Werner's theory

A blue that is not just copper

Dissolve some copper sulphate in water. The solution is pale blue. Add a few drops of ammonia solution, and a pale blue solid appears. Add more ammonia, and the solid vanishes. You are left with a deep blue solution, much darker than the one you began with.

The copper ion did not change its charge. It is still $\text{Cu}^{2+}$. Something new has bonded to it. Four ammonia molecules now surround each copper ion, and the group $[\text{Cu}(\text{NH}_{3})_{4}]^{2+}$ is a new ion with a colour of its own. Even the first pale blue solution is not bare $\text{Cu}^{2+}$. The copper ion is already surrounded by water molecules.

A metal ion with a fixed group of molecules or ions bonded round it is a complex. A compound that contains a complex is a coordination compound. Many of the colours you see in a chemistry laboratory belong to complexes.

Two kinds of salt that look alike

Think of three salts, and dissolve each in water:

• Mohr's salt, $\text{FeSO}_{4}\cdot (\text{NH}_{4})_{2}\text{SO}_{4}\cdot _{6}\text{H}_{2}\text{O}$
• The mineral carnallite, $\text{KCl}\cdot \text{MgCl}_{2}\cdot _{6}\text{H}_{2}\text{O}$
• Potassium hexacyanidoferrate(II), $\text{K}_{4}[\text{Fe}(\text{CN})_{6}]$

Add sodium hydroxide to the first solution. A green solid appears: iron(II) hydroxide. The iron is there as $\text{Fe}^{2+}$, and it behaves as it always does. In the same way, the carnallite solution gives the tests of $\text{K}^{+}$, $\text{Mg}^{2+}$ and $\text{Cl}^{-}$ one by one.

Now add sodium hydroxide to the third solution. Nothing appears. The solution does contain iron, but no $\text{Fe}^{2+}$ ions can be found. The iron is locked inside the ion $[\text{Fe}(\text{CN})_{6}]^{4-}$, which does not fall apart in water.

This is the difference:

• A double salt is two simple salts that crystallise together. In water it falls completely into the ions of its parts. It loses its identity.
• A coordination compound keeps its complex ion in water. The metal ion does not give its usual tests.

What Werner saw

Alfred Werner, a Swiss chemist, studied the compounds of cobalt(III) chloride with ammonia in the 1890s. He won the Nobel Prize in Chemistry in 1913 for this work. He made four compounds from the same three elements, and they differed in colour:

• $\text{CoCl}_{3}\cdot _{6}\text{NH}_{3}$ is yellow.
• $\text{CoCl}_{3}\cdot _{5}\text{NH}_{3}$ is purple.
• $\text{CoCl}_{3}\cdot _{4}\text{NH}_{3}$ is green.
• $\text{CoCl}_{3}\cdot _{3}\text{NH}_{3}$ is also a compound, and it does not dissolve to give ions.

Then he added excess silver nitrate to the solutions. Silver ions take free chloride out of solution as white $\text{AgCl}$. The yellow compound gave 3 mol of $\text{AgCl}$ for each mole of salt. The purple one gave 2 mol. The green one gave 1 mol. The last one gave none.

All four have 3 chloride ions for each cobalt. Yet the silver ions can reach only some of them. The others must be held somewhere that silver cannot reach. Measurements of electrical conductivity agreed. The yellow salt in water behaves like a salt with four ions. The purple salt has three ions, the green salt two, and the last one is a neutral molecule.

Werner's theory

Werner explained all of this with three ideas:

1. A metal has two kinds of valence. The primary valence is the oxidation state. It can be ionised, and it is satisfied by negative ions. The secondary valence is the number of groups bonded directly to the metal. It cannot be ionised, and it is satisfied by neutral molecules or by negative ions. Today we call it the coordination number.
2. Every metal in a given state has a fixed secondary valence. Cobalt(III) has a primary valence of 3 and a secondary valence of 6.
3. The groups that satisfy the secondary valence point in fixed directions in space. This gives each complex a shape: octahedral for 6, tetrahedral or square planar for 4, linear for 2.

We write the metal and the groups bonded to it inside square brackets. This is the coordination sphere. The ions outside the brackets are the counter ions. They are free in water.

Read the picture from the first compound to the last. In $[\text{Co}(\text{NH}_{3})_{6}]\text{Cl}_{3}$, ammonia fills all six places. The complex ion has a charge of +3, and three free chloride ions balance it. Silver finds 3 chloride ions, and there are four ions in all.

In the second compound, one chloride ion has moved inside. It now does two jobs: it helps to make up the charge of +3, and it fills one of the six places. That chloride ion is hidden from silver. Only two are free. In the third compound, two are hidden and one is free. In the last compound all three are inside, and the molecule is neutral.

The sim uses the rule of the picture. Cobalt(III) has six places, and chloride fills every place that ammonia leaves. The number of free chloride ions is the number of ammonia molecules minus three. Try 4 ammonia molecules and then 3. The curve goes flat at once for the neutral compound.

Practise with new numbers

Try it in Python

The program goes through Werner's series. For each number of ammonia molecules it works out the formula, the charge, the number of ions and the silver chloride.

The parts of a complex

Now we can name the parts. This is how every complex is built:

• The central metal atom or ion sits in the middle. It has empty orbitals that can accept electron pairs. It is a Lewis acid.
• The ligands are the molecules or ions bonded to it. Each ligand gives a lone pair of electrons to the metal. A ligand is a Lewis base. The bond is a coordinate bond.
• The coordination sphere is the metal and its ligands, in square brackets.
• The counter ions are outside the brackets.
• The coordination number is the number of donor atoms bonded to the metal.

Transition metals form complexes most easily. Their ions are small, they carry a high charge, and they have empty d orbitals to receive the lone pairs. You met oxidation numbers in the lessons on redox reactions. They tell you how much charge the metal carries in the complex.

Complexes at work

Complexes are not only laboratory curiosities. Look at where they appear:

• Haemoglobin has iron in a ring of four nitrogen atoms. The iron carries oxygen round the body.
• Chlorophyll has magnesium in a similar ring. It captures the light for photosynthesis.
• Vitamin B12 has cobalt.
• Cisplatin, a complex of platinum, is a drug used against some cancers.
• Nickel tetracarbonyl is a volatile liquid made from impure nickel and carbon monoxide. It breaks down on heating and leaves pure nickel.
• Silver and gold are taken out of their ores as cyanide complexes.

Common mistakes

• Counting every chloride ion as free. A chloride ion inside the square brackets is bonded to the metal. Silver ions cannot reach it.
• Mixing up the two valences. Cobalt(III) has a primary valence of 3 and a secondary valence of 6. The number of ammonia molecules has nothing to do with the +3.
• Thinking a complex falls apart in water. The complex ion stays whole. Only the counter ions leave.
• Forgetting that a complex ion is an ion. Count it when you count the ions in a solution.

Remember these

• Werner: primary valence is the oxidation state, secondary valence is the coordination number.
• The coordination sphere is the metal and its ligands, in square brackets. Counter ions are outside.
• Free chloride gives silver chloride at once. Chloride inside the sphere does not.
• A double salt gives all its ions. A complex keeps its complex ion.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which compound gives a white precipitate with silver nitrate at once: $[\text{Co}(\text{NH}_{3})_{6}]\text{Cl}_{3}$ or $[\text{Co}(\text{NH}_{3})_{3}\text{Cl}_{3}]$? Why?

2. How many ions does one formula unit of $\text{K}_{3}[\text{Fe}(\text{CN})_{6}]$ give in water?

3. Why does $\text{K}_{4}[\text{Fe}(\text{CN})_{6}]$ not give the test for $\text{Fe}^{2+}$ ions?

4. In $[\text{Co}(\text{NH}_{3})_{5}\text{Cl}]\text{Cl}_{2}$, the chloride ion inside the brackets satisfies which valence?