Ionisation Isomerism

Introduction

Ionisation isomerism rearranges which ion binds directly to a metal and which remains outside as a counter-ion. The two compounds retain the same overall composition but can produce different ions on dissolving. It is a particularly clear demonstration that a coordination formula needs brackets, not just an empirical list of atoms.

Core explanation

Compare [Co(NH₃)₅Br]SO₄ with [Co(NH₃)₅SO₄]Br as idealized isomers. In the first, one bromide ligand is coordinated and sulfate 2− balances the +2 complex [Co(NH₃)₅Br]²⁺. Co is +3 because five neutral NH₃ ligands and one Br⁻ give x − 1 = +2. In the second, sulfate is an inner ligand of charge −2; the bracketed complex [Co(NH₃)₅SO₄]⁺ is balanced by one outer Br⁻. Co remains +3 because x − 2 = +1. The overall ingredients match, but the direct donor ligand and counter-ion trade roles.

Their prompt solution tests differ. The second formula contains an external bromide and can provide Br⁻ for precipitation with Ag⁺ as AgBr, assuming the inner sphere remains intact over the test timescale. The first contains external sulfate instead; a suitable sulfate test can distinguish it under controlled conditions. A prolonged reaction may induce ligand exchange, so “will precipitate” needs a timescale and chemical-medium qualification.

The formula's bracket charge changes from +2 to +1 when the inner ligand changes from Br⁻ to SO₄²⁻, even though cobalt remains +3 and five NH₃ ligands remain. This is a good check against the mistaken notion that oxidation state equals complex charge. The charge difference is exactly accounted for by the different ligand charges.

Coordination number also deserves care. Monodentate bromide supplies one donor contact. A sulfate ligand can bind through one oxygen in the intended example, retaining a six-coordinate cobalt center with five ammines plus one sulfate O donor. In other compounds sulfate can use different binding modes or bridge metal centers. If a problem asks for coordination number, it should specify or imply the binding mode; do not infer that sulfate always contributes two merely because its charge is −2.

Ionisation isomerism is not simply swapping two ions in a crystal lattice. It changes a metal–ligand bond: an external ion becomes attached while a previously attached ligand becomes external. Nor is it oxidation or reduction, since cobalt remains +3 in the pair. It is a structural change at the coordination-sphere boundary.

For a proposed pair, verify exact compositional equality before naming it. Some superficially similar formulas have different ligand or counter-ion counts and are not isomers at all. Then inspect brackets, charge balance and likely released ions. This systematic route is more reliable than memorising the classic bromide/sulfate pair in isolation.

Step-by-step reasoning

1. Count every atom or ligand in both full salts to confirm matching composition. 2. Mark which anion is inside the bracket in each. 3. Calculate bracket and metal charges in both formulas. 4. Identify the outer ions available on prompt dissolution. 5. State an experimental distinction and its timescale assumption.

Visual explanation

Draw two boxes labelled Co(NH₃)₅. In the first, Br⁻ enters the box and SO₄²⁻ stays outside. In the second, SO₄²⁻ enters and Br⁻ stays outside. Arrows show the exchange across the bracket line, not a change in cobalt's oxidation state.

Real-world analogy

Two offices may employ the same number of people, but a person can switch from working inside one secure room to waiting outside as a visitor. The total roster stays the same while access and behaviour differ. The bracket boundary plays a similar accounting role.

Real-world example

An analytical chemist can compare prompt silver-halide precipitation of two samples. A salt with external bromide offers Br⁻ immediately, while a bromide ligand bound within an inert complex is less immediately available under the test conditions.

Why?

Why can both isomers contain Co(III) while their complex charges differ? The bound bromide is −1 but bound sulfate is −2; changing the inner ligand charge changes the bracketed total while external ions keep each salt neutral.

Common misconception

“The isomers differ only in where the printed symbols are placed.” Bracket placement represents a real change in metal–donor connectivity and predicts different initial dissolved ions.

Worked example

Determine which of the pair yields free Br⁻ promptly: [Co(NH₃)₅Br]SO₄ or [Co(NH₃)₅SO₄]Br. Only the second has Br outside the bracket, so it is the one expected to give prompt AgBr with Ag⁺ under no-substitution conditions. Both contain exactly one Br per formula unit; total bromine count alone could not answer.

Quick check

1. Does cobalt's formal oxidation state change between the two example isomers? Answer: No. It is +3 in both.

Exam focus

Compare the full formulas, not only their complex ions. Label inner ligand, outer counter-ion and prompt free ion. Specify sulfate's donor count if coordination number is required.

Advanced insight

The initial ion composition in water can change over time through ligand exchange. A fast analytical test probes the starting coordination assignment; equilibrium speciation may require a more complete reaction model.

Summary

Ionisation isomers have the same overall composition but exchange an inner anionic ligand with an outer counter-ion. Their complex charges and readily released ions may differ while the metal oxidation state remains the same.

Practice questions

1. Which example contains sulfate as the outer counter-ion? Answer: [Co(NH₃)₅Br]SO₄. 2. Which contains outer bromide? Answer: [Co(NH₃)₅SO₄]Br. 3. What is Co's oxidation state in [Co(NH₃)₅SO₄]⁺? Answer: +3, since x − 2 = +1. 4. Why is a rapid AgBr test conditional evidence? Answer: Coordinated bromide may be released later by substitution, so the initial result depends on reaction timescale and conditions.