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IUPAC additive names: cation before anion, ligands alphabetically with their prefixes, the metal with its oxidation state, and -ate for an anionic complex.
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By the end of this lesson you will be able to name a coordination compound from its formula, and read a name back into the metal's oxidation state, the complex's charge and the counter ions.
You can name simple ionic compounds, cation first and anion second, as in iron(III) chloride, and you know the Greek prefixes di-, tri-, tetra-, penta- and hexa- for two to six. From the last three lessons you can find the oxidation state of a metal in a complex, its coordination number, and which ions in a formula are ligands and which are counter ions. A name encodes all of that in words.
| Term | What it means |
|---|---|
| Additive name | The IUPAC name of a complex, built by adding the ligands' names to the metal's. |
| Multiplying prefix | di-, tri-, tetra-, penta-, hexa- for simple ligands; bis-, tris-, tetrakis- for ligands whose names already contain a number or are complex. |
| Stock number | The oxidation state in Roman numerals in parentheses after the metal, as in cobalt(III). |
| -ido ending | The ending of an anionic ligand's name: chlorido, cyanido, hydroxido, bromido. |
| -ate ending | The ending of the metal's name in an anionic complex: cobaltate, platinate. |
| Latin stem | The root used before -ate for some metals: ferrate for iron, cuprate for copper, argentate for silver, aurate for gold. |
| ammine | Ammonia as a ligand, spelled with two m's to distinguish it from an amine. |
The name of a coordination compound follows the same outer rule as any salt: the cation is named first, then the anion, with no numbers for how many of each, since the charges fix that. $\mathrm{[Co(NH_3)_6]Cl_3}$ is hexaamminecobalt(III) chloride and $\mathrm{K_3[Fe(CN)_6]}$ is potassium hexacyanidoferrate(III). The work is in naming the complex itself, which takes four parts in a fixed order.
1. The ligands, in alphabetical order. Each ligand has a name for use in complexes. Neutral ligands mostly keep special names: water is aqua, ammonia ammine, carbon monoxide carbonyl. Anionic ligands end in -ido or -o: chloride becomes chlorido, cyanide cyanido, hydroxide hydroxido, oxalate oxalato. The ligands are listed alphabetically by these names.
2. Prefixes for how many. di-, tri-, tetra-, penta- and hexa- count simple ligands. A ligand whose name already contains a number or is long, such as ethylenediamine, takes bis-, tris- or tetrakis- with its name in parentheses: tris(ethylenediamine). The prefixes do not count in the alphabetical order: tetraammine sorts under a and stays ahead of dichlorido.
3. The metal and its oxidation state. The metal's name follows the ligands with no space, and its oxidation state follows in Roman numerals in parentheses: cobalt(III), platinum(II).
4. The ending. If the whole complex is an anion, the metal's name ends in -ate: cobaltate, platinate, chromate. Several metals use their Latin stems before -ate: iron gives ferrate, copper cuprate, silver argentate, gold aurate, lead plumbate. A cationic or neutral complex keeps the plain metal name.
So $\mathrm{[Co(NH_3)_5Cl]Cl_2}$ is pentaamminechloridocobalt(III) chloride: ammine before chlorido alphabetically, cobalt with no -ate because the complex is a cation, and (III) because charge balance gives cobalt $+3$. Reading a name backwards gives everything you need: the numeral is the oxidation state, the ligands' charges added to it give the complex's charge, and that charge fixes the counter ions.
Another way: picture
Think of the name as a train. The engine is the cation, the caboose the anion. Inside the complex's car the passengers, the ligands, sit in alphabetical order by their own names, each carrying a sign for how many of them there are. The driver, the metal, sits at the back of the car wearing a badge with its oxidation state, and if the car is an anion the driver's name ends in -ate.
Another way: steps
| Ligand | Name in a complex | Charge |
|---|---|---|
| $\mathrm{NH_3}$ | ammine | $0$ |
| $\mathrm{H_2O}$ | aqua | $0$ |
| CO | carbonyl | $0$ |
| $\mathrm{Br^-}$ | bromido | $-1$ |
| $\mathrm{Cl^-}$ | chlorido | $-1$ |
| $\mathrm{CN^-}$ | cyanido | $-1$ |
| $\mathrm{F^-}$ | fluorido | $-1$ |
| $\mathrm{OH^-}$ | hydroxido | $-1$ |
| $\mathrm{NO_2^-}$ | nitrito-N (through N) or nitrito-O (through O) | $-1$ |
| $\mathrm{C_2O_4^{2-}}$ | oxalato | $-2$ |
| en | ethylenediamine | $0$ |
Older books, and many suppliers' catalogs, use the names IUPAC recommended before 2005: chloro, cyano, hydroxo, nitro for the N-bonded nitrite. Pentaamminechlorocobalt(III) chloride and pentaamminechloridocobalt(III) chloride are the same compound; the -ido forms make every anionic ligand end the same way. The alphabetical order is the same in both systems for every ligand in this table.
Reading a name is the reverse of writing it, and it checks both. Take potassium tetrachloridoplatinate(II):
Notice that the name never says dipotassium. The number of counter ions is not part of the name, because the charges determine it, and the same is true for the chloride in hexaamminecobalt(III) chloride. A name that adds such a prefix is a sign that the complex's charge has not been worked out.
A neutral complex, with no counter ions, is named as a single word: $\mathrm{[Co(NH_3)_3Cl_3]}$ is triamminetrichloridocobalt(III), and $\mathrm{[Ni(CO)_4]}$ is tetracarbonylnickel(0). Its oxidation state still goes in parentheses, even when it is zero.
Where a formula can describe more than one arrangement in space, the name adds a prefix in italics: cis- and trans- for two identical ligands next to or opposite each other, fac- and mer- for three, and $\Delta$ or $\Lambda$ for the two mirror-image forms of a chiral complex. Cisplatin's full name is cis-diamminedichloridoplatinum(II); the trans compound has the same formula and no anticancer activity. The next unit is about exactly these arrangements.
The surest check on a name is to turn it back into a formula. Read the ligands with their multiplying prefixes, the metal and its oxidation state, and write the bracket; then add up the charges. If the name was right, the charge you get matches the counter-ions the name lists, or is zero for a neutral complex.
Four quick checks help on the way. The ligands are listed in alphabetical order of their own names, ignoring the prefixes di, tri, tetra, bis and tris, so ammine comes before chlorido in tetraamminedichloridocobalt(III). Anionic ligands end in o: chlorido, cyanido, hydroxido, oxalato. Neutral water and ammonia keep the special names aqua and ammine. And the metal takes the ending -ate only when the whole complex ion is negative, with the Latin stem for a few metals: ferrate for iron, cuprate for copper, argentate for silver, aurate for gold.
The Roman numeral is the other point to check. It gives the oxidation state of the metal, not the charge of the complex, and the two differ whenever any ligand is charged. In potassium hexacyanidoferrate(III), iron is $+3$ but the complex ion is $3-$, because six cyanide ligands bring six negative charges. A numeral that equals the charge of an anionic complex is almost always a slip.
When the name comes first and the formula second, write the cation before the anion, as in any salt, and put the complex in square brackets whichever it is.
Spelling is a check too, because the rules leave no choice. The prefixes bis, tris and tetrakis replace di, tri and tetra when the ligand's own name already contains a number or is long, as in tris(ethylenediamine)cobalt(III); the ligand name is then written in parentheses. A name that says triethylenediamine means something else, triethylenediamine being a different molecule. And the whole name of a complex ion is written as one word, with no spaces between the ligands and the metal.
Many highway departments that spread rock salt in winter buy it treated with a few parts per million of an anticaking agent listed as sodium or potassium ferrocyanide, E535 and E536 in food-additive codes. The systematic names, sodium hexacyanidoferrate(II) and potassium hexacyanidoferrate(II), carry the chemistry that matters: iron(II) held by six cyanides so tightly that no free cyanide is released under normal conditions. The same compound is permitted in table salt in the European Union at up to $20$ mg/kg for that reason.
The name also distinguishes it from its close relative, potassium hexacyanidoferrate(III), the red prussiate used in blueprint paper, which is an oxidant and far more reactive in light. The two differ by one electron and by one Roman numeral, and a purchasing order that says ferricyanide instead of ferrocyanide orders the wrong chemical. Safety data sheets list both under their IUPAC names precisely so that numeral cannot be missed.
Hospital pharmacies handle three platinum drugs that are easily confused: cisplatin, cis-diamminedichloridoplatinum(II); carboplatin, diammine(cyclobutane-1,1-dicarboxylato)platinum(II); and oxaliplatin, which carries an oxalato ligand. All three are platinum(II), four-coordinate and neutral, and the names show it: no counter ion, no -ate, and (II) after the metal in each.
Their doses are very different: a typical cisplatin dose is about $75$ mg per square meter of body surface, carboplatin several hundred milligrams, and giving one in place of the other has caused fatal overdoses. The United States Pharmacopeia and the Institute for Safe Medication Practices list the platinum drugs among look-alike names, and many hospitals print the distinguishing parts of the name in capitals on labels. The ligand names this lesson teaches are the parts that differ, and reading them is part of dispensing safely.
Ordering by prefix or by charge. It is tempting to list anionic ligands first or to sort dichlorido under d. IUPAC orders ligands alphabetically by their own names, ignoring the multiplying prefixes, so tetraammine comes before dichlorido and aqua before chlorido.
Writing the complex's charge as the oxidation state. In $\mathrm{[Co(NH_3)_5Cl]Cl_2}$ the complex is $+2$ but cobalt is $+3$; the name is cobalt(III). The Roman numeral always belongs to the metal alone.
Getting -ate wrong. The ending depends on the complex's charge, not on the metal. $\mathrm{[CoCl_4]^{2-}}$ is tetrachloridocobaltate(II); $\mathrm{[Co(NH_3)_6]^{3+}}$ is hexaamminecobalt(III), never cobaltate. Adding -ate to a cation, or leaving it off an anion, names a different ion from the one in the bottle.
Identify the cation and the anion in $\mathrm{[Cr(H_2O)_6]Cl_3}$.
$\text{cation } \mathrm{[Cr(H_2O)_6]^{3+}}, \quad \text{anion } \mathrm{Cl^-}$
The bracketed complex is the cation here, so it is named first.
Name the ligands.
$6 \times \mathrm{H_2O} \to \text{hexaaqua}$
Water is aqua; six is hexa.
Find the oxidation state.
$x + 6 \times 0 = +3 \Rightarrow x = +3$
Water is neutral, so chromium carries the whole charge.
Name the metal.
$\text{chromium(III)}$
A cationic complex keeps the plain metal name.
Add the anion.
$\text{hexaaquachromium(III) chloride}$
The anion's name follows with no prefix for three.
List the ligands in $\mathrm{[Co(NH_3)_4Cl_2]Cl}$.
$4 \times \mathrm{NH_3}, \quad 2 \times \mathrm{Cl^-}$
Only the chlorides inside the brackets are ligands.
Name them for a complex.
$\text{ammine}, \quad \text{chlorido}$
Anionic chloride takes the -ido form.
Order them alphabetically, ignoring prefixes.
$\text{a(mmine)} < \text{c(hlorido)}$
The order is by ligand name, not by the prefix tetra- or di-.
Add the prefixes.
$\text{tetraammine}, \quad \text{dichlorido}$
Four ammines, two chloridos.
Find the oxidation state.
$x + 2 \times (-1) = +1 \Rightarrow x = +3$
The single chloride counter ion makes the complex $+1$.
Assemble the name.
$\text{tetraamminedichloridocobalt(III) chloride}$
Ligands, metal, numeral, then the anion.
Identify the ions in $\mathrm{K_4[Fe(CN)_6]}$.
$4\,\mathrm{K^+}, \quad \mathrm{[Fe(CN)_6]^{4-}}$
Potassium is the cation, so the complex is the anion.
Name the ligands.
$6 \times \mathrm{CN^-} \to \text{hexacyanido}$
Cyanide as a ligand is cyanido.
Find the oxidation state.
$x + 6 \times (-1) = -4 \Rightarrow x = +2$
Charge balance for the complex ion.
Name the metal as an anion.
$\text{iron} \to \text{ferrate(II)}$
An anionic complex takes -ate, and iron uses its Latin stem.
Put the cation first.
$\text{potassium hexacyanidoferrate(II)}$
No tetra- on potassium: the charges fix the number.
Check by reading it back.
$+2 + 6 \times (-1) = -4 \Rightarrow 4\,\mathrm{K^+}$
The name regenerates the formula exactly.
Name the ligands.
$3 \times \mathrm{en} \to \text{tris(ethylenediamine)}$
A ligand with a long name takes tris- and parentheses.
Find the oxidation state.
$x + 0 = +2 \Rightarrow \text{nickel(II)}$
En is neutral; two chlorides make the complex $+2$.
Assemble the name.
Which is the IUPAC name of $\mathrm{Na_2[CoCl_4]}$?
Complete the worked solution: find the oxidation state and the charge of the complex in potassium hexacyanidoferrate(III).
Read the Roman numeral after the metal.
$x =$ x
It is the metal's oxidation state.
Add the charges of the ligands the name lists.
$\text{complex charge} = x + (\text{ligand charges}) =$ c
Ammine and aqua add nothing; each chlorido or cyanido adds $-1$ and each oxalato $-2$.
Check the sign against the metal's name.
$\text{metal name ending in -ate} \iff \text{complex charge} < 0$
Only an anionic complex takes the -ate form of the metal's name, such as ferrate or cobaltate.
A complex has the ligands $\mathrm{Br^{-}}$, $\mathrm{CO}$, $\mathrm{F^{-}}$ and $\mathrm{C_2O_4^{2-}}$. Put their names in the order they appear in the complex's name.
Number the steps in order (write the number in the box):
Read these names: potassium tetrachloridoplatinate(II); diamminesilver(I) nitrate; triamminetrichloridocobalt(III). For each, in that order, fill in the metal's oxidation state, the charge of the complex and the counter ions per formula unit.
| oxidation state | charge of the complex | counter ions per formula unit | |
|---|---|---|---|
| the first name | |||
| the second name | |||
| the third name |
What is the charge of the complex in tris(ethylenediamine)nickel(II) chloride? Give it as a signed number, $0$ for a neutral complex.
Answer: charge of the complex
How many counter ions are there in one formula unit of potassium hexacyanidoferrate(III)? Every counter ion in it carries a single charge.
Answer: counter ions per formula unit
A chemistry stockroom's inventory lists three salts by their IUPAC names: sodium tetrachloridocobaltate(II); hexaaquachromium(III) chloride; potassium trioxalatoferrate(III). To check the shelf labels, which show only formulas, fill in for each, in that order, the metal's oxidation state, the complex's charge and the counter ions per formula unit.
| oxidation state | charge of the complex | counter ions per formula unit | |
|---|---|---|---|
| the first salt | |||
| the second salt | |||
| the third salt |
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
Read these names: potassium hexacyanidoferrate(III); potassium tetrachloridoplatinate(II); potassium trioxalatoferrate(III). For each, in that order, fill in the metal's oxidation state, the charge of the complex and the counter ions per formula unit.
| oxidation state | charge of the complex | counter ions per formula unit | |
|---|---|---|---|
| the first name | |||
| the second name | |||
| the third name |
You can name coordination compounds and read their names. Explain why $\mathrm{K_4[Fe(CN)_6]}$ is potassium hexacyanidoferrate(II) and not tetrapotassium hexacyanidoiron(IV).
15. Your turn: name $\mathrm{[Ni(en)_3]Cl_2}$., step 3
$\text{tris(ethylenediamine)nickel(II) chloride}$
A cationic complex keeps the plain metal name.