Lesson 1 of 13 · 9 min

Werner's theory

NCERT §5.1

Before he titrates anything, Farhan's teacher hands him four bottles, all labelled 'cobalt(III) chloride with ammonia', in yellow, purple, green and violet. Same elements, different colours, and they behave differently with silver nitrate.

The story this chapter follows: Farhan's well-water project

Farhan, a Class 12 student, wants to know why the water from his family's well leaves white crust in the kettle, so he tests it in the school lab. He titrates a 50.0 mL sample with 0.0100 M Na₂EDTA and needs 12.5 mL to reach the end point. Around him are a shelf of cobalt ammine salts, bottles of blue copper sulphate, a set of nickel solutions and a teacher who keeps asking why each one has the colour it has.

The lesson in notes

In short

A coordination compound holds a central metal atom or ion joined to a set of anions or neutral molecules through shared electron pairs. Chlorophyll (Mg), haemoglobin (Fe) and vitamin B12 (Co) are all coordination compounds.

Werner tested the cobalt(III) chloride-ammonia compounds with excess cold AgNO₃. Per mole: CoCl₃·6NH₃ (yellow) gave 3 mol AgCl, CoCl₃·5NH₃ (purple) gave 2, and both forms of CoCl₃·4NH₃ (green and violet) gave 1.

He explained this by keeping six groups (Cl⁻, NH₃ or both) fixed on the cobalt in a unit that does not break up: [Co(NH₃)₆]Cl₃ is a 1:3 electrolyte, [CoCl(NH₃)₅]Cl₂ a 1:2 electrolyte and [CoCl₂(NH₃)₄]Cl a 1:1 electrolyte, as their conductivities confirm.

Only chloride outside the square bracket is free to meet Ag⁺; chloride bonded to the metal inside the bracket is not precipitated.

The green and violet CoCl₃·4NH₃ share one empirical formula but differ in properties, so they are isomers.

Werner's postulates (1898): a metal shows primary and secondary valences. Primary valences are ionisable and satisfied by negative ions. Secondary valences are non-ionisable, satisfied by neutral molecules or anions, equal to the coordination number and fixed for a given metal.

The groups held by secondary valences have a definite arrangement in space for each coordination number, today called the coordination polyhedron. The bracketed species is the complex; ions outside are counter ions.

Common shapes: [Co(NH₃)₆]³⁺, [CoCl(NH₃)₅]²⁺ and [CoCl₂(NH₃)₄]⁺ are octahedral, [Ni(CO)₄] is tetrahedral and [PtCl₄]²⁻ is square planar.

A double salt such as carnallite (KCl·MgCl₂·6H₂O), Mohr's salt (FeSO₄·(NH₄)₂SO₄·6H₂O) or potash alum breaks up completely into simple ions in water. A complex does not: K₄[Fe(CN)₆] gives [Fe(CN)₆]⁴⁻, not Fe²⁺ and CN⁻.

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Werner's theory

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