Chemistry · Coordination Compounds · NEET
When a metal ion M reacts with ligands L in water, it forms a complex: M + 4L gives ML4. The stability constant Kf is the equilibrium constant for this reaction: Kf = [ML4] / ([M][L]^4). It measures how far the reaction goes toward making the complex. A large Kf means most of the metal ends up as the complex, so the complex is stable. It is also called the formation constant.
Yes. This is the key rule for NEET. The larger the stability constant, the more stable the complex in solution, and the harder it is to break apart. For example, [Cu(NH3)4]2+ has Kf around 10^12 and [Cu(CN)4]2- has an even larger Kf, so the cyanide complex is more stable. When a question asks 'which is most stable', you compare Kf values (or the ligand type if Kf is not given).
Ligands add one at a time, not all together. Each step has its own constant: K1 for the first ligand, K2 for the second, and so on. These are stepwise stability constants. The overall stability constant (beta) is the product of all the stepwise constants: beta4 = K1 x K2 x K3 x K4. So overall Kf covers the whole reaction from bare metal ion to the finished complex. Usually K1 > K2 > K3 > K4 because it gets harder to add more ligands.
A chelate is a ring formed when a bidentate or polydentate ligand grabs the metal at two or more points, like en (ethylenediamine) or EDTA. Complexes with chelate rings have much higher stability constants than similar complexes with only monodentate ligands like NH3. The main reason is entropy: one chelating ligand replaces several separate ligands, releasing more free molecules into solution, which increases disorder and makes the reaction favourable. This is exactly why the 2023 NEET answer was the en complex.
The instability constant (also dissociation constant) is for the complex breaking apart, which is the reverse reaction. It is simply 1/Kf. So a very stable complex has a very small instability constant. If a question gives an instability value, remember that a smaller instability constant means a more stable complex.
Stability constant is an equilibrium constant, so it links to Gibbs free energy by delta-G = -RT ln Kf. A large Kf means a large negative delta-G, meaning the complex forms spontaneously and is thermodynamically stable. For chelates, the extra stability comes mostly from a positive entropy change (delta-S), which makes delta-G more negative.
Which complex compound is the most stable?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It tells you the complex is very stable in solution. The metal ion holds its ligands strongly, the complex forms almost completely, and it resists breaking apart into free metal ion and ligands.
Yes. Stability constant and formation constant are two names for the same quantity, Kf. Its reciprocal, 1/Kf, is the instability or dissociation constant.
The en complex is more stable. en is a bidentate ligand that forms a chelate ring, so it has a higher stability constant than the ammonia complex with only monodentate NH3 ligands. This is the chelate effect.
Strong field ligands like CN- usually give complexes with high stability constants, so often yes. But for NEET comparison questions, the chelate effect (bidentate/polydentate ligands) is the most common deciding clue when Kf values are not provided.
EDTA is a hexadentate chelating ligand that forms very stable complexes with Ca2+ and Mg2+ (high stability constants). The difference in stability constants of the calcium and magnesium complexes even lets you estimate them selectively by titration, as stated in NCERT.
