Naming Coordination Salts

Introduction

A complex ion is only part of many coordination compounds. The full salt name also includes its counter-ion. As for ordinary ionic salts, the cation is named before the anion. If the complex is cationic, its name comes first; if the complex is anionic, the ordinary cation comes first. Brackets make this ordering straightforward once their charges are understood.

Core explanation

Take [Co(NH₃)₆]Cl₃. The complex cation is [Co(NH₃)₆]³⁺ because three outer Cl⁻ ions balance it. All ammonia ligands are neutral, so cobalt is +3. The complex is hexaamminecobalt(III), and the full salt name is hexaamminecobalt(III) chloride. The outer chloride is named simply chloride, not chlorido, because it is not a ligand in the bracketed entity. The formula's subscript three supplies charge balance; an ionic salt name does not usually say “trichloride” for those counter-ions.

Now consider K₄[Fe(CN)₆]. Potassium is the cation and the bracketed entity is a 4− anion. Six cyanido ligands total −6, so iron is +2. The complex anion is hexacyanidoferrate(II), and the full salt is potassium hexacyanidoferrate(II). Four potassium ions are implied by balancing +1 against −4. The potassium name comes first, even though the complex formula occupies most of the written symbols.

The contrast between ligand and counter-ion names is critical. In [Co(NH₃)₅Cl]Cl₂, one chlorido ligand is inside brackets and two chloride counter-ions are outside. The cation has charge +2; cobalt is +3 because one inner chloride contributes −1. Its complex name is pentaamminechloridocobalt(III), followed by chloride for the salt. The word appears in two roles in the full name: chlorido identifies attached Cl⁻, and chloride identifies outer Cl⁻. Older course styles may use chloro for the first role while preserving chloride for the second.

Charge balance tells how many counter-ions a correct formula needs, even when a name does not spell out their number. If a nitrate ion NO₃⁻ balances [Co(en)₃]³⁺, three nitrates are needed: Co(en)₃₃. The salt name places the complex cation first and nitrate second. The ligand name may be tris(ethane-1,2-diamine) under modern systematic style. The parentheses around the nitrate group in the formula make clear that the subscript applies to the entire polyatomic ion.

Some salts have both a complex cation and a complex anion. Name the complex cation first and complex anion second, determining each center's oxidation state independently. Never force one metal's Roman numeral to match the other's merely because their ion charges balance. A salt formula can have net charge zero while each complex ion and each metal have different charge or oxidation-state values.

The method is deliberately mechanical: find components, calculate charges, name within brackets, order components. It prevents confusion between a ligand name and an outer ion name. It also helps check a proposed formula: if the named counter-ion charges cannot balance the complex, the formula or oxidation-state assignment has been misread.

Step-by-step reasoning

1. Split the formula into bracketed entity and external ions. 2. Determine each component's charge and the metal's oxidation state. 3. Name ligands alphabetically and then the metal with its Roman numeral. 4. Apply -ate if the complex is anionic. 5. Name the cation first and the anion second for the complete salt.

Visual explanation

Draw two formula cards. On [Co(NH₃)₆]Cl₃, point first to the left bracketed cation, then the right chloride anion. On K₄[Fe(CN)₆], point first to left potassium cations, then the bracketed ferrate anion.

Real-world analogy

An address gives a building name and a street name in a specified order, even though the building may be much larger than the street sign. Naming a coordination salt follows component order, not the visual length of its formula.

Real-world example

Clear naming matters in analytical reagents: potassium hexacyanidoferrate(II) and potassium hexacyanidoferrate(III) differ by iron oxidation state and potassium ratio. Their systematic names reveal the distinction without relying on historical colour-based labels.

Why?

Why is the external Cl⁻ in [Co(NH₃)₆]Cl₃ called chloride rather than chlorido? It is an outer counter-ion, not a ligand coordinated directly to cobalt within the square brackets.

Common misconception

“The number of external ions must be spoken as a molecular prefix.” Standard ionic naming uses charge balance rather than saying trichloride for three counter-ions in this context.

Worked example

Name K₃[Fe(CN)₆]. Three K⁺ ions require a 3− complex. Six cyanides are −6 total, so Fe is +3. The anion is hexacyanidoferrate(III). Name the cation first to obtain potassium hexacyanidoferrate(III). The Roman numeral is III, not based on the three potassium ions by coincidence but on the metal charge equation.

Quick check

1. Which component is named first in K₄[Fe(CN)₆]? Answer: Potassium, the cation.

Exam focus

Differentiate chlorido inside brackets from chloride outside. Check component charges before naming. Use cation-before-anion order even when the complex anion is the chemically interesting part.

Advanced insight

Names can be extended with stereochemical descriptors for cis/trans or optical forms. The base composition name is necessary but may not distinguish every physically different complex of the same formula.

Summary

Name a coordination salt by naming its cation, then its anion. First solve the bracketed entity's charge and the metal oxidation state; then use ligand names and -ate rules. Outer ions retain their ordinary ionic names.

Practice questions

1. What is the full name of [Co(NH₃)₆]Cl₃? Answer: Hexaamminecobalt(III) chloride. 2. What is the full name of K₄[Fe(CN)₆] in current ligand spelling? Answer: Potassium hexacyanidoferrate(II). 3. Is outer chloride in [Co(NH₃)₅Cl]Cl₂ named chlorido? Answer: No. Inner chloride is chlorido; outer chloride is chloride. 4. How many nitrate counter-ions balance [Co(en)₃]³⁺? Answer: Three NO₃⁻ ions.