thermodynamic stability versus kinetic lability
Picture two locked boxes. One has a flimsy lock that any key opens in a second, yet inside it the contents are perfectly happy to stay put. The other has a heavy vault door that takes hours to crack, but the moment you do, everything inside tumbles out because it wanted to leave all along. The first box is stable but easy to open; the second is unstable but hard to open. Coordination complexes behave exactly like this, and the two questions — how badly does it want to fall apart, and how fast can it fall apart — have completely separate answers.
Thermodynamic stability is about the destination: it asks whether the complex sits lower in energy than its separated metal ion and free ligands, measured by the formation constant. A large formation constant means the assembled complex is strongly favoured at equilibrium. Kinetic lability is about the journey: it asks how quickly a ligand can be swapped out, set by the height of the activation barrier for substitution. A labile complex exchanges ligands in well under a minute; an inert one may take hours or days. The classic illustration is the hexacyanoferrate ions. [Fe(CN)6]4- and [Fe(CN)6]3- are both extremely stable thermodynamically and also inert, so cyanide does not leak out even though free cyanide is deadly. By contrast [Ni(CN)4]2- is thermodynamically very stable yet kinetically labile — its cyanides exchange with labelled cyanide in solution within seconds.
Keeping these two ideas apart is one of the most important habits in coordination chemistry, because confusing them leads to wrong predictions. A complex being inert does not mean it is at its most stable arrangement — it may simply be trapped behind a tall barrier, like a diamond that is metastable but takes geological time to turn into graphite. Conversely, a very stable complex can still swap its ligands rapidly. When a chemist says a platinum drug is inert, they mean it reacts slowly enough to survive a trip through the bloodstream, not that it is the lowest-energy thing possible.
[Co(NH3)6]3+ is thermodynamically unstable in acid (the equilibrium favours breakdown to Co3+ and ammonium) yet so inert that it survives in acid solution for days, because the cobalt(III) low-spin d6 centre exchanges ligands agonizingly slowly.
A complex can be doomed by thermodynamics yet saved by a slow clock — stability and lability are independent.
The single most common mistake here is treating stable and inert (or unstable and labile) as synonyms. They describe different axes: thermodynamics says where equilibrium lies, kinetics says how fast you get there. The four combinations all exist in real complexes.