Chapter 5 · Introduction

Chapter 5 — Coordination Compounds

Metal ions wrapped in ligands: how Werner decoded them, how to name them, why they come in isomers, and why they are coloured and magnetic.

In the early 1700s a Berlin paint-maker trying to make a red pigment accidentally produced a brilliant blue instead. "Prussian blue" became the first modern synthetic pigment, used by painters from Hokusai to Picasso, and later in the blueprints of engineers. Its secret, understood only two centuries later, is iron held inside a framework of cyanide groups. The same kind of chemistry gives blood its red (iron in haemoglobin), leaves their green (magnesium in chlorophyll), and sapphire and ruby their colours. It lets us measure the hardness of drinking water, extract gold from crushed rock, and treat cancer with a small platinum compound, cisplatin.

All of these are coordination compounds: a central metal atom or ion bound to a set of molecules or ions called ligands. In this chapter you will follow Werner's experiments that revealed their structure, learn their vocabulary and the IUPAC rules for naming them, discover why one formula can describe several different compounds (isomers), and use two bonding theories, valence bond theory and crystal field theory, to explain their shapes, magnetism and colour. You will finish with the many ways coordination compounds are used in analysis, industry, biology and medicine.

Werner's theory→Ligands, denticity, coordination number→IUPAC names→Isomerism→Valence bond theory→Crystal field splitting→Colour, magnetism and uses

Question	Key idea	Section
How many ions does a complex give?	only ions outside $[\ ]$ ionise	5.1
What is the oxidation state of the metal?	$x+\sum(\text{ligand charges})=\text{charge}$	5.1
How many isomers are there?	cis/trans, fac/mer, mirror images	5.2
Why is it magnetic, and what shape is it?	$d^{2}sp^{3}$ vs $sp^{3}d^{2}$; $\mu=\sqrt{n(n+2)}$	5.3
Why is it coloured?	$\Delta_{o}=N_{A}hc/\lambda$	5.3

🎯 By the end of this chapter you can…

explain Werner's theory of primary and secondary valence and use precipitation and conductivity data to deduce the formula of a complex;
use the terms coordination entity, central atom, ligand, denticity, chelate, ambidentate ligand, coordination number, coordination sphere, coordination polyhedron, homoleptic and heteroleptic, and find the oxidation number of the central atom;
write the formula of a mononuclear complex from its IUPAC name and name it from its formula;
identify and draw geometrical (cis/trans, fac/mer) and optical isomers, count stereoisomers, and recognise linkage, coordination, ionisation and hydrate isomers;
use valence bond theory to predict hybridisation, shape and magnetic moment, and crystal field theory to explain high- and low-spin complexes, CFSE and colour;
describe bonding in metal carbonyls and the importance of coordination compounds in analysis, metallurgy, biology and medicine.

Six identical ligands surround a chromium(III) ion. Choose the ligand: as you move along the series from fluoride to cyanide, the ligands split the metal's d orbitals more strongly (Section 5.3), so the complex absorbs light of shorter wavelength and its colour changes. The spectrum bar shows the absorbed band; the tube shows (roughly) the colour you would see. The band positions are approximate literature values for the lowest-energy absorption; real spectra have two or three bands. Notice that the number of unpaired electrons never changes. Press Play to step through the ligands.

Ligand:

splitting $\Delta_{o}$—

light absorbed—

colour seen—

unpaired electrons (Cr³⁺, d³)—

See 5.3 Bonding: Valence Bond and Crystal Field Theories →

Sections in this chapter

5.1

Werner's Theory, Terminology and IUPAC Nomenclature

Primary and secondary valence, ligands, denticity and chelates, coordination number, oxidation number, and the IUPAC rules for formulas and names.

$\mathrm{[CoCl(NH_{3})_{5}]Cl_{2}}$

Open section →5.2

Isomerism in Coordination Compounds

Geometrical (cis/trans, fac/mer) and optical isomers, counting stereoisomers, and linkage, coordination, ionisation and hydrate isomerism.

cis / trans · fac / mer

Open section →5.3

Bonding: Valence Bond and Crystal Field Theories

Hybridisation and magnetism, crystal field splitting, high- and low-spin complexes, CFSE, colour, metal carbonyls and applications.

$\Delta_{t}=\tfrac49\Delta_{o}$

Open section →

✅ Before you start

You will need the electronic configurations of the transition metals and their ions, their common oxidation states and the spin-only magnetic moment $\mu=\sqrt{n(n+2)}\ \mathrm{BM}$ (Chapter 4, The d- and f-Block Elements); oxidation numbers (Class 11 Chemistry, Redox Reactions); the shapes of the d orbitals and Hund's rule (Class 11 Chemistry, Structure of Atom); and hybridisation ($sp^{3}$, $dsp^{2}$, $sp^{3}d^{2}$) and molecular shapes (Class 11 Chemistry, Chemical Bonding and Molecular Structure). Werner's experiments use mole calculations and molar concentration (1.1 Types of Solutions, Concentration and Solubility) and the idea that more ions in solution conduct electricity better (Chapter 2, Conductance of Electrolytic Solutions). Section 5.3 uses $E=hc/\lambda$ for the energy of light.

← 4.3 Lanthanoids, Actinoids and Applications 5.1 Werner's Theory, Terminology and IUPAC Nomenclature →