A coordination complex is a substance in which a metal atom or ion accepts electrons from (and thus associates with) a group of neutral molecules or anions called ligands. A complex can be an anion, a cation ion, or a neutral molecule. 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 for experiment 5 in this course.
The coordination compounds are named in the following way.
A. When naming coordination compounds, always name the cation before the anion. This rule holds regardless of whether the complex ion is the cation or the anion. (This is just like naming an ionic compound.)
B. In naming the complex ion:
1. Name the ligands first, in alphabetical order, and then name the central metal. Note: In the chemical formula the central metal is written before the ligands.
2. The names of some common ligands are listed in Table 1.
Table 1. Names of Some Common Ligands
3. The Greek prefixes di-, tri-, tetra-, etc. are used to designate the number of each type of ligand in the complex ion. If the ligand already contains a Greek prefix (e.g. ethylenediamine) or if it is a polydentate ligand (i.e. it can attach at more than one coordination site), the prefixes bis-, tris-, tetrakis-, and pentakis- are used instead. (See examples 3 and 4.) The numerical prefixes are listed in Table 2.
Table 2. Numerical Prefixes
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 called 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 and not ironate).
Table 3: Name of Metals in Anionic Complexes
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.
Give the systematic names for the following coordination compounds:
Answer: triamminetriaquachromium(III) chloride
Answer: pentaamminechloroplatinum(IV) bromide
Answer: dichlorobis(ethylenediamine)platinum(IV) chloride
Answer: tris(ethylenediamine)cobalt(III) sulfate
Answer: potassium hexacyanoferrate(II)
Answer: sodium tetrachloronickelate(II)
Answer: ammonium diaquabis(oxalato)nickelate(II)
Solution: The oxalate ion is a bidentate ligand.
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. pentaamminebromocobalt(III) sulfate
6. hexaammineiron(III) hexacyanochromate (III)
7. pentaamminesulfatocobalt(III) ion
8. pentaaquahydroxoiron(III) ion
Updated On: 7/12/2011 (G Noelken)