Course 25.2Lesson 1 of 3
Isomers have the same formula and a different structure. A complex can change its structure in four ways. Ligands and counter ions can swap places (ionisation and hydrate isomers). A ligand can bond through another atom (linkage isomers). Two complex ions can swap ligands (coordination isomers). Silver nitrate and a conductivity meter tell many of them apart.
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Two jars on a laboratory shelf hold crystals of chromium chloride with water. Both jars carry the same formula, . The crystals in one jar are violet. The crystals in the other jar are dark green. How can one formula describe two different substances?
You met this puzzle in Class 11 with carbon compounds. Isomers have the same molecular formula and different structures. In structural isomers the atoms are joined in a different order. In stereoisomers the atoms are joined in the same order but sit differently in space. Complexes show both kinds. This lesson is about the structural kind.
A complex has more to its structure than an organic molecule. Look at . The square brackets enclose the coordination sphere: the metal ion and the ligands bonded to it. The ions outside the brackets are counter ions. So the structure of a complex answers three questions.
Each answer can change while the overall formula stays the same. This gives four kinds of structural isomerism.
Two complex salts can swap a ligand with a counter ion. Take and . Count the atoms in each salt: one platinum, four ammonia, two chlorine and two bromine. The overall formula is the same. But in the first salt the chlorine is inside the bracket and the bromine is outside. In the second salt it is the other way round.
These are ionisation isomers. They give different ions in water.
Silver nitrate tells them apart. Ligands inside the bracket are held by coordinate bonds, so they do not leave. Only the free ions outside precipitate. The first salt gives a pale yellow precipitate of silver bromide. The second gives a white precipitate of silver chloride.
Here is a second pair, with sulphate: and . Barium chloride gives a white precipitate of barium sulphate with the second salt only, because only there is the sulphate ion free.
Notice what a conductivity meter would say about the first pair. Each salt gives three ions in water, one complex ion and two simple ions. The conductivity is about the same. So a precipitation test, not a conductivity test, separates this pair.
Water can sit inside the bracket as a ligand, or outside it as water of crystallisation. Hydrate isomers (also called solvate isomers) differ in how the water is shared. The classic case is . It has three isomers.
Every isomer holds six ligands round chromium. More chloride inside the bracket means less chloride outside and more water outside. The overall formula never changes.
In a fresh, cold solution, chromium(III) holds its ligands so firmly that the chloride inside the bracket does not react. Silver nitrate therefore precipitates only the free chloride. Count the ions too. The violet isomer gives four ions in water. The pale green one gives three, and the dark green one gives two. A conductivity measurement counts the ions, so it separates all three.
Choose each isomer in turn. The bar for the violet isomer is three times the bar for the dark green one, because it has three free chloride ions against one.
Some ligands have two different atoms that can each donate a lone pair. Such a ligand is ambidentate. The nitrite ion is one. It can bond to the metal through nitrogen or through oxygen. The thiocyanate ion can bond through sulphur or through nitrogen. The cyanide ion can bond through carbon or through nitrogen.
Two complexes that differ only in the donor atom are linkage isomers. The standard pair is this.
The red form changes slowly into the yellow form, so the nitrogen-bonded form is the more stable one. The thiocyanate ligand works the same way, as thiocyanato-S or thiocyanato-N.
Do not mix up two words. An ambidentate ligand bonds through one atom or the other, never both at once. A bidentate ligand bonds through two atoms at the same time. The nitrate ion cannot show linkage isomerism, because all its donor atoms are oxygen.
Linkage isomers give the same ions in water. A conductivity meter and a silver nitrate test cannot tell them apart. Their colours differ, and so do their infrared spectra.
This kind needs a salt in which both the cation and the anion are complexes. The ligands can swap between the two metals. Take . Copper holds the ammonia and platinum holds the chlorides. Now swap the ligands: . Both salts hold one copper, one platinum, four ammonia and four chlorine. Each is a coordination isomer of the other. The charges balance in both: with .
The metals can also be the same element in two oxidation states. has platinum(II) in the cation and platinum(IV) in the anion. is its coordination isomer.
Ask what moved.
The program checks two things for a pair of formulas. First it adds up everything in each formula, wherever it sits. Isomers must have the same total. Then it counts the ions each formula gives in water and what silver nitrate precipitates.
Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.
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