Read Ligands and the Coordination Sphere First
Before counting isomers or splitting energies, nail down what actually counts as a ligand, how many donor atoms it uses, and what stays inside the coordination sphere versus what ionises away as a counter-ion.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
What makes something a ligand
A ligand needs a lone pair (or π system) it can donate to the metal. NH4+ has no lone pair on nitrogen to give — all four are used in N–H bonds — so it cannot coordinate. Hydrazine (H2N–NH2) has two nitrogen lone pairs but under normal conditions donates through only one at a time, acting as a monodentate ligand rather than a bridging bidentate one here. Choose B for the hydrazine question.
Counting donor atoms: denticity
Ethylenediamine (en) is simultaneously neutral, bidentate (two N donors) and chelating — 'all of the above' is correct precisely because these three descriptions aren't competing, they're compatible layers of the same fact. EDTA goes further: four carboxylate oxygens plus two amine nitrogens make it hexadentate, so a single EDTA molecule alone fills all six sites of an octahedral Ca2+ complex — you don't need two.
Double salts lose their identity; coordination compounds don't
Mohr's salt, carnallite and alum are classic double salts: dissolve them in water and they dissociate completely into their simple constituent ions — Fe2+, NH4+, SO4²- for Mohr's salt — with no complex ion surviving. That's the dividing line from a true coordination compound, where the complex ion (like [Fe(CN)6]⁴-) stays intact in solution.
What's inside the coordination sphere, and what isn't
Coordination number counts donor atoms bonded directly to the metal — not every atom in the formula. In CuSO4·5H2O, only 4 of the 5 waters are coordinated to Cu2+; the 5th is held by hydrogen bonding to the sulfate, which is why the formula is better written [Cu(H2O)4]SO4·H2O. Similarly, when all three Cl- in CoCl3·3NH3 sit inside the coordination sphere, no free counter-ion remains outside to ionise, so the compound behaves as a non-electrolyte in solution.
OlympIQ exam lens
- ✓A ligand must have a donor lone pair available — NH4+ fails this test.
- ✓Denticity = number of donor atoms bonded to the same metal centre; EDTA is hexadentate, en is bidentate.
- ✓Double salts (Mohr's salt, alum, carnallite) fully dissociate; coordination compounds keep their complex ion intact.
- ✓Coordination number counts only atoms directly bonded to the metal, not every atom in the formula.
- ✓A compound with zero counter-ions outside the coordination sphere is a non-electrolyte.
Don't assume every nitrogen- or oxygen-rich species is automatically a good ligand or automatically polydentate — check that a lone pair is actually free to donate, and count donor atoms directly rather than atoms in the formula.
Ready to try the pattern?
100% coverage across the 11 linked questions.
Practise 11 linked questions