Read Why Chelates Are Stable and Where Complexes Show Up
Stability comparisons almost always come down to the chelate effect, while the 'applications' half of this topic is really just recognising the same coordination chemistry at work in biology, medicine and classic qualitative tests.
Stability & Real-World Applications is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
The chelate effect: rings beat singles
Between EDTA (hexadentate), dien (tridentate), ethane-1,2-diamine (bidentate) and ammonia (monodentate) as ligands for Co³⁺, EDTA gives the most stable complex — more chelate rings mean a larger favourable entropy change on binding, which is the real driver of the chelate effect, not just 'more bonds.' Choose A. The same logic ranks [Fe(C₂O₄)₃]3- (three bidentate oxalates, most stable) above [Fe(H₂O)₆]3+, [Fe(NH₃)₆]3+ and [FeCl₆]3- (all weaker/monodentate).
Classic qualitative tests are coordination chemistry
The brown ring test for NO₃⁻ forms [Fe(H₂O)5(NO)]2+, where the unusual apparent oxidation state of iron works out to +1 once NO is treated as NO⁺. Dimethylglyoxime (DMG) is the standard reagent for Ni²⁺, forming a bis-chelate through two O–H···O hydrogen bonds between ligand units. Sodium nitroprusside reacts with S²⁻ to give a violet colour, a classic qualitative test for sulfide. Photographic 'fixing' is itself a coordination reaction: unexposed AgBr dissolves as a soluble thiosulfate complex, Na3[Ag(S₂O₃)₂], which is what allows it to be washed away.
Coordination chemistry in biology and medicine
Magnesium is the central metal ion in chlorophyll — plant biology's photosynthetic pigment depends on a coordination compound. Cisplatin, cis-[Pt(NH₃)₂Cl₂], is used in cancer chemotherapy for its ability to bind DNA; Zeise's salt, by contrast, is a landmark organometallic compound with no medicinal role, so don't confuse the two just because both contain platinum.
OlympIQ exam lens
- ✓The chelate effect is fundamentally an entropy effect — more free small molecules released means a more stable chelate complex.
- ✓EDTA (hexadentate) and oxalate/en (bidentate, times 3) rank above monodentate ligand sets for the same metal.
- ✓Brown ring test (NO₃⁻), DMG test (Ni₂⁺), and sodium nitroprusside test (S₂⁻) are named coordination-chemistry qualitative tests worth memorising directly.
- ✓Cisplatin (chemotherapy) and Zeise's salt (organometallic landmark) both contain Pt but serve completely different roles — don't conflate them.
- ✓Mg²⁺ is the coordinating metal in chlorophyll, the biology-adjacent fact this topic most often tests.
Don't rank stability by counting total ligand atoms or bonds alone — the chelate effect is about entropy from ring formation, which is why a hexadentate EDTA beats six separate monodentate ligands even though the raw bond count could look similar.
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