The metal complexes are named based on certain rules. These rules were recommended by the International Union of Pure and Applied Chemistry (IUPAC). The names thus given to the complexes are called systematic names.
Complex Cations
The name of a complex cation has to begin with the number of ligands followed by the name of the ligand bonded to the central metal atom or ion.
The name of the metal follows this.
The oxidation number (Stock number) of the metal is indicated by a Roman numeral in parentheses.
For example, [Cr(NH3)6]³+ is named as hexaammine chromium(III) ion.
There should be no spaces between the ligand name and the metal name; and similarly, between the metal name and the parenthesis. [Cr(NH3)6]3+ is considered as a single entity analogous to the Na+ ion in NaCl. NaCl Sodium chloride
[Cr(NH3)6]Cl3 Hexaamminechromium(III) chloride
BaCl2 is named barium chloride and not barium dichloride because the divalency of barium is implied. Similarly, [Cr(NH3)6]Cl3 is not named a trichloride but simply a chloride. The first part of the name of the complex, hexaamminechromium(III), automatically indicates that it is a trichloride. Some other examples are given below.
[Cu(NH3)4]SO4 Tetraamminecopper(II) sulphate
[Fe(H₂O)6]C13 Hexaaquariron(III) chloride
[CoSO4(NH3)4]NO3 Tetraamminesulphatocobalt(III) nitrate
For a metal in zero oxidation state, the symbol (0) is used.
Ni(CO)4 Tetracarbonylnickel(0)
Fe(CO)5 Pentacarbonyliron (0)
Square brackets are not required to represent the formulae of simple uncharged carbonyls
Complex anions
When the ligands attached to the metal ion produce a complex negative ion like [Fe(CN)6]4-, the suffix ate is attached to the name of the metal.
K4[Fe(CN)6] Potassium Hexacyanoferrate (II)
Na₂[ZnCl4] Sodiun Tetrachlorozincate (II)
[PtC16]2- Hexachloroplatinate(IV) ion
The names of some complex anions with different metals are given in the following table:
| Metal | Ending in the name of the complex anion | Metal | Ending in the name of the complex anion |
|---|---|---|---|
| Ag | Argentate | Sn | Stannate |
| Au | Aurate | Os | Osmate |
| Cr | Chromate | W | Tungstate |
| Co | Cobaltate | Zn | Zincate |
| Cu | Cuprate | Pt | Platinate |
| Fe | Ferrate | Mo | Molybdate |
| Pb | Plumbate | V | Vanadate |
| Mn | Manganate | Ti | Titanate |
| Ni | Nickelate |
The name of the complex begins with the name of the cation, whether the cation is simple ion such as Na+ or K+ or is itself a complex cation such as [Co(NH3)6]3+
The complex ion is enclosed in a bracket. [ ]. It is one of the ions expected to be formed in solution when the complex is dissolved in a solvent.
Naming Ligands
Negative Ligands
The name of a negative ligand should end in -o. Some such ligands are listed in the following table.
| Formula | Name |
|---|---|
| F- | Fluoro |
| Cl- | Chloro |
| O2- | Oxo |
| -SCN | Thiocyanato |
| C2O42- | Oxalato |
| H- | Hydrido |
| HO- | Hydroxo |
| CN- | Cyano |
| S2O32- | Thiosulphato |
| H₂N- | Amido |
| HS- | Mercapto |
| Br- | Bromo |
| I- | Iodo |
| CH₂COO- | Acetato |
| HN²- | Imido |
Neutral Ligands
The neutral ligands are named as such without any special ending
| Formula | Name |
|---|---|
| NH₂ | Ammine |
| H₂O | Aqua |
| CO | Carbonyl |
| NO | Nitrosyl |
| O₂ | Dioxygen |
| N₂ | Dinitrogen |
| H₂NCH₂CH₂NH₂ | Ethylenediamine |
| C3H5N | Pyridine |
Positive Ligands
Positive ligands are named with an ending –ium.
NH2–NH3+ Hydrazinium
This ligand, though positive can bind through the uncharged nitrogen.
Order of Ligands
When a complex contains more than one kind of ligand, then, the ligands are listed in alphabetical order.
[CrCl3(NO₂)3]³– Trichlorotrinitrochromate(III) ion
[Cr(NH3)3(H₂O)3]³+ Triamminetriaquachromium(III) ion
[CrBrCl₂F3]³– Bromodichlorotri fluorochromate(III) ion
[Co(NO₂)3(NH3)3] Triamminetrinitrocobalt(III)
[PtCl2(H₂O)(NH₂NH3)]+ Aquadichlorohydraziniumplatinum(II) ion
[PtCl₂(Ph₂PH)(tu)] Dichloro(diphenylphosphine)thioureaplatinum(II)ion
Na[PtBrCl(NO₂)(NH3)] Sodium amminebromochloronitroplatinate(II)
The prefixes (di, tri and so on) are not considered in choosing the alphabetical order; numerical prefixes do not affect the alphabetical order.
The prefixes, di, tri, tetra, penta and hexa are used to indicate the number of ligands of the same kind present in the complex.
[Ag(NH3)2] + Diamminesilver(I) ion
However, when the name of the ligand itself has a prefix such as di, tri, tetra, or if it is a polydentate ligand, then to avoid confusion, Greek prefixes such as bis (= di), tris (= tri) and tetrakis (= tetra) are used to indicate the number of such ligands present in the complex. In these cases, the names of the ligands are written in parentheses.
[Cu(en)2]SO4 Bis(ethylenediamine)copper(II) sulphate
[Cr(en)3]Cl3 Tris(ethylenediamine) chromium(III) chloride
[Ni(PF3)4] Tetrakis(phosphorustrifluoride)nickel(0)
When these Greek prefixes are used, the name of the ligand is placed in parenthesis.
Naming Isomeric Ligands
A type of isomerism among complexes involves attachment of a ligand to the metal through different donor atoms. For example, the ligand NO₂ may be linked to the metal M through N or O. Then, such differences in linkages are indicated by different ligand names.
M-NO₂ Nitro (linked through the N)
M-ONO Nitrito (linked through the O)
[CO(NH3)5NO₂]²+ Pentaaminenitrocobalt(III) ion
[CO(NH3)5ONO]²+ Pentaaminenitritocobalt(III) ion
M-SCN Thiocyanato (linked through the S)
M-NCS Isothiocyanato (linked through the N)
Examples are (CO), Mn-SCN and (CO), Mn-NCS
