A complex is a substance in which a metal atom or ion is associated with a group of neutral molecules or anions called ligands. Coordination compounds are neutral substances (i.e. uncharged) in which at least one ion is present as a complex. You will learn more about coordination compounds in the lab lectures of experiment 4 in this course.
The coordination compounds are named in the following way. (At the end of this tutorial we have some examples to show you how coordination compounds are named.)
A. To name a coordination compound, no matter whether the complex ion is the cation or the anion, always name the cation before the anion. (This is just like naming an ionic compound.)B. In naming the complex ion:
1. Name the ligands first, in alphabetical order, then the metal atom or ion. Note: The metal atom or ion is written before the ligands in the chemical formula.2. The names of some common ligands are listed in Table 1.
� For anionic ligands end in "-o"; for anions that end in "-ide"(e.g. chloride), "-ate" (e.g. sulfate, nitrate), and "-ite" (e.g. nirite), change the endings as follows: -ide-o; -ate
� For neutral ligands, the common name of the molecule is used e.g. H2NCH2CH2NH2 (ethylenediamine). Important exceptions: water is called ‘aqua’, ammonia is called ‘ammine’, carbon monoxide is called ‘carbonyl’, and the N2 and O2 are called ‘dinitrogen’ and ‘dioxygen’.
Table 1. Names of Some Common Ligands
Anionic Ligands
|
Names
|
Neutral Ligands
|
Names
| |
Br-
|
bromo
|
NH3
|
ammine
| |
F-
|
fluoro
|
H2O
|
aqua
| |
O2-
|
oxo
|
NO
|
Nitrosyl
| |
OH-
|
Hydroxo
|
CO
|
Carbonyl
| |
CN-
|
cyano
|
O2
|
dioxygen
| |
C2O42-
|
oxalato
|
N2
|
dinitrogen
| |
CO32-
|
carbonato
|
C5H5N
|
pyridine
| |
CH3COO-
|
acetato
|
H2NCH2CH2NH2
|
ethylenediamine
|
3. Greek prefixes are used to designate the number of each type of ligand in the complex ion, e.g. di-, tri- and tetra-. If the ligand already contains a Greek prefix (e.g. ethylenediamine) or if it is polydentate ligands (ie. can attach at more than one binding site) the prefixes bis-, tris-, tetrakis-, pentakis-, are used instead. (See examples 3 and 4.) The numerical prefixes are listed in Table 2.
Table 2. Numerical Prefixes
Number
|
Prefix
|
Number
|
Prefix
|
Number
|
Prefix
|
1
|
mono
|
5
|
penta (pentakis)
|
9
|
nona (ennea)
|
2
|
di (bis)
|
6
|
hexa (hexakis)
|
10
|
deca
|
3
|
tri (tris)
|
7
|
hepta
|
11
|
undeca
|
4
|
tetra (tetrakis)
|
8
|
octa
|
12
|
dodeca
|
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
Name of Metal
|
Name in an Anionic Complex
|
Iron
|
Ferrate
|
Copper
|
Cuprate
|
Lead
|
Plumbate
|
Silver
|
Argenate
|
Gold
|
Aurate
|
Tin
|
Stannate
|
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)63- and Fe(CN)64- 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)
Solution: potassium is the cation and the complex ion is the anion.
Since there are 4 K+ binding with a complex ion, the charge on the complex ion must be - 4.
Since each ligand carries –1 charge, the oxidation number of Fe must be +2.
The common name of this compound is potassium ferrocyanide.
6. Na2[NiCl4]
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, eventhough 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.
Can you give the molecular formulas of the following coordination compounds?
1. hexaammineiron(III) nitrate
2. ammonium tetrachlorocuprate(II)
3. sodium monochloropentacyanoferrate(III)
4. potassium hexafluorocobaltate(III)
Can you give the name of the following coordination compounds?
5. [CoBr(NH3)5]SO4
6. [Fe(NH3)6][Cr(CN)6]
7. [Co(SO4)(NH3)5]+
8. [Fe(OH)(H2O)5]2+
Answers:
1. [Fe(NH3)6](NO3)3
2. (NH4)2[CuCl4]
3. Na3[FeCl1(CN)5]
4. K3[CoF6]
5. pentaamminebromocobalt(III) sulfate
6. hexaammineiron(III) hexacyanochromate (III)
7. pentaamminesulfatocobalt(III) ion
8. pentaaquahydroxoiron(III) ion
Naming Coordination Compounds