Q.The stability constant K of the [Ag(CN)₂]⊖ is 5.5 × 10¹⁸ while that for the corresponding [Ag(NH₃)₂]⊕ is 1.6 × 10⁷. Explain why [Ag(CN)₂]²⊖ is more stable.

A coordination complex's stability can be described thermodynamically (the free energy change, ΔG, of its formation reaction) or kinetically (how fast it undergoes ligand substitution -- a labile complex substitutes rapidly, an inert complex substitutes slowly or not at all; these two notions of 'stability' are independent of each other).

Quantitatively, stability is measured by the STABILITY CONSTANT (β), which describes the equilibrium Mⁿ⁺ + xL ⇌ [MLx]ⁿ⁺: β = [MLx]/([M][L]ˣ). The larger β is, the more the equilibrium favours the intact complex, and the greater the complex's stability. The reciprocal relationship, β = 1/α, connects β to the dissociation (INSTABILITY) constant α = [M][L]ˣ/[MLx], which describes the same equilibrium read in the opposite (dissociating) direction. Comparing published values shows [Hg(CN)₄]²⁻ (β ≈ 10⁴¹) is far more stable than [Fe(SCN)]²⁺ (β ≈ 10³). …