Coordination Compounds: Terms, Nomenclature and Isomerism — Part 1: Werner’s theory and ligands: visual lesson, ISC Class 12 Chemistry

Lesson

Metal ions with company

Part 1 — compounds in which a central metal is bonded to a set of ions or molecules by coordinate bonds.

In this lesson you will

Distinguish a coordination compound from a double salt using ionisation in water
Explain primary and secondary valencies according to Werner’s theory
Classify ligands by denticity and identify chelating and ambidentate ligands
Calculate the coordination number and oxidation state of the metal in a complex

Table of coordination compounds and where they matter
Coordination compound	Where it matters
Haemoglobin (Fe)	Oxygen transport in blood
Chlorophyll (Mg)	Photosynthesis
Na₃[Ag(S₂O₃)₂]	Fixing in black-and-white photography
[Ag(CN)₂]⁻, [Au(CN)₂]⁻	Extraction of silver and gold
Ca–EDTA	Estimating hardness of water

Concept

Double salts and coordination compounds

Both are made by combining stoichiometric amounts of two or more salts. A double salt exists only in the solid and dissociates completely into simple ions in water. A coordination compound keeps its complex ion intact in water, so its constituents do not all give their usual tests.

Table contrasting double salts with coordination compounds
Compound	Type	Ions in water
Potash alum, K₂SO₄·Al₂(SO₄)₃·24H₂O	Double salt	K⁺, Al³⁺, SO₄²⁻
Carnallite, KCl·MgCl₂·6H₂O	Double salt	K⁺, Mg²⁺, Cl⁻
K₃[Fe(CN)₆]	Coordination compound	K⁺ and [Fe(CN)₆]³⁻ only
[Ag(NH₃)₂]Cl	Coordination compound	[Ag(NH₃)₂]⁺ and Cl⁻

Concept

Werner’s theory

Werner (1898): (1) a metal shows primary valency (ionisable, satisfied by anions, equal to the oxidation state) and secondary valency (non-ionisable, satisfied by ligands, equal to the coordination number). (2) Every metal has a fixed secondary valency. (3) Secondary valencies are directed in space, giving a definite geometry: 6 → octahedral; 4 → tetrahedral or square planar.

Table of cobalt ammine chlorides with ions per formula unit and moles of silver chloride precipitated
Werner formula	Ions per formula unit	Moles AgCl per mole
[Co(NH₃)₆]Cl₃	4	3
[CoCl(NH₃)₅]Cl₂	3	2
[CoCl₂(NH₃)₄]Cl	2	1
[CoCl₃(NH₃)₃]	0 (non-electrolyte)	0

Cookies