Home / General Chemistry II / 8. Coordination Chemistry

8. Coordination Chemistry

Transition metals love company: a metal ion surrounded by molecules or ions called ligands forms a coordination complex — the chemistry behind gemstones, haemoglobin, and catalysts.

Complexes and Ligands

The coordination number is the number of donor atoms bonded to the metal (commonly 4 or 6). Ligands donate a lone pair: monodentate (NH3, H2O, Cl−), bidentate (ethylenediamine, "en"), or polydentate chelators (EDTA, hexadentate).

Example: $[\text{Co}(\text{NH}_3)_6]\text{Cl}_3$ — cobalt(III) with six ammonia ligands, coordination number 6, octahedral geometry. Its name: hexaamminecobalt(III) chloride.

Crystal Field Theory and Colour

Ligands split the metal's five d orbitals into two energy levels. An electron jumping the gap absorbs visible light — the colour you see is the complement of the colour absorbed.

Example: $[\text{Cu}(\text{H}_2\text{O})_6]^{2+}$ is blue because its d–d transition absorbs orange-red light. $[\text{Zn}(\text{H}_2\text{O})_6]^{2+}$ is colourless: Zn2+ is d10, so no d–d transition is possible.

Magnetism

Unpaired d electrons make a complex paramagnetic (attracted to a magnet); all-paired complexes are diamagnetic. Strong-field ligands pair electrons up; weak-field ligands leave them spread out.

Practice Problems

  1. Name [Fe(CN)6]4−.
  2. What is the coordination number of [Ni(en)3]2+?
  3. Why is [Zn(H2O)6]2+ colourless?
  4. How many donor atoms does EDTA use? What is this kind of ligand called?

Answers: 1. Hexacyanoferrate(II)   2. 6 (three bidentate ligands × 2 donor atoms)   3. Zn2+ is d10 — no d–d transition can occur   4. Six — a hexadentate (chelating) ligand

ChemistNate • Released to the Public Domain. Feel free to use, share, and build upon these educational resources freely.