What a learner can do afterwards
- Defines a ligand and a coordination number and picks both out of a given formula
- Predicts octahedral, tetrahedral or square planar geometry from the ligand and its size
- Separates monodentate from bidentate ligands and explains why the larger ones hold on more tightly
- Explains cis-trans isomerism in a named complex and why it matters for a drug such as a platinum complex
1 · Read
A ligand donates a lone pair into an empty orbital on the metal, and the coordination number counts those ligand to metal bonds. [Cu(H2O)6]2+ holds 6 waters so the number is 6, while [CuCl4]2- holds 4 chlorides so the number is 4.
Six small waters fit around copper in an octahedron, but four bulky chlorides only fit as a tetrahedron. Size decides how many ligands fit, and the count sets the shape you get.
Water, ammonia and chloride each bind through one atom, so they are monodentate. Ethane-1,2-diamine binds through two atoms, so it is bidentate, and that double grip makes its complexes far harder to pull apart.
In square planar complexes, matching ligands side by side is cis and opposite corners is trans. Cisplatin, the cis form, fights tumours, while the trans form does not, so the shape really matters.
Lone pairs set the count, size sets the shape, double grips hold tighter, and cis versus trans can save lives.
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8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.