Nomenclature

Understanding the Nomenclature of Coordination Compounds

Nomenclature in chemistry is the systematic naming of chemical compounds. For coordination compounds, which consist of a central metal atom or ion bonded to a set of ligands, the nomenclature becomes particularly important due to the complexity and variety of these compounds. The International Union of Pure and Applied Chemistry (IUPAC) provides rules for naming coordination compounds to ensure consistency and clarity.

Key Terms

  • Coordination Compound: A compound that contains a coordination complex, which is a central atom or ion (usually metallic) surrounded by a number of bound molecules or anions (ligands).
  • Ligand: An ion or molecule that binds to a central metal atom to form a coordination complex.
  • Coordination Sphere: The central atom/ion plus its attached ligands.
  • Oxidation State: The charge that an atom would have if all bonds to atoms of different elements were 100% ionic.
  • Coordination Number: The number of ligand atoms that are bonded to the central atom/ion.

Rules for Naming Coordination Compounds

The IUPAC rules for naming coordination compounds are as follows:

  1. Name the ligands first, in alphabetical order, followed by the central metal.
  2. For anionic ligands, the ending is changed to '-ido' or '-ate', depending on the ligand.
  3. Neutral ligands are named as the molecule, with some exceptions (e.g., water is named 'aqua').
  4. The oxidation state of the metal is indicated by Roman numerals in parentheses.
  5. If the coordination entity is an anion, the name of the metal ends with the suffix '-ate'.
  6. The prefix 'bis-', 'tris-', 'tetrakis-', etc., are used when the ligands are polydentate or if there are multiple identical ligands that require prefixes like di-, tri-, etc., to avoid confusion.

Table of Common Ligands and Their Names

Ligand Formula Ligand Type Ligand Name in Coordination Compounds
Cl- Anionic Chlorido
NH3 Neutral Ammine
CO Neutral Carbonyl
H2O Neutral Aqua
CN- Anionic Cyanido
OH- Anionic Hydroxido
O2- Anionic Oxido
EDTA Polydentate Ethylenediaminetetraacetato

Examples

Let's look at some examples to understand the application of these rules:

Example 1: [Co(NH3)6]Cl3

  • Ligands: NH3 (Ammine)
  • Central Metal: Co (Cobalt)
  • Oxidation State of Co: +3

Name: Hexaamminecobalt(III) chloride

Example 2: K4[Fe(CN)6]

  • Ligands: CN- (Cyanido)
  • Central Metal: Fe (Iron)
  • Oxidation State of Fe: +2

Name: Potassium hexacyanidoferrate(II)

Example 3: [Cr(H2O)4Cl2]Cl

  • Ligands: H2O (Aqua), Cl- (Chlorido)
  • Central Metal: Cr (Chromium)
  • Oxidation State of Cr: +3

Name: Tetraaquadichloridochromium(III) chloride

Example 4: [Pt(NH3)2Cl2]

  • Ligands: NH3 (Ammine), Cl- (Chlorido)
  • Central Metal: Pt (Platinum)
  • Oxidation State of Pt: +2

Name: Diamminedichloridoplatinum(II)

Practice Problems

  1. Name the following coordination compound: [Cu(NH3)4]SO4
  2. What is the IUPAC name for [Co(en)3]Cl3, where 'en' is ethylenediamine?
  3. Determine the name for [Ag(NH3)2]+ ion.

Answers

  1. Tetraamminecopper(II) sulfate
  2. Tris(ethylenediamine)cobalt(III) chloride
  3. Diamminesilver(I) ion

By understanding and applying these rules, you can systematically name any coordination compound you encounter. Remember that practice is key to mastering chemical nomenclature, so make sure to work through plenty of examples as you study for your exams.