Nomenclature of Coordination Compounds

The nomenclature of coordination compounds follows specific rules established by the International Union of Pure and Applied Chemistry (IUPAC). This systematic approach helps accurately name coordination compounds by specifying the number, type, and arrangement of ligands around a central metal atom or ion.

The IUPAC nomenclature system for coordination compounds is based on guidelines developed from Alfred Werner's coordination chemistry theories and is recommended by the Inorganic Nomenclature Committee of the IUPAC. This system ensures that the names of coordination compounds are systematic, unambiguous, and consistent across the field of inorganic chemistry. Below are the detailed rules for naming coordination compounds:

1.0Identify the Components of the Coordination Compound

A coordination compound consists of a central metal ion (or atom) surrounded by ligands. Ligands are ions or molecules that donate a pair of electrons to the metal to form a coordinate bond. The coordination sphere refers to the central metal ion and its attached ligands. The parts outside the brackets are called counterions, balancing the charge of the complex ion.

Ex. K₄[Fe(CN)₆] the naming of this complex starts with potassium.

1. Naming the Ligands

Ligands are named before the central metal ion in the formula. Different rules apply depending on whether the ligands are neutral molecules or anions:

Here are the details of neutral ligands, anionic ligands, and polydentate ligands presented in tabular format for clarity:

(a) Neutral Ligands

(b) Anionic Ligands

(c) Polydentate Ligands

2. Using Prefixes for the Number of Ligands

Prefixes indicate the number of each type of ligand:

• "Mono-" (1, rarely used)
• "Di-" (2)
• "Tri-" (3)
• "Tetra-" (4)
• "Penta-" (5)
• "Hexa-" (6)

For polydentate ligands or if the ligand name already contains a numerical prefix (like ethylenediamine), use prefixes like "bis-" (2), "tris-" (3), "tetrakis-" (4) with the ligand name in parentheses.

3. Naming the Central Metal Atom/Ion

• Neutral Complexes: If the complex is neutral, the name of the metal is written as it is.
• Anionic Complexes: If the complex ion is an anion, the metal name ends with the suffix "-ate." For example:
• • Iron becomes "ferrate."
  • Copper becomes "cuprate."
  • Silver becomes "argentate."
  • Gold becomes "aurate."
  • Lead becomes "plumbate."

4. Oxidation State of the Central Metal

The oxidation state of the central metal ion is indicated by a Roman numeral in parentheses immediately following the name of the metal.

5. Naming the Entire Coordination Complex

The complete name of the coordination compound is written as follows:

• Cation before Anion: In the full name, the cation is named first, followed by the anion, regardless of which is the complex ion.
• List Ligands Alphabetically: Within the coordination sphere, ligands are listed alphabetically by their name, not prefix. For example, "ammine" would come before "chloro," even if the complex contains more "chloro" ligands than "ammine."
• Write the Central Metal Name: After naming all the ligands, the central metal's name is written, followed by its oxidation state in parentheses.

2.0Examples of Naming Coordination Compounds

Example 1: Naming the Complex Ion [CrCl₂(H₂O)₄]⁺

Solution:

Identify the Ligands:

• The ligands in this complex are chloride (Cl⁻) and water (H₂O).
• The IUPAC names for these ligands are:
• Cl⁻: "chloro"
• H₂O: "aqua"

Order of Ligands:

• Ligands are named in alphabetical order, irrespective of their charge.
• "Aqua" comes before "chloro" alphabetically.

Specify the Number of Each Ligand:

• There are 4 H₂O (aqua) ligands and 2 Cl⁻ (chloro) ligands.
• Use the prefixes "tetra-" for 4 and "di-" for 2.
• The ligand names with prefixes: "tetraaqua" and "dichloro."

Name the Central Metal:

• The central metal is chromium (Cr).
• Since this is a cationic complex, the metal is named as "chromium."

Determine the Oxidation State of the Metal:

• To find the oxidation state of chromium:
• Let the oxidation state of Cr be xxx.
• For each Cl⁻, the oxidation number is −1, contributing −2 for two chlorides.
• H₂O is neutral, contributing 000.
• The overall charge of the complex ion is +1.
• Equation: x+(−2)+0=+1⇒x−2=+1⇒x=+3

Form the Complete Name:

• The full name of the complex is "tetraaquadichlorochromium(III) ion."
• "(III)" indicates the oxidation state of the chromium ion.

Example 2: K₄[Fe(CN)₆]

• The complex ion is [Fe(CN)₆]⁴⁻, with K⁺ as the counterion.
• Ligands: CN⁻ is "cyano."
• Central metal: Fe is "iron," but because it’s an anionic complex, it becomes "ferrate."
• Oxidation state of iron: +2.
• Name: Potassium hexacyanoferrate(II).

Example 3: [Co(en)₃]Cl₃

• The complex ion is [Co(en)₃]³⁺, with Cl⁻ as the counterion.
• Ligands: "en" (ethylenediamine) is a neutral ligand.
• Prefix: "tris-" is used because there are three ethylenediamine ligands.
• Central metal: Co is "cobalt."
• Oxidation state of cobalt: +3.
• Name: Tris(ethylenediamine)cobalt(III) chloride.

Example 4: Na[Cr(H₂O)₂Cl₄]

• The complex ion is [Cr(H₂O)₂Cl₄]⁻.
• Ligands: H₂O is "aqua" and Cl⁻ is "chloro."
• Central metal: Cr is "chromium."
• Oxidation state of chromium: +3.
• Name: Sodium diaquatetrachlorochromate(III).

The naming of some of the complexes is done as follows – (as per IUPAC)