Naming the Metal and Oxidation State

Introduction

After naming ligands, a coordination name identifies the central metal and its oxidation state. A cationic or neutral complex commonly keeps the usual metal name; an anionic complex changes the metal name to an -ate form. The Roman numeral records the metal's formal oxidation state, not the complex ion's charge or coordination number.

Core explanation

For [Co(NH₃)₆]³⁺, all ammonia ligands are neutral, so cobalt is +3. The name of the complex cation is hexaamminecobalt(III). The numeral III means cobalt's oxidation state +3. It does not mean three ammonia molecules or a coordination number of three: there are six ligands and six donor contacts.

For neutral [Pt(NH₃)₂Cl₂], two chlorido ligands total −2 and ammonia contributes zero, so platinum is +2. Alphabetically, ammine precedes chlorido, giving diamminedichloridoplatinum(II) in current IUPAC-style ligand spelling. The coordination entity has zero net charge; the Roman numeral remains II because platinum's formal state is +2. A neutral overall formula can therefore have a positive metal oxidation state.

Anionic coordination entities take an -ate ending on the metal component. In [Fe(CN)₆]⁴⁻, iron is +2 because x − 6 = −4. A current-style name is hexacyanidoferrate(II), using the traditional Latin-derived metal root ferrate. In [CuCl₄]²⁻, x − 4 = −2 gives Cu(II), and the anionic metal form is cuprate, producing tetrachloridocuprate(II). Several metals have traditional roots such as ferrate for iron and cuprate for copper. A reference table should be used for unusual metal roots rather than inventing one.

An anionic complex's Roman numeral is not the magnitude of its negative charge. [Fe(CN)₆]³⁻ is ferrate(III) because six cyanides total −6 and the complex is 3−, requiring Fe(+3). The 3− overall charge and (III) metal oxidation state happen to have the same magnitude in this case, but [Fe(CN)₆]⁴⁻ immediately shows they need not.

The positive, neutral or negative nature of the entire coordination entity determines the metal-name ending. External counter-ions do not change the internal naming rule. K₄[Fe(CN)₆] is an ionic salt with a 4− bracketed anion, so its inner complex is ferrate(II) even though the full salt is neutral. A cationic ammine complex paired with chloride retains cobalt, not cobaltate.

Oxidation-state notation follows formal charge accounting. It is not a claim that all metal–ligand bonds are perfectly ionic. The numeral remains useful for distinguishing complexes with the same ligand set but different redox states. If a ligand is redox-active and its formal charge uncertain, advanced nomenclature may require more care; introductory problems generally supply conventional ligand states.

Step-by-step reasoning

1. Determine complex charge from the bracket or counter-ions. 2. Calculate metal oxidation state from ligand charges. 3. Name and alphabetise the ligands. 4. Use the usual metal name for cationic or neutral complexes, -ate for anionic ones. 5. Append the metal's oxidation state as a Roman numeral in parentheses.

Visual explanation

Draw a three-row chart: [Co(NH₃)₆]³⁺ → cobalt(III), [Pt(NH₃)₂Cl₂] → platinum(II), [Fe(CN)₆]⁴⁻ → ferrate(II). Colour the sign of complex charge separately from the Roman numeral to prevent conflation.

Real-world analogy

A person's job title and team score are different labels: a team can have a negative net score while a member holds rank two. The -ate ending reports the complex's negative status, while the Roman numeral reports the metal's formal state.

Real-world example

The names of ferrocyanide and ferricyanide salts historically distinguish [Fe(CN)₆]⁴⁻ and [Fe(CN)₆]³⁻. Systematic ferrate(II) and ferrate(III) naming makes the formal iron oxidation-state difference explicit.

Why?

Why is [Fe(CN)₆]⁴⁻ named ferrate(II) rather than ferrate(IV)? The six cyanide ligands contribute −6, so iron must contribute +2 to give a net complex charge of −4.

Common misconception

“A complex with charge 3− must contain a metal in oxidation state −3.” The complex charge includes all ligand charges. Solve the full charge equation before choosing a Roman numeral.

Worked example

Name [CuCl₄]²⁻ in current ligand spelling. Four Cl⁻ ligands are tetrachlorido. Charge balance x + 4(−1) = −2 gives Cu(+2). The complex is anionic, so the metal uses cuprate. The resulting name is tetrachloridocuprate(II). Coordination number four is not part of the Roman numeral.

Quick check

1. What Roman numeral belongs to [Fe(CN)₆]³⁻? Answer: (III), because iron is +3.

Exam focus

Compute oxidation state before naming. Decide the metal's -ate ending from the bracketed entity's charge, not the full salt's neutrality. Use a trusted list for traditional anionic metal roots.

Advanced insight

IUPAC nomenclature is designed to encode composition and formal state, while stereochemical descriptors can additionally specify geometry. A correct basic name may still be incomplete if cis/trans or linkage isomerism is central to the question.

Summary

The metal follows the ligand names and carries its formal oxidation state in Roman numerals. Anionic complex names use an -ate metal form, such as ferrate or cuprate. Complex charge and metal oxidation state are distinct quantities.

Practice questions

1. What is cobalt's Roman numeral in [Co(NH₃)₆]³⁺? Answer: (III). 2. Is neutral [Pt(NH₃)₂Cl₂] platinum(0)? Answer: No. Two chloride ligands make Pt(II). 3. What metal ending is used for an anionic iron complex? Answer: Ferrate. 4. Name the metal part of [Fe(CN)₆]⁴⁻. Answer: Ferrate(II), because iron is +2.