Coordination Compounds

Lesson

Part 1 — compounds in which a metal ion is surrounded by a set of molecules or ions held by coordinate bonds.

In this lesson you will

Explain Werner’s primary and secondary valencies and use precipitation data to write the formula of a complex
Define ligand, coordination number, denticity, chelate, ambidentate ligand and related terms
Name mononuclear coordination compounds by IUPAC rules and write formulas from names
Identify structural and stereoisomers (geometrical and optical) of coordination compounds

Table of five familiar coordination compounds and where they are met: haemoglobin, chlorophyll, vitamin B12, cisplatin and EDTA complexes
Compound	Where you meet it
Haemoglobin (iron complex)	Carries oxygen in blood
Chlorophyll (magnesium complex)	Photosynthesis in leaves
Vitamin B12 (cobalt complex)	Prevents anaemia
Cisplatin (platinum complex)	Cancer treatment
EDTA complexes	Testing hardness of water

Concept

Double salts and complexes

A double salt exists only in the solid; in water it gives all its ions (Mohr’s salt gives Fe²⁺, NH₄⁺ and SO₄²⁻). A coordination compound keeps its complex ion intact in water.

Table contrasting double salts, which give all their simple ions in water, with complexes, which keep the complex ion intact
Compound	Type	Ions in water
FeSO₄·(NH₄)₂SO₄·6H₂O (Mohr’s salt)	Double salt	$\ce{Fe^2+}$ , $\ce{NH4+}$ , $\ce{SO4^2-}$
KCl·MgCl₂·6H₂O (carnallite)	Double salt	$\ce{K+}$ , $\ce{Mg^2+}$ , $\ce{Cl-}$
$\ce{K4[Fe(CN)6]}$	Complex	$\ce{K+}$ and $\ce{[Fe(CN)6]^4-}$ only (no free $\ce{Fe^2+}$ or $\ce{CN-}$ )
$\ce{[Cu(NH3)4]SO4}$	Complex	$\ce{[Cu(NH3)4]^2+}$ and $\ce{SO4^2-}$

Concept

Werner’s theory

Werner (1898) proposed that a metal shows two kinds of valency. Primary valency: ionisable, satisfied by negative ions; equals the oxidation state. Secondary valency: non-ionisable, satisfied by ligands; equals the coordination number and is fixed in direction, giving a definite shape.

Table of three cobalt chloride ammonia compounds, the moles of silver chloride each precipitates per mole and their Werner formulas
Compound	Moles of AgCl per mole with AgNO₃	Werner formula
CoCl₃·6NH₃ (yellow)	3	$\ce{[Co(NH3)6]Cl3}$
CoCl₃·5NH₃ (purple)	2	$\ce{[CoCl(NH3)5]Cl2}$
CoCl₃·4NH₃ (green or violet)	1	$\ce{[CoCl2(NH3)4]Cl}$

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