Introduction

Building on the Previous Unit

The previous unit on the d- and f-block elements established that transition metals readily form a large number of complex compounds, in which a metal atom or ion is bound to a set of surrounding anions or neutral molecules through shared electron pairs. In current terminology these are called coordination compounds, and this unit turns to them as a subject in their own right. Coordination chemistry is one of the more active and demanding areas of modern inorganic chemistry — the bonding models and structural ideas it has produced have, among other things, helped explain how such compounds function as essential components of living systems.

Why Coordination Compounds Matter

Coordination compounds are not a narrow, specialised topic — they turn up across chemistry and biology:

• Biological systems: chlorophyll (built around magnesium), haemoglobin (built around iron), and vitamin B12 (built around cobalt) are all, at their core, coordination compounds of a transition-metal ion.
• Industry: many metallurgical extraction processes, industrial catalysts, and analytical reagents rely on coordination compounds.
• Everyday applications: electroplating, textile dyeing, and several areas of medicinal chemistry also make direct use of coordination chemistry.

Because of this reach, coordination compounds are often described as underpinning much of modern inorganic and bio-inorganic chemistry, and by extension a good part of the chemical industry built on them.

What This Unit Covers

By the end of this unit, the goal is to be able to:

• explain the postulates of Werner's theory of coordination compounds;
• correctly use the core vocabulary — coordination entity, central atom/ion, ligand, coordination number, coordination sphere, coordination polyhedron, oxidation number, and the homoleptic/heteroleptic distinction;
• apply the IUPAC rules of nomenclature for coordination compounds, both to name a compound and to write its formula;
• recognise and classify the different types of isomerism shown by coordination compounds;
• describe how Valence Bond Theory and Crystal Field Theory account for the bonding in coordination compounds; and
• appreciate why coordination compounds matter, both in the natural world and in industrial and everyday applications.

The unit begins, in the next section, with the historical starting point for all of this: Alfred Werner's own theory of how these compounds are put together.