# Nomenclature of Coordination Complexes

Learning Objectives

• To learn the basis for complex ion and compound nomenclature

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.

1. Ligands are named first in alphabetical order.
2. The name of the metal comes next.
3. 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).

Table $$\PageIndex{1}$$: Anionic Monodentate Ligands
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.

Table $$\PageIndex{2}$$: Select Neutral Monodentate Ligands. Note: Ammine is spelled with two m's when referring to a ligand. Amines are a class of organic nitrogen-containing compounds.
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.

Table $$\PageIndex{3}$$: Select Polydentate ligands
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.

Table $$\PageIndex{4}$$: Prefixes for indicating number of ligands in a complex.
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.

Example $$\PageIndex{1}$$

What is the name of this complex ion: $$[CrCl_2(H_2O)_4]^+$$?

Solution

Let's start by identifying the ligands. The ligands here are Cl and H2O. Therefore, we will use the monodentate ligand names of "chloro" and "aqua". Alphabetically, aqua comes before chloro, so this will be their order in the complex's name. There are 4 aqua's and 2 chloro's, so we will add the number prefixes before the names. Since both are monodentate ligands, we will say "tetra[aqua]di[chloro]".

Now that the ligands are named, we will name the metal itself. The metal is Cr, which is chromium. Therefore, this coordination complex is called tetraaquadichlorochromium(III) ion. See the next section for an explanation of the (III).

Exercise $$\PageIndex{1}$$

What is the name of this complex ion: $$[CoCl_2(en)_2]^+$$?

Solution

We take the same approach. There are two chloro and ethylenediamine ligands. The metal is Co, cobalt. We follow the same steps, except that $$en$$ is a polydentate ligand with a prefix in its name (ethylenediamine), so "bis" is used instead of "bi", and parentheses are added. Therefore, this coordination complex is called dichlorobis(ethylenediamine)cobalt(III) ion.

### 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:

Table $$\PageIndex{5}$$: Latin terms for Select Metal Ion
Transition Metal Latin
Iron Ferrate
Copper Cuprate
Tin Stannate
Silver Argentate
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.

Example $$\PageIndex{2}$$

What is the name of [Cr(OH)4]- ?

Solution

Immediately we know that this complex is an anion. There is only one monodentate ligand, hydroxide. There are four of them, so we will use the name "tetrahydroxo". The metal is chromium, but since the complex is an anion, we will have to use the "-ate" ending, yielding "chromate". The oxidation state of the metal is 3 (x+(-1)4=-1). Write this with Roman numerals and parentheses (III) and place it after the metal to get tetrahydroxochromate(III) ion.

Exercise $$\PageIndex{2}$$

What is the name of [CuCl4]2- ?

Solution

tetrachlorocuprate(II) ion

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.

Example $$\PageIndex{3}$$

What is the name of [Pt(NH3)4)][Pt(Cl)4] ?

Solution

NH3 is neutral, making the first complex positively charged overall. Cl has a -1 charge, making the second complex the anion. Therefore, you will write the complex with NH3 first, followed by the one with Cl (the same order as the formula). This coordination compound is called tetraammineplatinum(II) tetrachloroplatinate(II).

Example $$\PageIndex{4}$$

What is the name of [CoCl(NO2)(NH3)4]+ ?

Solution

This coordination complex is called tetraamminechloronitrito-N-cobalt(III). N comes before the O in the symbol for the nitrite ligand, so it is called nitrito-N. If an O came first, as in [CoCl(ONO)(NH3)4]+, the ligand would be called nitrito-O, yielding the name tetraamminechloronitrito-O-cobalt(III).

Nitro (for NO2) and nitrito (for ONO) can also be used to describe the nitrite ligand, yielding the names tetraamminechloronitrocobalt(III) and tetraamminechloronitritocobalt(III).

## 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.

Examples $$\PageIndex{5}$$

1. Amminetetraaquachromium(II)
2. Amminesulfatochromium(II)
3. Amminetetraaquachromium(II) sulfate
4. Potassium hexacyanoferrate(III)

Solution

1. Amminetetraaquachromium(II) ion would be written as [Cr(H2O)4(NH3)]+2. Both ligands are neutral, so they are ordered alphabetically with H2O before NH3. Their order in the formula is the opposite of that in the complex's name since one uses their chemical symbols and the other uses the names of the ligands.
2. Amminesulfatochromium(II) is written as [Cr(SO4)(NH3)]. SO4 is an anion, so it comes before NH3.
3. Amminetetraaquachromium(II) sulfate -> Try this on your own. Did you get [Cr(H2O)4(NH3)]SO4? If you did, you are correct.
4. Potassium hexacyanoferrate(III) -> Try this on your own. Did you get K3[Fe(CN)6]? Remember to balance the K!

## References

1. Petrucci, Ralph H. General Chemistry Principles and Modern Applications. 9th ed. Upper Saddle River: Pearson Prentice Hall, 2002.