6.3.1 Naming Complex Compounds

In the previous section, we learned how complex compounds are formed. Because these compounds can have a variety of structures, chemists use a systematic set of rules developed by IUPAC to name them. This ensures that every complex compound has a unique and unambiguous name, allowing scientists worldwide to communicate clearly. This section outlines the IUPAC nomenclature rules for naming complex compounds.

Main Idea

• The ligands attached to the central metal ion.
• The central metal ion and its oxidation state.
• The overall charge of the complex (and any counter-ions).

The IUPAC nomenclature of complex compounds recommends the following rules.

Core Concept

The core concept is to follow a systematic set of rules to build the name of a complex from its chemical formula. The process involves identifying the ligands, naming them in alphabetical order, specifying the metal and its oxidation state, and indicating the overall charge. The order of naming is generally: ligands (alphabetical) → central metal (with oxidation state) → anion (if any).

Detailed Explanation

The textbook outlines the following rules for naming complex compounds:

IUPAC Nomenclature Rules

i. Name cation before anion: Whether it is a simple ion or a complex ion, the cation is named first, followed by the anion.

ii. Ligands first in alphabetical order: While naming the coordination sphere, the ligands are named first in alphabetical order, then the transition metal ion.

• ferrate for iron
• cuprate for copper
• argentate for silver

iv. Prefixes for ligand number: We use the prefixes di, tri, tetra, etc., for ligands to show their number. For polydentate ligands, prefixes bis (two), tris (three), tetrakis (four), and pentakis (five) are used.

• chloro (Cl^-)
• hydroxo (OH^-)
• carbonato (CO_3^{2-})
• aqua (H_2O)
• ammine (NH_3)
• carbonyl (CO)

vi. Oxidation state in Roman numerals: The oxidation state of the central ion is represented by Roman numerals and put in parenthesis after the name of the metal atom.

Examples of Complex Compounds

The textbook provides the following examples:

Important Note from the Textbook: While naming complex compounds, the number of counter ions is not counted.

Working Out the Charge on Complexes and Their Formulae

The total charge on a complex ion depends upon the oxidation state of the metal ion and the number and nature of ligands. The oxidation state of the metal ion can be calculated from the charge on the complex ion. Remember that oxidation state on a transition metal ion equals the charge on the complex minus the sum of charges on the ligands.

The ions outside the coordination sphere neutralizing the complex ion are called counter ions.

Worked Examples from the Textbook

The textbook provides two worked examples:

Example 1: Deduce the charge on the tetraamminedichlorocobalt(III) complex ion. What is its formula?

• Analysis: The formula has two chloride ions and four ammonia molecules, so the ligands contribute two negative charges (-2). The central cobalt cation has an oxidation number of +3, so it contributes three positive charges (+3).
• Calculation: Total or net charge on the complex ion = (+3) + (-2) = +1.
• Formula: The formula is therefore [CoCl_2(NH_3)_4]⁺.

Example 2: Deduce the oxidation state of copper in the [CuCl_4]^2- complex ion. What is its name?

• Analysis: The complex ion is an anion with a total or net charge of -2. The formula has four chloride ions acting as ligands, which contribute a total of four negative charges (-4).
• Calculation: Hence, the central copper ion must contribute two positive charges so its oxidation state is +2.
• Name: This ion is the tetrachlorocuprate(II) ion.

Intermediate Level

The IUPAC nomenclature of complex compounds is a systematic method. To name a complex, you first identify the ligands and name them in alphabetical order, using prefixes (di, tri, tetra, etc.) to indicate their number. Anionic ligands get the suffix "-o" (e.g., chloro, hydroxo), while neutral ligands keep their names (e.g., aqua, ammine, carbonyl). The central metal ion is named next, followed by its oxidation state in Roman numerals in parentheses. If the complex ion is an anion, the metal's name gets the suffix "-ate" (e.g., ferrate, cuprate). The cation is always named before the anion. The rules also specify that for polydentate ligands, prefixes like bis, tris, tetrakis are used.

Important Points

• - Cation before anion in the name.
• - Ligands are named alphabetically before the metal.
• - Prefixes: di, tri, tetra, etc., for simple ligands; bis, tris, tetrakis for polydentate.
• - Anionic ligands end in "-o" (e.g., chloro, hydroxo, carbonato).
• - Neutral ligands keep their names (e.g., aqua, ammine, carbonyl).
• - Oxidation state of metal in Roman numerals in parentheses.
• - Suffix "-ate" for anionic complexes (e.g., ferrate, cuprate).
• - Number of counter ions is not counted in the name of the complex.
• - The charge on the complex = oxidation state of metal + sum of ligand charges.

Remember

• - Ligands first, alphabetical.
• - Prefixes for number: di, tri, tetra, etc.
• - Anionic ligands: end in "-o" (chloro, hydroxo).
• - Neutral ligands: keep their names (aqua, ammine).
• - Metal name, then oxidation state in Roman numerals.
• - Anionic complex: metal name + "-ate" (cuprate, ferrate).
• - Cation first, anion second.
• - Counter ions are not included in the complex name.

Common Mistakes

• - Not listing ligands in alphabetical order. This is a key rule. For example, in [Pt(NH_3)_4BrCl]Cl₂, "bromochloroplatinum" is correct because bromo comes before chloro alphabetically.
• - Forgetting the "-ate" suffix for anionic complexes. For example, [CuCl_4]^2- should be "cuprate", not "copper".
• - Using the wrong Roman numeral for the oxidation state. This often happens if the charge on the complex is not calculated correctly.
• - Incorrectly counting the number of counter ions. The name of the complex ion does not include the number of counter ions outside the brackets.
• - Using the wrong prefix for polydentate ligands. For example, using "di" instead of "bis" for two bidentate ligands. The textbook example [Cr(en)_2Cl_2]Cl uses "bis(1,2-diaminoethane)".

Quick Revision

The IUPAC nomenclature of complex compounds follows systematic rules. The ligands are named first in alphabetical order, using prefixes (di, tri, tetra) to indicate their number. Anionic ligands end in "-o" (e.g., chloro, hydroxo), while neutral ligands keep their names (e.g., aqua, ammine). The central metal ion is named next, followed by its oxidation state in Roman numerals in parentheses. If the complex ion is an anion, the metal's name gets the suffix "-ate". The cation is always named before the anion. The number of counter ions is not included in the name of the complex. For example, [Cu(NH_3)_2]²⁺ is diamminecopper(II) ion, and K_4[Fe(CN)_6] is potassium hexacyanoferrate(II).

60-Second Revision

When naming complex compounds: ligands first in alphabetical order, using prefixes (di, tri, tetra). Anionic ligands end in "-o" (chloro, hydroxo); neutral ligands keep their names (aqua, ammine). Then name the metal with its oxidation state in Roman numerals. If the complex is an anion, add the suffix "-ate" to the metal. The cation is named before the anion. Counter ions are not counted.

Frequently Asked Questions (FAQs)

Q1. What is the IUPAC nomenclature system for complex compounds? Answer: The IUPAC nomenclature system provides a set of rules for naming complex compounds unambiguously. The rules include naming the ligands alphabetically, using prefixes to indicate their number, specifying the metal and its oxidation state in Roman numerals, and adding the suffix "-ate" if the complex is an anion. The cation is named before the anion.

Q2. How do you name the ligands in a complex compound? Answer: Ligands are named in alphabetical order before the metal. Anionic ligands end in "-o" (e.g., chloro for Cl^-, hydroxo for OH^-). Neutral ligands usually retain their names (e.g., aqua for H_2O, ammine for NH_3, carbonyl for CO). The number of ligands is indicated by prefixes such as di, tri, tetra, etc.

Q3. What is the difference between a ligand name and a metal name in a complex? Answer: The ligand name indicates the molecule or ion attached to the metal (e.g., chloro, aqua). The metal name indicates the central metal ion, followed by its oxidation state in Roman numerals in parentheses (e.g., copper(II)). If the complex is anionic, the metal name gets the suffix "-ate" (e.g., cuprate).

• Q4. What is the IUPAC name of the complex [Cu(NH_3)_2]²⁺?
• Answer: The ligands are two ammonia molecules (ammine), and the metal is copper in the +2 oxidation state. The name is diamminecopper(II) ion.

Q5. What is the IUPAC name of the complex K_4[Fe(CN)_6]? Answer: The cation is potassium (K⁺), and the complex anion is [Fe(CN)_6]^4-. The ligands are six cyanide ions (cyano), and the metal is iron in the +2 oxidation state. Since the complex is anionic, the metal name becomes "ferrate". The name is potassium hexacyanoferrate(II).

• Q6. What is the IUPAC name of the complex [Ni(CO)_4]?
• Answer: The ligands are four carbon monoxide molecules (carbonyl), and the metal is nickel in the 0 oxidation state. The name is tetracarbonylnickel(0).

Q7. What is the IUPAC name of the complex [Co(NO_2)_3(NH_3)_3]? Answer: The ligands are three nitro groups (NO_2^-) and three ammonia molecules (NH_3). They are named alphabetically: ammine before nitro. The metal is cobalt in the +3 oxidation state. The name is triamminetrinitrocobalt(III).

Q8. How do you deduce the oxidation state of the central metal ion in a complex? Answer: The oxidation state of the central metal ion can be calculated from the overall charge of the complex ion and the charges of the ligands. The sum of the oxidation state of the metal and the charges of the ligands must equal the overall charge of the complex. For example, in [CuCl_4]^2-, the charge is -2, and the four chloride ligands contribute -4. Therefore, the copper ion must be +2.

Q9. What is the IUPAC name of the complex [Cr(en)_2Cl_2]Cl? Answer: The ligands are two 1,2-diaminoethane (en) and two chloride (chloro). They are named alphabetically: chloro before 1,2-diaminoethane. The metal is chromium in the +3 oxidation state. The name is dichlorobis(1,2-diaminoethane)chromium(III) chloride.

Multiple Choice Questions (MCQs)

• Q1. What is the IUPAC name of the complex [Cu(NH_3)_2]²⁺?
• A. Diamminecopper(II) ion
• B. Copper diammine(II) ion
• C. Diamminecuprate(II) ion
• D. Copper(II) diammine ion
• Correct Answer: A
• Explanation: The ligands are two ammonia molecules (ammine), and the metal is copper in the +2 oxidation state. The name is diamminecopper(II) ion.

• Q2. What is the name of the ligand Cl^- in a complex?
• A. Chlorine
• B. Chloride
• C. Chloro
• D. Chlorido
• Correct Answer: C
• Explanation: Anionic ligands end in "-o". Therefore, Cl^- is named "chloro" in a complex.

• Q3. What is the oxidation state of iron in K_4[Fe(CN)_6]?
• A. +1
• B. +2
• C. +3
• D. +4
• Correct Answer: B
• Explanation: The complex ion is [Fe(CN)_6]^4-. Six cyanide ligands contribute -6. The overall charge is -4, so the iron must be +2.

• Q4. What is the IUPAC name of [Ni(CO)_4]?
• A. Tetracarbonylnickel(0)
• B. Nickel tetracarbonyl(0)
• C. Tetracarbonylnickelate(0)
• D. Nickel(0) tetracarbonyl
• Correct Answer: A
• Explanation: The ligands are four carbonyl groups, and the metal is nickel in the 0 oxidation state. The name is tetracarbonylnickel(0).

Q5. Which of the following is the correct name for [Co(NH_3)_4Cl_2]⁺? A. Tetraamminedichlorocobalt(III) ion B. Dichlorotetraamminecobalt(III) ion C. Tetraamminedichlorocobaltate(III) ion D. Tetraamminedichlorocobalt(II) ion Correct Answer: A Explanation: The ligands are four ammine and two chloro. They are named alphabetically: ammine before chloro. The metal is cobalt in the +3 oxidation state. The name is tetraamminedichlorocobalt(III) ion.

• Q6. What is the suffix used for the metal in an anionic complex?
• A. -ite
• B. -ate
• C. -ide
• D. -ous
• Correct Answer: B
• Explanation: The suffix "-ate" is added to the name of the central metal atom if the coordination sphere is anionic.

• Q7. What is the name of the ligand H_2O in a complex?
• A. Water
• B. Hydroxo
• C. Aqua
• D. Hydrate
• Correct Answer: C
• Explanation: The neutral ligand H_2O is named "aqua" in a complex.

Q8. What is the IUPAC name of the complex [Pt(NH_3)_4BrCl]Cl₂? A. Tetraamminebromochloroplatinum(IV) chloride B. Tetraamminebromochloroplatinate(IV) chloride C. Bromochlorotetraammineplatinum(IV) chloride D. Tetraamminebromochloroplatinum(II) chloride Correct Answer: A Explanation: The ligands are four ammine, one bromo, and one chloro. They are named alphabetically: ammine, bromo, chloro. The metal is platinum in the +4 oxidation state. The name is tetraamminebromochloroplatinum(IV) chloride.

• Q9. What is the oxidation state of copper in [CuCl_4]^2-?
• A. +1
• B. +2
• C. +3
• D. +4
• Correct Answer: B
• Explanation: The complex ion has a charge of -2. Four chloride ligands contribute -4. Therefore, the copper ion must be +2.

Q10. What is the correct IUPAC name for [Cr(H_2O)_4Cl_2]Cl? A. Tetraaquadichlorochromium(III) chloride B. Hexaaquachromium(III) chloride C. Dichlorotetraaquachromium(III) chloride D. Tetraaquadichlorochromate(III) chloride Correct Answer: A Explanation: The complex ion is [Cr(H_2O)_4Cl_2]⁺. The ligands are four aqua and two chloro. They are named alphabetically: aqua before chloro. The metal is chromium in the +3 oxidation state. The name is tetraaquadichlorochromium(III) chloride. (Note: The textbook has a typo and incorrectly calls it "hexaaquachromium(III) chloride" for a formula with only 4 water molecules.)

Multiple Choice Questions (MCQs)

• Q1. What is the IUPAC name of the complex [Cu(NH_3)_2]²⁺?
• A. Diamminecopper(II) ion
• B. Copper diammine(II) ion
• C. Diamminecuprate(II) ion
• D. Copper(II) diammine ion
• Correct Answer: A
• Explanation: The ligands are two ammonia molecules (ammine), and the metal is copper in the +2 oxidation state.

• Q2. What is the name of the ligand Cl^- in a complex?
• A. Chlorine
• B. Chloride
• C. Chloro
• D. Chlorido
• Correct Answer: C
• Explanation: Anionic ligands end in "-o". Therefore, Cl^- is named "chloro".

• Q3. What is the oxidation state of iron in K_4[Fe(CN)_6]?
• A. +1
• B. +2
• C. +3
• D. +4
• Correct Answer: B
• Explanation: The complex ion is [Fe(CN)_6]^4-. Six cyanide ligands contribute -6. The overall charge is -4, so the iron must be +2.

• Q4. What is the IUPAC name of [Ni(CO)_4]?
• A. Tetracarbonylnickel(0)
• B. Nickel tetracarbonyl(0)
• C. Tetracarbonylnickelate(0)
• D. Nickel(0) tetracarbonyl
• Correct Answer: A
• Explanation: The ligands are four carbonyl groups, and the metal is nickel in the 0 oxidation state.

• Q5. Which of the following is the correct name for [Co(NH_3)_4Cl_2]⁺?
• A. Tetraamminedichlorocobalt(III) ion
• B. Dichlorotetraamminecobalt(III) ion
• C. Tetraamminedichlorocobaltate(III) ion
• D. Tetraamminedichlorocobalt(II) ion
• Correct Answer: A
• Explanation: The ligands are four ammine and two chloro. They are named alphabetically: ammine before chloro. The metal is cobalt in the +3 oxidation state.

• Q6. What is the suffix used for the metal in an anionic complex?
• A. -ite
• B. -ate
• C. -ide
• D. -ous
• Correct Answer: B
• Explanation: The suffix "-ate" is added to the name of the central metal atom if the coordination sphere is anionic.

• Q7. What is the name of the ligand H_2O in a complex?
• A. Water
• B. Hydroxo
• C. Aqua
• D. Hydrate
• Correct Answer: C
• Explanation: The neutral ligand H_2O is named "aqua" in a complex.

Q8. What is the IUPAC name of the complex [Pt(NH_3)_4BrCl]Cl₂? A. Tetraamminebromochloroplatinum(IV) chloride B. Tetraamminebromochloroplatinate(IV) chloride C. Bromochlorotetraammineplatinum(IV) chloride D. Tetraamminebromochloroplatinum(II) chloride Correct Answer: A Explanation: The ligands are four ammine, one bromo, and one chloro. They are named alphabetically: ammine, bromo, chloro. The metal is platinum in the +4 oxidation state.

• Q9. What is the oxidation state of copper in [CuCl_4]^2-?
• A. +1
• B. +2
• C. +3
• D. +4
• Correct Answer: B
• Explanation: The complex ion has a charge of -2. Four chloride ligands contribute -4. Therefore, the copper ion must be +2.

Q10. What is the correct IUPAC name for [Cr(H_2O)_4Cl_2]Cl? A. Tetraaquadichlorochromium(III) chloride B. Hexaaquachromium(III) chloride C. Dichlorotetraaquachromium(III) chloride D. Tetraaquadichlorochromate(III) chloride Correct Answer: A Explanation: The complex ion is [Cr(H_2O)_4Cl_2]⁺. The ligands are four aqua and two chloro. They are named alphabetically: aqua before chloro. The metal is chromium in the +3 oxidation state. The name is tetraaquadichlorochromium(III) chloride.