Kinetic studies of cyanide exchange on [M(CN)4]2- square-planar complexes (M = Pt, Pd, and Ni) were performed as a function of pH by 13C NMR. The [Pt(CN)4]2- complex has a purely second-order rate law, with CN- as acting as the nucleophile, with the following kinetic parameters: (k2Pt,CN)298 = 11 ± 1 s-1 mol-1 kg, ΔH2? Pt,CN = 25.1 ± 1 kJ mol-1, ΔS2? Pt,CN = -142 ± 4 J mol-1 K-1, and ΔV2? Pt,CN = -27 ± 2 cm3 mol-1. The Pd(II) metal center has the same behavior down to pH 6. The kinetic parameters are as follows: (k2Pd,CN)298 = 82 ± 2 s-1 mol-1 kg, ΔH2? Pd,CN = 23.5 ± 1 kJ mol-1, ΔS2? Pd,CN = -129 ± 5 J mol-1 K-1, and ΔV2? Pd,CN = -22 ± 2 cm3 mol-1. At low pH, the tetracyanopalladate is protonated (pKa Pd(4,H) = 3.0 ± 0.3) to form [Pd(CN)3HCN]-. The rate law of the cyanide exchange on the protonated complex is also purely second order, with (k2PdH,CN)298 = (4.5 ± 1.3) × 103 s-1 mol-1 kg. [Ni(CN)4]2- is involved in various equilibrium reactions, such as the formation of [Ni(CN)5]3-, [Ni(CN)3HCN]-, and [Ni(CN)2(HCN)2] complexes. Our 13C NMR measurements have allowed us to determine that the rate constant leading to the formation of [Ni(CN)5]3- is k2Ni(4),CN = (2.3 ± 0.1) × 106 s-1 mol-1 kg when the following activation parameters are used: ΔH2? Ni,CN = 21.6 ± 1 kJ mol-1, ΔS2? Ni,CN = -51 ± 7 J mol-1 K-1, and ΔV2? Ni,CN = -19 ± 2 cm3 mol-1. The rate constant of the back reaction is k-2Ni(4),CN = 14 × 106 s-1. The rate law pertaining to [Ni(CN)2(HCN)2] was found to be second order at pH 3.8, and the value of the rate constant is (k2 Ni(4,2H),CN,)298 = (63 ± 15) × 106 s-1 mol-1 kg when ΔH2? Ni(4,2H),CN = 47.3 ± 1 kJ mol-1, ΔS2? Ni(4,2H),CN = 63 ± 3 J mol-1 K-1, and ΔV2? Ni(4,2H),CN = -6 ± 1 cm3 mol-1. The cyanide-exchange rate constant on [M(CN)4]2- for Pt, Pd, and Ni increases in a 1:7:200 000 ratio. This trend is modified at low pH, and the palladium becomes 400 times more reactive than the platinum because of the formation of [Pd(CN)3HCN]-. For all cyanide exchanges on tetracyano complexes (A mechanism) and on their protonated forms (IIIa mechanisms), we have always observed a pure second-order rate law: first order for the complex and first order for CN-. The nucleophilic attack by HCN or solvation by H2O is at least nine or six orders of magnitude slower, respectively than is nucleophilic attack by CN- for Pt(II), Pd(II), and Ni(II), respectively.