1.3 Nomenclature of Coordination Compounds

Nomenclature of Coordination Compounds

Coordination compounds consist of a central metal atom or ion surrounded by molecules or ions called ligands. Naming these compounds follows a precise set of IUPAC rules that let chemists communicate structure, composition, and oxidation state from the name alone. This section covers how to build those names, determine oxidation states, and write correct formulas.

Components of Coordination Compounds

Every coordination compound has two key parts: the central metal and its ligands. The name is built from both, plus the metal's oxidation state.

• Ligands are always named before the central metal in the compound's name.
• Anionic ligands get the suffix -o appended to their root name: chloride → chloro, cyanide → cyano, hydroxide → hydroxo, oxalate → oxalato.
• Neutral ligands generally keep their molecular name, but a few have special names you need to memorize:
  • Water → aqua
  • Ammonia → ammine (note the double "m")
  • Carbon monoxide → carbonyl
  • Nitric oxide → nitrosyl
• The oxidation state of the central metal is written as a Roman numeral in parentheses right after the metal name, with no space: cobalt(III), iron(II).
• Within the coordination sphere, ligands are listed in alphabetical order by their ligand name (ignoring any multiplying prefixes like di- or tris-).

Prefixes and Suffixes in Nomenclature

When a complex contains more than one of the same ligand, you indicate the count with a prefix.

• For simple ligand names, use the standard Greek prefixes: di-, tri-, tetra-, penta-, hexa-.
  • Example: six ammine ligands → hexaammine
• If the ligand name already contains a Greek prefix or is complex (like ethylenediamine), use the alternative prefixes bis-, tris-, tetrakis-, etc., and enclose the ligand name in parentheses.
  • Example: three ethylenediamine ligands → tris(ethylenediamine), not triethylenediamine

For anionic complexes, the metal name takes the suffix -ate, and for certain metals a Latin-derived root is used:

• Iron → ferrate
• Copper → cuprate
• Gold → aurate
• Silver → argentate
• Tin → stannate
• Lead → plumbate

So $[Fe(CN)_6]^{4-}$ as a potassium salt is named potassium hexacyanoferrate(II), not potassium hexacyanoironate(II).

Cationic or neutral complexes use the ordinary English metal name with no special suffix.

Oxidation States in Coordination Compounds

Determining Oxidation States

The oxidation state of the central metal ties the name to the electronic structure of the complex. You find it using a charge-balance equation:

$\text{Oxidation state of metal} + \text{Sum of all ligand charges} = \text{Overall charge of the complex ion}$

Neutral ligands (aqua, ammine, carbonyl) contribute 0. Anionic ligands contribute their ionic charge (chloro = −1, cyano = −1, oxalato = −2).

Calculating Oxidation States: Step-by-Step

1. Identify the complex ion and its overall charge. Counter-ions outside the brackets tell you this charge.
2. List every ligand and assign each its charge.
3. Multiply each ligand charge by the number of that ligand present.
4. Solve for the metal's oxidation state.

Example 1: $[Co(NH_3)_6]Cl_3$

• Three $Cl^-$ counter-ions mean the complex ion $[Co(NH_3)_6]$ has a +3 charge.
• Six $NH_3$ ligands are neutral (total ligand charge = 0).
• Co oxidation state: $+3 - 0 = +3$. This is cobalt(III).

Example 2: $K_4[Fe(CN)_6]$

• Four $K^+$ counter-ions mean the complex ion $[Fe(CN)_6]$ has a −4 charge.
• Six $CN^-$ ligands contribute $6 \times (-1) = -6$.
• Fe oxidation state: $-4 - (-6) = +2$. This is iron(II).

Ligands and Prefixes in Nomenclature

Common Ligands

You'll encounter these ligands repeatedly. Knowing their names and charges by heart saves a lot of time.

Polydentate ligands (also called chelating ligands) bind through more than one donor atom. Ethylenediamine (en) binds through two nitrogen atoms, and EDTA binds through six donor atoms. Their multi-point attachment is why the bis-/tris-/tetrakis- prefixes exist: the ligand names are already complex enough that stacking a di- or tri- on the front would be confusing.

Prefixes for Multiple Ligands: Quick Reference

Formula Writing for Coordination Compounds

Writing Formulas: Step-by-Step

1. Write the metal symbol first inside square brackets.
2. List the ligands after the metal. Convention: anionic ligands before neutral ligands, and within each group, alphabetical order by chemical formula.
3. Use subscripts to show the number of each ligand: $[Co(NH_3)_6]$, $[CoCl_2(en)_2]$.
4. Place the overall charge of the complex as a superscript outside the square brackets: $[Fe(CN)_6]^{4-}$.
5. Write the counter-ions outside the brackets to balance the charge. Cation comes first in the full formula: $K_4[Fe(CN)_6]$, $[Co(NH_3)_6]Cl_3$.

Anionic Complexes and Naming as Salts

When the coordination complex itself is an anion, the full compound is named as a salt: cation name first, then the anionic complex name (which uses the -ate suffix on the metal).

• $K_3[Fe(CN)_6]$ → potassium hexacyanoferrate(III)
• $Na_2[PtCl_6]$ → sodium hexachloroplatinate(IV)

The oxidation state is not written explicitly in the formula. You determine it from the counter-ions and ligand charges, as shown in the oxidation state section above.