Determination of formation constants for complexes of very high stability: log β4 for the [Pd(CN)4]2− ion

The formation constant, log β4 = 62.3 for [Pd(CN)4]2− is reported at 25 °C in 0.1 M NaClO4. This value of log β4 was determined using a competition reaction, monitored using UV–Vis spectroscopy and 1H NMR. The competition reaction used was with the tetraamine ligand 2,3,2-tet(1,4,8,11-tetraazaundecane), for which log K1 = 47.8 at 25 °C in 0.1 M NaClO4 was determined by competition with thiocyanate, as described by earlier workers (Q.Y. Yan, G. Anderegg, Inorg. Chim. Acta 105 (1985) 121.). Also reported is a value of log β4 for the [Pd(SCN)4]2− ion of 27.2 in 0.1 M NaClO4, determined by competition with 2,2,2-tet. Measurement of log K1 for cyclam with Pd(II) was attempted using a competition reaction with cyanide, combined with the very high value of log β4 for [Pd(CN)4]2− measured here. It appeared that the equilibrium being followed was actually [Pd(cyclam)]2+ + 2CN− ⇄ [Pd(cyclam)(CN)2], for which a constant of log K = 5.2 was obtained. 1H NMR and IR studies suggested that the complex [Pd(cyclam)(CN)2] was prone to oxidation to Pd(IV), followed by disproportionation to [Pd(cyclam)]2+ and, presumably, (CN)2. The very high value of log β4 for [Pd(CN)4]2− found here appears to be the highest formation constant known for any metal ion.