ligands end in the letter -o (cyano, chloro, fluoro, carbonato, oxalato, etc.). Neutral ligands do not follow this pattern — aqua and ammine are the prime examples. This is a convenient check when decoding an unfamiliar ligand name.
7 Key Takeaways
- In a complex ion name, ligands are listed first in alphabetical order; the metal comes last. This is the reverse of the formula order (metal first, ligands second).
- Use standard numerical prefixes (di-, tri-, tetra-, penta-, hexa-) for simple monodentate ligands; use bis-, tris-, tetrakis- for polydentate ligands. Omit the prefix entirely when only one ligand of that type is present.
- Complex cations and neutral complexes use the English metal name (e.g., iron, gold, chromium) followed by the Roman numeral oxidation state in parentheses.
- Complex anions use the metal name with an -ate suffix. Select metals with Latin-derived symbols — Fe (ferrate), Cu (cuprate), Ag (argentate), Au (aurate), Pb (plumbate), Sn (stannate) — use their Latin root rather than the English name.
- The metal's oxidation state is determined by subtracting the total ligand charge from the overall complex ion charge. All negatively charged ligands end in -o; neutral ligands (aqua, ammine) do not.
- Complete coordination compounds are named cation first, anion second, with no numerical prefix on the counter-ion and no "ion" at the end of the name.
- Converting a name to a formula requires identifying the charge on each complex ion, then finding the smallest integer ratio that yields a neutral compound — identical in logic to balancing any simple ionic formula.