Bonding in Coordination Compounds

Werner had already described what a coordination compound looks like — a central metal surrounded by a fixed number of ligands in a definite geometry — but his theory was silent on why this happens. It left three basic questions unanswered:

• Why do only certain metals show this remarkable tendency to form coordination compounds?
• Why do the bonds around the metal have fixed directions in space, rather than being random?
• Why do coordination compounds show such distinctive magnetic and optical (colour) behaviour?

Explaining these features needs a proper bonding model. Over time chemists have proposed several such models for coordination entities:

• Valence Bond Theory (VBT)
• Crystal Field Theory (CFT)
• Ligand Field Theory (LFT)
• Molecular Orbital Theory (MOT)

Of these, this section develops only an elementary picture based on VBT and CFT — the two approaches that give the clearest qualitative feel for geometry, magnetism, and colour without needing the full machinery of molecular orbital theory.

What each theory is built to explain

Valence Bond Theory treats the metal–ligand bond essentially as a covalent bond formed by orbital hybridisation on the metal, with each ligand donating an electron pair into an empty hybrid orbital.

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