Writing formulas and IUPAC names
In a formula the metal comes first; in a name the ligands come first, in alphabetical order, then the metal with its oxidation state in Roman numerals.
Rules for writing the formula
- Write the central atom first.
- Then write the ligands in alphabetical order; the charge of a ligand does not matter. By the strict rule you sort by the ligand's formula or abbreviation: Cl before NH₃, H₂O before NH₃, and en under e.
- Put polyatomic ligands and abbreviations in round brackets: (NH₃), (CN), (en), (C₂O₄).
- Enclose the whole coordination entity in square brackets, with no spaces inside.
- For a charged entity written alone, put the charge outside the bracket as a right superscript, number first: [Co(CN)₆]³⁻, [Cr(H₂O)₆]³⁺.
- The charges of the cation and the anion must balance in the full compound.
- Your textbook uses both styles. Some formulas follow the strict rule, such as [CoCl(NH₃)₅]²⁺. Others list the ligands in the order of their names: [Co(NH₃)₅Cl]Cl₂, [Co(NH₃)₄(H₂O)₂]Cl₃, [Co(NH₃)₅Br]SO₄. Both show the same compound and both are accepted.
Names of common ligands
| Ligand | Formula | Name in a complex | Charge |
|---|---|---|---|
| Chloride | Cl⁻ | chlorido | −1 |
| Bromide | Br⁻ | bromido | −1 |
| Cyanide | CN⁻ | cyanido | −1 |
| Hydroxide | OH⁻ | hydroxido | −1 |
| Nitrite, bonded through N | NO₂⁻ | nitrito-N | −1 |
| Nitrite, bonded through O | ONO⁻ | nitrito-O | −1 |
| Thiocyanate | SCN⁻ (through S) or NCS⁻ (through N) | thiocyanato-S or thiocyanato-N | −1 |
| Oxalate | C₂O₄²⁻ | oxalato | −2 |
| Sulphate | SO₄²⁻ | sulphato | −2 |
| Carbonate | CO₃²⁻ | carbonato | −2 |
| Water | H₂O | aqua | 0 |
| Ammonia | NH₃ | ammine | 0 |
| Carbon monoxide | CO | carbonyl | 0 |
| Nitrogen monoxide | NO | nitrosyl | 0 |
| Ethane-1,2-diamine | H₂NCH₂CH₂NH₂ (en) | ethane-1,2-diamine | 0 |
How to name a complex, step by step
Worked on [CoCl₂(en)₂]Cl.
- 1Name the cation first, then the anion, as for any salt.Here the complex [CoCl₂(en)₂]⁺ is the cation; the anion Cl⁻ is 'chloride'.
- 2Inside the complex, list the ligands in alphabetical order, ignoring number prefixes.chlorido (c) comes before ethane-1,2-diamine (e).
- 3Add prefixes: di, tri, tetra for simple ligands; bis, tris, tetrakis for ligands whose names already contain a number, with that name in brackets.dichlorido and bis(ethane-1,2-diamine)
- 4Name the metal as the element in a cation or neutral complex; add -ate in an anionic complex.Here the complex is a cation, so: cobalt.
- 5Add the metal's oxidation state in Roman numerals in brackets, with no space.x + 2(−1) + 2(0) = +1, so x = +3: cobalt(III).
- 6★dichloridobis(ethane-1,2-diamine)cobalt(III) chlorideThe complex is written as one word; the anion follows after a space.
★ key step
Worked names to learn
| Formula | IUPAC name | Oxidation state of metal |
|---|---|---|
| [Co(NH₃)₆]Cl₃ | hexaamminecobalt(III) chloride | +3 |
| [CoCl(NH₃)₅]Cl₂ | pentaamminechloridocobalt(III) chloride | +3 |
| K₄[Fe(CN)₆] | potassium hexacyanidoferrate(II) | +2 |
| K₃[Fe(CN)₆] | potassium hexacyanidoferrate(III) | +3 |
| [Ni(CO)₄] | tetracarbonylnickel(0) | 0 |
| [PtCl₂(NH₃)₂] | diamminedichloridoplatinum(II) | +2 |
| K₂[PtCl₆] | potassium hexachloridoplatinate(IV) | +4 |
| [Cu(NH₃)₄]SO₄ | tetraamminecopper(II) sulphate | +2 |
| [Co(en)₃]Cl₃ | tris(ethane-1,2-diamine)cobalt(III) chloride | +3 |
| Na₃[Co(NO₂)₆] | sodium hexanitrito-N-cobaltate(III) | +3 |
| K[Ag(CN)₂] | potassium dicyanidoargentate(I) | +1 |
| [Pt(NH₃)₄][PtCl₄] | tetraammineplatinum(II) tetrachloridoplatinate(II) | +2 in both |
| K₃[Al(C₂O₄)₃] | potassium trioxalatoaluminate(III) | +3 |
| [Zn(OH)₄]²⁻ | tetrahydroxidozincate(II) ion | +2 |
Writing a formula from a name
Write the formulas of (a) diamminesilver(I) chloride (b) pentaamminenitrito-O-cobalt(III) sulphate (c) potassium tetrachloridonickelate(II) (d) hexaaquairon(II) sulphate.
- (a) Ag⁺ with two neutral NH₃ gives [Ag(NH₃)₂]⁺. One Cl⁻ balances it: [Ag(NH₃)₂]Cl.
- (b) Co³⁺ + five NH₃ (0) + one ONO⁻ (−1) gives a charge of +2. One SO₄²⁻ balances it: [Co(NH₃)₅(ONO)]SO₄. Writing ONO shows the bond is through O.
- (c) '-ate' means an anion. Ni²⁺ + 4Cl⁻ gives −2, so two K⁺ balance it: K₂[NiCl₄].
- (d) Fe²⁺ + six neutral H₂O gives +2. One SO₄²⁻ balances it: [Fe(H₂O)₆]SO₄.
[Ag(NH₃)₂]Cl, [Co(NH₃)₅(ONO)]SO₄, K₂[NiCl₄] and [Fe(H₂O)₆]SO₄.
Common mistakes that cost marks
- Writing amine for NH₃. In a complex it is ammine, with a double m.
- Forgetting -ate for an anionic complex: K₃[Fe(CN)₆] is hexacyanidoferrate(III), not hexacyanidoiron(III).
- Alphabetising by the prefix: in 'tetraammine' and 'dichlorido' sort by ammine and chlorido, not by t and d.
- Leaving a space before the oxidation state: write cobalt(III), not cobalt (III).
- Writing di or tri with ethane-1,2-diamine: write bis(ethane-1,2-diamine) and tris(ethane-1,2-diamine).
- Writing the charge of the whole complex instead of the oxidation state of the metal.
Pharmacy and research labs name drugs by these rules, for example the anticancer drug cisplatin, cis-diamminedichloridoplatinum(II). So a chemist anywhere in the world gets the same compound from the name.
2 marks: “Write the IUPAC names of (i) [CrCl₂(en)₂]Cl (ii) K₃[Co(C₂O₄)₃].” (i) dichloridobis(ethane-1,2-diamine)chromium(III) chloride (ii) potassium trioxalatocobaltate(III).
2 marks: “Write the formulas of (i) tetraamminezinc(II) nitrate (ii) sodium hexacyanidoferrate(III).” (i) [Zn(NH₃)₄](NO₃)₂ (ii) Na₃[Fe(CN)₆].
3 marks: “Write the IUPAC names of [Pt(NH₃)₄][PtCl₄], [Ni(CO)₄] and Na₃[Co(NO₂)₆], and state the oxidation state of the metal in each.” tetraammineplatinum(II) tetrachloridoplatinate(II), Pt +2 in both; tetracarbonylnickel(0), Ni 0; sodium hexanitrito-N-cobaltate(III), Co +3.
Formula: metal first. Name: metal last. Ligands always A to Z; an anionic complex ends in -ate; the Roman numeral sits right after the metal with no space.