State the main postulates of Werner's theory. Use them to explain why CoCl₃·6NH₃ and CoCl₃·5NH₃ give 3 and 2 moles of AgCl per mole respectively.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · State the two valences. A metal shows a primary valence: ionisable, satisfied by anions, equal to the oxidation state. It also shows a secondary valence: non-ionisable, satisfied by neutral molecules or anions, equal to the coordination number.
2 · Add the spatial postulate. Groups held by secondary valences have a fixed arrangement in space: octahedral for coordination number 6.
3 · Apply it to CoCl₃·6NH₃. Six NH₃ fill all six secondary valences: [Co(NH₃)₆]Cl₃. All three Cl⁻ are outside, so 3 AgCl form.
4 · Apply it to CoCl₃·5NH₃. Five NH₃ and one Cl⁻ fill six positions: [CoCl(NH₃)₅]Cl₂. Only two Cl⁻ are ionisable, so 2 AgCl form.
5 · Show the structure. Draw an octahedron with Co at the centre and the six groups at the corners; write the ionisable Cl⁻ outside the square bracket.
6 · Close in one line. So the number of ionisable chlorides shows which groups lie inside the coordination sphere.
Scoring tip: Always write the modern formula with square brackets. Many marking schemes give the mark for putting the bracket in the right place.
