Foundations — Coordination number and geometry — 2 (linear), 4 (tetrahedral - square planar), 6 (octahedral)

This page builds every symbol, word, and picture the parent note leans on — assuming you have seen none of them. Read top to bottom; each block earns the next.

1. The central metal ion — what "${\text{M}}^{n+}$" means

Picture: a ball in the middle of the drawing. Because it is missing electrons, it is electron-hungry — it wants to borrow electron pairs from its neighbours. That hunger is the whole reason ligands attach.

Why the topic needs it: the metal is the planet at the centre. Its charge and its electron count decide how many moons it collects and what shape they make.

2. Electron pairs and the coordinate bond — the "grab"

Picture: think of a lone pair as a sticky hand. The metal has empty pockets; a lone-pair hand plugs into a pocket. One hand plugged in = one coordinate bond.

Why the topic needs it: every line drawn from a ligand to the metal is a coordinate bond. Counting these bonds is literally what coordination number does.

3. Ligand and donor atom — the difference that trips everyone

Why the topic needs it: the parent's biggest warning — CN counts donor atoms, not ligand molecules — only makes sense once these two words are separate in your head.

4. Denticity — how many hands one ligand has

Picture (figure above): a monodentate ligand is a stick with one hand; a bidentate ligand is a clip that clamps with two hands at once; EDTA is a whole octopus of six hands wrapping the metal. See Chelation and Denticity for how multi-handed ligands lock on so tightly.

Why the topic needs it: denticity is the multiplier that turns "3 en molecules" into "6 grabs".

5. Coordination number — the total count

Reading the symbols out loud: "for each kind of ligand, multiply how many of them by how many hands each has, then add all those products." That total is the number of hands touching the metal — the coordination number (CN).

Why the topic needs it: CN is the first thing you compute for any complex; the geometry follows from it.

6. Electron-pair repulsion — why shapes happen

Picture: blow up a balloon for each donor pair, all tied at the metal. They shove until they reach a stable, symmetric spread.

• 2 balloons → point opposite ways → a straight line, angle $180°$.
• 4 balloons → cannot lie flat without crowding; they lift into 3D corners of a tetrahedron, angle $109.5°$.
• 6 balloons → four around the middle + one up + one down → an octahedron, all neighbours $90°$ apart.

Why the topic needs it: this is the engine. CN sets how many balloons; repulsion decides where they sit.

7. Angles and geometry names — reading the shapes

8. The d-electron twist — why CN 4 has two answers

Why the topic needs it: pure balloon-repulsion says CN 4 should always be a tetrahedron. But a $d^8$ metal with a strong-field ligand gains extra stability by flattening into a square — the field energy overrules the repulsion. That flip (and the Magnetic properties of complexes it changes) is why $[{\text{NiCl}}_4{]}^{2-}$ and $[{\text{Ni(CN)}}_4{]}^{2-}$ have different shapes despite the same metal.

Prerequisite map

This map is the road into the parent topic (English index 3.4.4). Later branches also feed Isomerism in coordination compounds.

Equipment checklist

Self-test: cover the right side and try to answer before revealing.