Question

Name each of the following complex ions and determine the oxidation number of the metal: (a) \(\left[\mathrm{Fe}(\mathrm{CN})_{6}\right]^{4-}\); (b) \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right]^{3+}\); (c) \(\left[\mathrm{Co}(\mathrm{CN})_{5}\left(\mathrm{OH}_{2}\right)\right]^{2-}\); (d) \(\left[\mathrm{Co}\left(\mathrm{SO}_{4}\right)\left(\mathrm{NH}_{3}\right)_{5}\right]^{+}\).

Short answer

(a) Hexacyanoferrate(II), Fe is +2. (b) Hexaamminecobalt(III), Co is +3. (c) Pentaammineaquacobalt(III), Co is +3. (d) Pentaammine(sulfato)cobalt(III), Co is +3.

Step-by-step solution

Identifying the Ligands for Each Complex Ion

Identify and list all the ligands in each complex ion, noting their charges. For example, CN⁻ has a charge of -1, NH₃ is neutral, OH₂ (water) is neutral, and SO₄²⁻ has a charge of -2.

Naming the Ligands

Name the ligands in alphabetical order, ignoring any prefixes that specify quantity. Cyano for CN⁻, ammine for NH₃, aqua for H₂O, and sulfate for SO₄²⁻.

Determining the Oxidation Number of the Metal

Use the charge of the complex ion and the known charges of the ligands to determine the oxidation state of the metal. The sum of the oxidation states of the metal and the charges of the ligands must equal the overall charge of the complex ion.

Naming the Complex Ions

Combine the names of the ligands with the metal. For cationic complexes, the metal keeps its elemental name. For anionic complexes, the metal ends with the suffix '-ate'. The oxidation state of the metal is denoted in Roman numerals in parentheses after the metal's name.

Writing the Names and Oxidation States for Each Complex

(a) Hexacyanoferrate(II) with oxidation number +2. (b) Hexaamminecobalt(III) with oxidation number +3. (c) Pentaammineaquacobalt(III) with oxidation number +3. (d) Pentaammine(sulfato)cobalt(III) with oxidation number +3.

Key concepts

Ligands Identification

Identifying ligands, which are the ions or molecules attached to the central metal atom in a coordination compound, is crucial in understanding and naming complex ions. Let's take for example the complex ion \(\[\mathrm{Fe}(\mathrm{CN})_{6}\]^{4-}\); here, the ligand is cyanide (CN⁻). Ligands can be charged or neutral, and this affects the overall charge of the complex ion.

Neutral ligands, like ammonia (NH₃) and water (OH₂), do not affect the complex's overall charge. However, charged ligands, such as cyanide (CN⁻) and sulfate (SO₄²⁻), contribute negative charges to the complex. It's important for students to recognize that ligands should be listed in alphabetical order when naming a complex ion, disregarding any prefixes. For instance, in \(\[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\]^{3+}\), ammine (NH₃) is the ligand.

Types of Ligands

In coordination compounds, ligands can be:

• Unidentate: bonding through a single donor atom, e.g., CN⁻ and NH₃.
• Bidentate: bonding through two donor atoms, such as ethylenediamine (en).
• Polydentate: bonding through multiple donor atoms, e.g., EDTA.

Students should be comfortable identifying these ligands as they form the basis of how complex ions are named and understood.

Oxidation Number Determination

Determining the oxidation number of the metal in a complex ion involves using the overall charge of the ion and the known charges of the ligands. The sum of the oxidation states of all atoms in a complex ion is equal to the complex ion’s charge. For example, in \(\[\mathrm{Fe}(\mathrm{CN})_{6}\]^{4-}\), the cyanide ligands each have a charge of -1, and there are six of them, contributing a total charge of -6. Because the complex ion has an overall charge of -4, the iron must have an oxidation number of +2 to balance the charges.

Oxidation number rules are the same as in other chemical contexts: the oxidation number of neutral elements is zero, and for monoatomic ions, it is equal to the ion's charge. This can be more challenging when polyatomic ligands are involved, but the basic principle of charge balancing remains the same.

Calculating the Oxidation Number

The general formula to find the metal's oxidation number is:

• Let x be the oxidation number of the metal.
• Sum up charges of all ligands and the overall charge of the complex.
• Create an equation where x plus the ligands' charges equals the complex's charge.
• Solve for x, which will be the metal's oxidation number.

By practicing these steps, students can proficiently deduce the oxidation state of the metal in various complexes.

Coordination Compound Nomenclature

Naming coordination compounds requires a systematic approach. The names of the ligands are given first, followed by the metal. In our examples, compounds like \(\[\mathrm{Fe}(\mathrm{CN})_{6}\]^{4-}\) and \(\[\mathrm{Co}(\mathrm{NH}_{3})_{6}\]^{3+}\) follow these rules. When the complex ion is an anion (negatively charged), the metal's name ends with the suffix '-ate', with Latin or English alternative names used for some metals; for example, iron becomes ferrate. The oxidation state of the metal in the complex is indicated by Roman numerals in parentheses right after the metal's name.

For positively charged complex ions, the metal name is not altered, and the oxidation state still follows in parentheses. Ligands are named with specific prefixes and suffixes: for example, 'cyano' for CN⁻ and 'ammine' for NH₃.

Naming Order and Specific Terms

• Anionic ligands end in '-o', neutral ligands keep their names with certain exceptions (water becomes 'aqua').
• Ligands are named in alphabetical order, without considering numerical prefixes.
• The metal name follows ligands, with its oxidation state in Roman numerals.
• In polynuclear complexes, prefixes like 'bis-' and 'tris-' are used to indicate the number of identical ligands.

Understanding these rules helps students to accurately name coordination compounds and interpret complex chemical formulas.