Introduction

The previous unit noted that transition metals have a tendency to form complexes, or coordination compounds -- a name built from the Latin complexus ('hold') and 'coordinate' ('to arrange'). These complexes behave quite differently from simple ionic or covalent compounds: chromium(III) chloride hexahydrate, CrCl3.6H2O, for instance, can exist as a purple, pale green or dark green compound depending on how it's prepared. Some non-metals form coordination compounds too, though far less readily than the d-block elements.

Coordination compounds are not just a laboratory curiosity -- they are essential to life and industry alike. Haemoglobin, the oxygen carrier in human blood, is a coordination compound of iron; cobalamin (vitamin B12) is a coordination compound of cobalt; and chlorophyll, the pigment that drives photosynthesis in plants, is a coordination compound too. Industrially, complexes such as Wilkinson's compound and the Ziegler-Natta catalyst are workhorses of large-scale chemical manufacturing.