Read How Chemists Proved Werner's Structures
Werner's coordination theory wasn't guessed — it was proved with two simple experiments: how many ions a solution's conductance implies, and how much AgCl precipitates instantly with AgNO3. Learn to read both.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Conductance tells you how many ions form
Treating CrCl3·6H2O with excess AgNO3 gives 3 mol of AgCl, meaning all three chlorides are ionisable — so the correct formula places every water inside the coordination sphere and every chloride outside it: [Cr(H2O)6]Cl3. Choose D. The same logic ranks a conductance series: more ionisable chloride in a PtCl4·nNH3 series means higher measured conductance.
Only some chlorides precipitate — that's the clue
If AgNO3 precipitates only a fraction of the total chlorine instantly, the rest must be coordinated (bonded to the metal, not free). When 1/3 of CrCl3·6H2O's chlorine precipitates, two of the three chlorides are still attached to Cr, giving [CrCl2(H2O)4]Cl·2H2O. The same reasoning applies to CoBr2Cl·4NH3: if AgNO3 precipitates AgCl instantly but not AgBr, the chloride is the free counter-ion and both bromides stay coordinated — [CoBr2(NH3)4]Cl.
From ion count to coordination number and oxidation number
In [Pt(O2)(en)2Br]2+, work the coordination number and platinum's oxidation number from charge balance across every ligand (peroxide, two neutral bidentate en, one Br-) rather than from a memorised default oxidation state for platinum.
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- ✓Ions that precipitate instantly with AgNO3 are the free counter-ions; unprecipitated chlorine is coordinated.
- ✓More free (ionisable) ions means higher solution conductance.
- ✓Total conductance-derived ion count = coordination sphere charge + number of counter-ions, not the total atom count in the formula.
- ✓Always rebuild oxidation number from overall charge balance across all ligands, not from a memorised 'default' oxidation state.
Don't assume the formula as written shows the true coordination sphere — always re-derive it from the conductance/precipitation data given, since Werner-era formulas like 'CrCl3·6H2O' deliberately hide which atoms are actually bonded to the metal.
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