Naming Coordination Compounds

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:

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.

Examples Give the systematic names for the following coordination compounds:

1. [Cr(NH3)3(H2O)3]Cl3

2. [Pt(NH3)5Cl]Br3

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

 


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