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 C-13 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: (k(2)(Pt,CN))(298) = 11 +/- S-1 mol(-1) kg, DeltaH(2)(double dagger) (Pt,CN) = 25.1 +/- 1 kJ mol(-1), DeltaS(2)(double dagger) (Pt,CN) = -142 +/- 4 J mol(-1) K-1, and DeltaV(2)(double dagger) (Pt,.CN) = -27 +/- 2 cm(3) mol(-1). The Pd(II) metal center has the same behavior down to pH 6. The kinetic parameters are as follows: (k(2)(Pd,CN))(298) = 82 +/-2 s(-1) mol(-1) kg, DeltaH(2)(double dagger) (Pt,CN) = 23.5 +/-1 kJ mol(-1), DeltaS(2)(double dagger) (Pd,CN) = -129 +/- 5 J mol(-1) K-1, and DeltaV(2)(double dagger) (Pd,CN) = -22 +/- 2 cm(3) mol(-1). At low pH, the tetracyanopalladate is protonated (pK(a)(Pd(4,H)) = 3.0 +/- 0.3) to form [Pd(CN)(3)HCN](-). The rate law of the cyanide exchange on the protonated complex is also purely second order, with (k(2)(PdH,CN))(298) = (4.5 +/-1.3) x 10(3) 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)(3)HCN](-), and [Ni(CN)(2)(HCN)(2)] complexes. Our C-13 NMR measurements have allowed us to determine that the rate constant leading to the formation of [Ni(CN)(5)](3-) is k(2)(Ni(4),CN) = (2.3 +/- 0.1) x 10(6) S-1 mol(-1) kg when the following activation parameters are used: DeltaH(2)(double dagger) (NI,CN) = 21.6 +/- 1 kJ mol(-1), DeltaS(2)(double dagger) (Ni,CN) = -51 +/- 7 J mol(-1) K-1, and DeltaV(2)(double dagger) (Ni,CN) = -19 +/- 2 cm(3) mol(-1). The rate constant of the back reaction is k(-2) (Ni(4),CN) = 14 x 10(6) 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 (k(2)(Ni(4,2H),CN))(298) = (63 +/- 15) x 10(6) s(-1) mol(-1) kg when DeltaH(2)(double dagger) (Ni(4,2H),CN) = 47.3 +/- 1 kJ mol(-1), DeltaS(2)(double dagger) (Ni,(4,2H),CN) = 63 +/- 3 J mol(-1) K-1, and DeltaV(2)(double dagger) (Ni(4,2H),CN) = -6 +/- 1 cm(3) 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)(3)HCN](-). For all cyanide exchanges on tetracyano complexes (A mechanism) and on their protonated forms (///(a) mechanisms), we have always observed a pure second-order rate law: first order for the 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.