Thermodynamics, kinetics and mechanism for Palladium(II) oxalate complex formation


         Přemysl Lubal ^1,2, Radomír Pilný ^1, Romana Michalicová ^1 , Lars Ivar Elding ^3

^1 Department of Chemistry, Faculty of Science, Masaryk University, Kotlářská 2, 611 37 Brno, Czech
Republic

^2 Central European Institute of Technology (CEITEC), Masaryk University, Kamenice 5, 625 00 Brno,
Czech Republic

^3 Department Chemistry, Faculty of Science, Lund University, PO Box 124, 221 00 Lund, Sweden


Spectrophotometric study of complex formation between [Pd(H[2]O)[4]]^2+ and oxalate ions
(ox=C[2]O[4]^2-) was performed in aqueous solution under variable temperature and ionic strength.
Thermodynamic parameters (log[10] K[1H] = 3.38 ± 0.08, DH^0 = -33 ± 3 kJ.mol^-1,          DS^0 =
-48 ± 11 J.K^-1.mol^-1, T = 298.2 K, I = 1.00 M HClO[4]) were determined for following reaction:

Stability constants of complexes [Pd(H[2]O)[2](ox)] and [Pd(ox)[2]]^2- (log[10] K[1]^0 = 9.04 ±
0.06, log[10] K[2]^0 = 13.1 ± 0.3) were determined under conditions of zero ionic strength and
temperature 298.2 K and calculated using SIT theory from their dependence on ionic strength of the
solution. Formation of complex [Pd(H[2]O)[2](ox)] can be described by two-step reaction mechanism,
where the rate-determining step is formation of monodentate complex [Pd(H[2]O)[3](ox)]^+ with
H[2]ox and Hox^- species. This can be described by parameters of equilibrium reaction (log[10]
K[1H]’ = 1.15,[ ]H^0 = -3 kJ.mol^-1, S^0 = -12 J.K^-1.mol^-1), which are of the same reaction
order as thermodynamic data for Pd(II) complexes with other monodentate ligands containing
carboxylate group^1,2. This reaction step is followed by closing of the chelate ring and
thermodynamically stable 1:1 complex is created (K[1H,rc] » 2.19). Chelate effect as a result of
increased stability of complex [Pd(H[2]O)[2](ox)] can be explained by dominance of enthalpic
contribution (~ 75 %) over entropic contribution (~ 25 %) which is the opposite behaviour than is
usually observed for most of the metal complexes.


REFERENCES

1.   Shi T., Elding L. I., Inorg. Chem. 36, 528 (1997)

2.   Lubal P., Elding L. I., J. Sol. Chem. 35, 769 (2006).