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. [Co(H2O)6]2+ holds 6 waters, so its number is 6, while [CoCl4]2- holds 4 chlorides, so its number is 4.
Six small water molecules fit around Co2+ in an octahedron, so [Co(H2O)6]2+ is octahedral. Chloride ions are larger, so only four fit, and [CoCl4]2- is tetrahedral. The same size argument works for other metal ions, such as copper(II). Four ligands can also sit at the corners of a flat square around the metal. This square planar shape is common for platinum(II), as in Pt(NH3)2Cl2.
Water, ammonia and chloride each bind through one atom, so they are monodentate. Ethane-1,2-diamine (en) binds through both its nitrogen atoms, so it is bidentate. When two en replace four waters on copper, three particles become five: [Cu(H2O)6]2+ + 2en -> [Cu(en)2(H2O)2]2+ + 4H2O. More free particles means entropy rises, so the bidentate complex is more stable. This is the chelate effect.
In a square planar complex with two pairs of matching ligands, the matching pair can sit side by side (cis) or across the square (trans). These are two different compounds. Cisplatin is cis-Pt(NH3)2Cl2 and is used as an anticancer drug: its side-by-side positions let the platinum bind to two neighbouring sites on DNA. The trans form is not an effective drug, so shape decides whether it works.
Lone pairs set the coordination number, ligand size sets the shape, bidentate ligands give extra stability, and cis or trans can decide whether a drug works.
Original chemistry lesson and assessment text written for os-taxonomy.
The rewritten reading on ligands and coordination number was written for this dataset. The readings on shapes and on bidentate ligands and cis-trans isomers are original writing for os-taxonomy.
2 · Watch
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Optional connections
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.