Determination of formation constants for complexes of very high stability:: logβ4 for the [Pd(CN)4]2- ion

The formation constant, log beta(4) = 62.3 for [Pd(CN)(4)](2-) is reported at 25 degrees C in 0.1 M NaClO4. This value of log beta(4) was determined using a competition reaction, monitored using UV-Vis spectroscopy and H-1 NMR. The competition reaction used was with the tetraamine ligand 2,3,2-tet(1,4,8,11-tetraazaundecane), for which logK(1) = 47.8 at 25 degrees 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 beta(4) 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 logK(1) for cyclam with Pd(II) was attempted using a competition reaction with cyanide, combined with the very high value of log beta(4) for [Pd(CN)(4)](2-) measured here. It appeared that the equilibrium being followed was actually [Pd(cyclam)](2+) + 2CN(-) reversible arrow [Pd(cyclam)(CN)(2)], for which a constant of logK = 5.2 was obtained. H-1 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 beta(4) for [Pd(CN)(4)](2-) found here appears to be the highest formation constant known for any metal ion. (c) 2005 Elsevier B.V. All rights reserved.