Coordination Compounds · Lesson 6 of 12
Structural Isomerism
“Distinguish changes in donor connectivity and coordination-sphere membership through formulas and solution tests.”
• Identify linkage, coordination, ionisation and solvate isomerism. • Write solution species and balanced diagnostic precipitation equations. • Separate coordinated water from lattice water. • State assumptions before making an exhaustive isomer count.
When the bonds themselves change
Geometrical and optical isomers retain the same donor connectivity. Structural isomers change that connectivity: a ligand may bind through a different atom, move between two metal centres, or exchange its coordinated role with a counter ion or solvent molecule. Classification therefore begins with the coordination sphere rather than with colour. Two salts can have different colours for several reasons, and colour alone does not establish which bonds differ.
Linkage isomerism: choose another donor atom
Nitrite can bind through nitrogen to give M–NO₂ or through oxygen to give M–ONO. These are linkage forms with the same NO₂⁻ composition and formal charge. In [Co(NH₃)₅(NO₂)]Cl₂, the N-bound form is commonly yellow and the O-bound form red. Nitrito-κN and nitrito-κO specify the donor; neither should be confused with nitrate, NO₃⁻. Thiocyanate similarly gives M–NCS and M–SCN, named thiocyanato-κN and thiocyanato-κS.
A linkage change preserves the total donor count for these monodentate modes, but changes which atom is directly attached and therefore changes the bonding environment. Do not represent an N-bound form by merely reversing letters in a caption while drawing the bond to oxygen. In the diagram, the short connecting line always ends at the actual donor atom.
Coordination isomerism: redistribute ligands between complex ions
A salt can contain both a complex cation and a complex anion. In [Co(NH₃)₆][Cr(CN)₆], neutral ammonia surrounds Co(III) and six CN⁻ surround Cr(III). The entity charges are +3 and −3. In [Cr(NH₃)₆][Co(CN)₆], the ligand sets have exchanged metal partners. The total elemental composition remains the same, but the metal–ligand bonds differ. This is coordination isomerism, not the mere movement of a counter ion outside one bracket.
Ionisation isomerism: coordinated versus free anion
Problem
Distinguish [CoBr(NH₃)₅]SO₄ from [Co(NH₃)₅(SO₄)]Br in rapid aqueous tests.
- 1.The first salt gives [CoBr(NH₃)₅]²⁺ and SO₄²⁻. Free sulfate gives Ba²⁺ + SO₄²⁻ → BaSO₄(s). Bromide remains bound on the test timescale, so an immediate AgBr test does not count it as free bromide.
- 2.The second gives [Co(NH₃)₅(SO₄)]⁺ and Br⁻. Free bromide gives Ag⁺ + Br⁻ → AgBr(s). Sulfate is coordinated and is not immediately available for the corresponding sulfate test under the stipulated conditions.
- 3.Both retain Co(III), total composition and six donor sites when sulfate is monodentate. The differing free ions identify an exchange across the coordination-sphere boundary.
The chloride analogue works the same way: [CoCl(NH₃)₅]SO₄ releases sulfate and yields rapid BaSO₄, whereas [Co(NH₃)₅(SO₄)]Cl releases chloride and yields rapid AgCl. These net ionic equations conserve both atoms and charge. A table that switches from chloride in the question to bromide in the answer must not be copied without matching the actual species.
Hydrate isomerism: where is the water?
| Formula | Coordinated water / chlorido | External chloride | Lattice water |
|---|---|---|---|
| [Cr(H₂O)₆]Cl₃ | 6 / 0 | 3 | 0 |
| [CrCl(H₂O)₅]Cl₂·H₂O | 5 / 1 | 2 | 1 |
| [CrCl₂(H₂O)₄]Cl·2H₂O | 4 / 2 | 1 | 2 |
These formulas all contain one chromium, three chlorines and six waters overall. What changes is whether water directly coordinates the metal or occupies the crystal lattice, accompanied here by chloride entering the sphere. Each usual octahedral entity has six donors and Cr +3. The hexaaqua form is violet, and a pentaaquachlorido hydrate is grey-green under the described solid-state conditions. Lattice water is written after a dot and does not contribute to CN. Solvate is the broader term; hydrate is used when the solvent is water.
Combine classifications without inventing counts
K[Cr(H₂O)₂(C₂O₄)₂] has a −1 octahedral entity, Cr(III), two bidentate oxalates and two monodentate waters. The two waters may be cis or trans; cis gives an optical pair. [PtCl₂(NH₃)(H₂O)] is instead square planar with a cis/trans distinction for the two chlorides, but no optical pair in the ideal planar model. [Co(en)₃]Cl₃ has optical isomerism without a geometrical pair. Classification depends on the specific composition and geometry, not just on the existence of brackets.
Problem
Identify defensible isomerism in [Co(NH₃)₅(NO₂)](NO₃)₂ without assuming extra ligands or changing CN.
- 1.With a fixed pentaammine octahedral sphere, coordinated nitrite can attach through N or O, giving two linkage forms. Their two counter ions are nitrate.
- 2.Exchange nitrite with one nitrate: [Co(NH₃)₅(ONO₂)](NO₂)(NO₃), taking nitrate as a monodentate O donor. This is an ionisation isomer; the entity remains +2.
- 3.A pentaammine sphere with only one different ligand has no cis/trans pair: all six positions for that one ligand are rotationally equivalent. Therefore a claim of ten isomers is not supported by these fixed assumptions. Additional binding modes or coordination numbers would need explicit specification and evidence.
An exhaustive count is a stronger claim than identifying examples. State whether connectivity, CN, geometry, donor modes and lattice species are fixed before counting. Otherwise expanding the allowed binding modes silently changes the question. This is especially relevant to nitrate, nitrite and sulfate, which can display more complicated coordination outside the simple monodentate examples used here.
Quick Quiz
Quiz
Changing M–NO₂ to M–ONO is which kind of isomerism?
The donor atom changes within the same ambidentate ligand.
Which free ion is present in [CoBr(NH₃)₅]SO₄?
Sulfate is written outside the bracket and balances the +2 complex.
What does the dot H₂O in [CrCl(H₂O)₅]Cl₂·H₂O represent?
The water outside the bracket is not counted among directly bonded donors.
Which pair illustrates coordination isomerism?
Ligand sets exchange between the two different metal-containing entities.
Why does octahedral MA₅B have no cis/trans pair?
One distinct monodentate ligand does not define two inequivalent relative arrangements.
Practice Problems
- Give an example of each of the six isomerism categories taught in the chapter.
- Write balanced net ionic tests for the chloride/sulfate ionisation pair.
- For the three chromium hydrate formulas in the table calculate CN and the rapid AgCl yield.
- Classify K[Cr(H₂O)₂(C₂O₄)₂], [Co(en)₃]Cl₃ and [PtCl₂(NH₃)(H₂O)].
- A student claims ten isomers for [Co(NH₃)₅(NO₂)](NO₃)₂. What assumptions must be checked?
- Explain why ionisation isomers need not retain identical solution tests despite identical elemental composition.
1. Geometrical: cis/trans [PtCl₂(NH₃)₂]. 2. Optical: the two [Co(en)₃]³⁺ helicities. 3. Linkage: M–NO₂/M–ONO pentaamminecobalt(III) salts. 4. Coordination: [Co(NH₃)₆][Cr(CN)₆] and exchanged metal partners. 5. Ionisation: [CoBr(NH₃)₅]SO₄/[Co(NH₃)₅(SO₄)]Br. 6. Hydrate: [Cr(H₂O)₆]Cl₃/[CrCl(H₂O)₅]Cl₂·H₂O. Each pair changes the feature specified by its category.
Key Takeaways
• Structural isomers differ in connectivity or coordination-sphere membership. • Ambidentate donor changes produce linkage isomerism. • Coordination isomerism redistributes ligands between complex ions. • Ionisation isomers release different counter ions. • Hydrate isomers differ in coordinated and lattice water. • Diagnostic equations must use the actual ions and conserve charge. • Exhaustive isomer counts require explicit geometry and binding assumptions.