5.3: Nomenclature of Coordination Chemistry
- Page ID
- 206950
Coordination complexes have their own classes of isomers, different magnetic properties and colors, and various applications (photography, cancer treatment, etc), so it makes sense that they would have a naming system as well. Consisting of a metal and ligands, their formulas follow the pattern [Metal Anions Neutrals]±Charge, while names are written Prefix Ligands Metal(Oxidation State).
Introduction
According to the Lewis base theory, ligands are Lewis bases since they can donate electrons to the central metal atom. The metals, in turn, are Lewis acids since they accept electrons. Coordination complexes consist of a ligand and a metal center cation. The overall charge can be positive, negative, or neutral. Coordination compounds are complex or contain complex ions, for example:
- Complex Cation: \([CO(NH_3)_6]^{3+}\)
- Complex Anion: \([CoCl_4(NH_3)_2]^-\)
- Neutral Complex: \([CoCl_3(NH_3)_3]\)
- Coordination Compound: \(K_4[Fe(CN)_6]\)
A ligand can be an anion or a neutral molecule that donates an electron pair to the complex (NH3, H2O, Cl-). The number of ligands that attach to a metal depends on whether the ligand is monodentate, bidentate, or polydentate. For more information, see Ligands and Chelation. To begin naming coordination complexes, here are some things to keep in mind.
- Ligands are named first in alphabetical order.
- The name of the metal comes next.
- The oxidation state of the metal follows, noted by a Roman numeral in parentheses (II, IV).
Rule 1: Anionic Ligands
Ligands that act as anions which end in "-ide" are replaced with an ending "-o" (e.g., Chloride → Chloro). Anions ending with "-ite" and "-ate" are replaced with endings "-ito" and "-ato" respectively (e.g., Nitrite → Nitrito, Nitrate → Nitrato).
Molecular Formula | Ligand Name | Molecular Formula | Ligand Name |
---|---|---|---|
F- | Fluoro | OH- | Hydroxo |
Cl- | Chloro | SO42- | Sulfato |
Br- | Bromo | S2O32- | Thiosulfato |
I- | Iodo | NO2- | Nitrito-N-; Nitro |
O2- | Oxo | ONO- | Nitrito-O-; Nitrito |
CN- | Cyano | SCN- | Thiocyanato-S-; Thiocyanato |
NC- | Isocyano | NCS- | Thiocyanato-N-; Isothiocyanato |
Rule 2: Neutral Ligands
Most neutral molecules that are ligands carry their normal name. The few exceptions are the first four on the chart: ammine, aqua, carbonyl, and nitrosyl.
Molecular Formula of Ligand | Ligand Name |
---|---|
NH3 | Ammine |
H2O | Aqua |
CO | Carbonyl |
NO | Nitrosyl |
CH3NH2 | Methylamine |
C5H5N | Pyridine |
Polydentate ligands follow the same rules for anions and neutral molecules.
Short name | Extended name |
---|---|
en | Ethylenediamine |
ox2- | Oxalato |
EDTA4- | Ethylenediaminetetraacetato |
Rule 3: Ligand Multiplicity
The number of ligands present in the complex is indicated with the prefixes di, tri, etc. The exceptions are polydentates that have a prefix already in their name (en and EDTA4- are the most common). When indicating how many of these are present in a coordination complex, put the ligand's name in parentheses and use bis, tris, and tetrakis.
Number of Ligands | Monodentate Ligands | Polydentate Ligands |
---|---|---|
1 | mono | - |
2 | di | bis |
3 | tri | tris |
4 | tetra | tetrakis |
5 | penta | - |
6 | hexa | - |
Prefixes always go before the ligand name; they are not taken into account when putting ligands in alphabetical order. Note that "mono" often is not used. For example, [FeCl(CO)2(NH3)3]2+ would be called triamminechlorodicarbonyliron(III) ion. Remember that ligands are always named first, before the metal is.
Rule 4: The Metals
When naming the metal center, you must know the formal metal name and the oxidation state. To show the oxidation state, we use Roman numerals inside parenthesis. For example, in the problems above, chromium and cobalt have the oxidation state of +3, so that is why they have (III) after them. Copper, with an oxidation state of +2, is denoted as copper(II). If the overall coordination complex is an anion, the ending "-ate" is attached to the metal center. Some metals also change to their Latin names in this situation. Copper +2 will change into cuprate(II). The following change to their Latin names when part of an anion complex:
Transition Metal | Latin |
---|---|
Iron | Ferrate |
Copper | Cuprate |
Tin | Stannate |
Silver | Argentate |
Lead | Plumbate |
Gold | Aurate |
The rest of the metals simply have -ate added to the end (cobaltate, nickelate, zincate, osmate, cadmate, platinate, mercurate, etc. Note that the -ate tends to replace -um or -ium, if present).
Finally, when a complex has an overall charge, "ion" is written after it. This is not necessary if it is neutral or part of a coordination compound (Example \(\PageIndex{3}\)). Here are some examples with determining oxidation states, naming a metal in an anion complex, and naming coordination compounds.
A last little side note: when naming a coordination compound, it is important that you name the cation first, then the anion. You base this on the charge of the ligand. Think of NaCl. Na, the positive cation, comes first and Cl, the negative anion, follows.
Writing Formulas of Coordination Complexes
The formula of a coordination complex is written in a different order than its name. The chemical symbol of the metal center is written first. The ligands are written next, with anion ligands coming before neutral ligands. If there is more than one anion or neutral ligand, they are written in alphabetical order according to the first letter in their chemical formula.
In a coordination compound's name, when one of the ions is just an element, the number of atoms is not indicated with a prefix. Since it still has to be written in the formula, it is determined by balancing the overall charge of the compound. (For example, tetrafluorochromium(VI) chloride becomes [CrF4]Cl2.
References
- Petrucci, Ralph H. General Chemistry Principles and Modern Applications. 9th ed. Upper Saddle River: Pearson Prentice Hall, 2002.
Contributors
- Justin Hosung Lee (UCD), Sophia Muller (UCD)