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Table 2. Numerical Prefixes

4. After naming the ligands, name the central metal. If the complex ion is a cation, the metal is named same as the element. For example, Co in a complex cation is call cobalt and Pt is called platinum. (See examples 1-4). If the complex ion is an anion, the name of the metal ends with the suffix –ate. (See examples 5 and 6.). For example, Co in a complex anion is called cobaltate and Pt is called platinate. For some metals, the Latin names are used in the complex anions e.g. Fe is called ferrate (not ironate).

Table 3: Name of Metals in Anionic Complexes

5. Following the name of the metal, the oxidation state of the metal in the complex is given as a Roman numeral in parentheses.

C. To name a neutral complex molecule, follow the rules of naming a complex cation. Remember: Name the (possibly complex) cation BEFORE the (possibly complex) anion. See examples 7 and 8.

For historic reasons, some coordination compounds are called by their common names. For example, Fe(CN)₆^3- and Fe(CN)₆^4- are named ferricyanide and ferrocyanide respectively, and Fe(CO)₅ is called iron carbonyl.

Examples Give the systematic names for the following coordination compounds:

1. [Cr(NH₃)₃(H₂O)₃]Cl₃

Answer: triamminetriaquachromium (III) chloride

Solution: The complex ion is inside the parentheses, which is a cation.

The ammine ligands are named before the aqua ligands according to alphabetical order.

Since there are three chlorides binding with the complex ion, the charge on the complex ion must be +3 (since the compound is electrically neutral).

From the charge on the complex ion and the charge on the ligands, we can calculate the oxidation number of the metal. In this example, all the ligands are neutral molecules. Therefore, the oxidation number of chromium must be same as the charge of the complex ion, +3.

2. [Pt(NH₃)₅Cl]Br₃

Answer: pentaamminechloroplatinum (IV) bromide

Solution: The complex ion is a cation, the counter anion is the 3 bromides.

The charge of the complex ion must be +3 since it bonds with 3 bromides.

The NH₃ are neutral molecules while the chloride carries  1 charge. Therefore, the oxidation number of platinum must be +4.

3. [Pt(H₂NCH₂CH₂NH₂)₂Cl₂]Cl₂

Answer: dichlorobis(ethylenediamine)platinum (IV) chloride

Solution: ethylenediamine is a bidentate ligand, the bis- prefix is used instead of di-

4. [Co(H₂NCH₂CH₂NH₂)₃]₂(SO₄)₃

Answer: tris(ethylenediamine)cobalt (III) sulfate

Solution: The sulfate is the counter anion in this molecule. Since it takes 3 sulfates to bond with two complex cations, the charge on each complex cation must be +3.

Since ethylenediamine is a neutral molecule, the oxidation number of cobalt in the complex ion must be +3.

Again, remember that you never have to indicate the number of cations and anions in the name of an ionic compound.

5. K₄[Fe(CN)₆]

Answer: potassium hexacyanoferrate (II)

Answer: sodium tetrachloronickelate (II)

Solution: The complex ion is the anion so we have to add the suffix –ate in the name of the metal.

7. Pt(NH₃)₂Cl₄

Answer: diamminetetrachloroplatinum (IV)

Solution: This is a neutral molecule because the charge on Pt⁺⁴ equals the negative charges on the four chloro ligands.

If the compound is [Pt(NH₃)₂Cl₂]Cl₂, even though the number of ions and atoms in the molecule are identical to the example, it should be named: diamminedichloroplatinum(II) chloride, a big difference.

8. Fe(CO)₅

Answer: pentacarbonyliron (0)

Solution: Since it is a neutral complex, it is named in the same way as a complex cation. The common name of this compound, iron carbonyl, is used more often.

9. (NH₄)₂[Ni(C₂O₄)₂(H₂O)₂]

Answer: ammonium diaquabis(oxalato)nickelate(II)

Solution: The oxalate ion is a bidentate ligand.

10. [Ag(NH₃)₂][Ag(CN)₂]

Answer: diamminesilver(I) dicyanoargentate(I)

You can have a compound where both the cation and the anion are complex ions. Notice how the name of the metal differs even though they are the same metal ions.