Bis(oxalato)palladate(II) is an anionic coordination complex of palladium in the +2 oxidation state with two oxalate units as ligands on it. It therefore can be represented by the formula [Pd(C2O4)2]2−. Salts containing the ion include potassium bis(oxalato)palladate(II), K2[Pd(C2O4)2], and silver bis(oxalato)palladate(II), Ag2[Pd(C2O4)2].
Preparation Bis(oxalato)palladate(II) salts can be prepared by substitution of the chloride ligands in tetrachloropalladate(II) with oxalate. Potassium bis(oxalato)palladate(II) has been prepared from potassium tetrachloropalladate(II) and oxalate in aqueous solution.
[PdCl4]2− + 2 C2O42− → [Pd(C2O4)2]2− + 4 Cl−
Potassium bis(oxalato)palladate has also been prepared from hydrated palladium(II) oxide and oxalic acid.
Other salts can be obtained by metathesis. Silver bis(oxalato)palladate(II), Ag2[Pd(C2O4)2], has for example been prepared and used as a precursor to silver–palladium colloids.
Properties Structure Palladium(II) in [Pd(C2O4)2]2− has the characteristic square-planar coordination geometry of many d8 Pd(II) complexes. Each oxalate ligand acts as a bidentate ligand through two oxygen atoms, producing two five-membered chelate rings.
The structure has been determined in potassium bis(oxalato)palladate(II) tetrahydrate, K2[Pd(C2O4)2]·4H2O. The salt crystallizes in the monoclinic space group P21/n, with lattice parameters a = 11.03 Å, b = 14.86 Å, c = 3.70 Å and β = 93.8°.
The [Pd(C2O4)2]2− groups are essentially planar. Oxygen atoms belonging to neighboring complexes lie above and below the palladium coordination plane as the nearest out-of-plane neighbors of Pd.
Stability and reactions Potassium bis(oxalato)palladate(II) is commonly isolated as a hydrate. Preparative work describes the material as sufficiently sensitive that it is normally stored in a cool, dark place.
The oxalate ligands can be displaced by other donor ligands. K2[Pd(C2O4)2] has therefore been widely used as a starting material for preparing other Pd(II) oxalato complexes.
For example, reaction with substituted adenine ligands gives complexes of the general form [Pd(ox)(L)2]·nH2O, in which one oxalate ligand remains coordinated while the second is replaced by nitrogen donors.
Silver salt and photochemical reduction Silver bis(oxalato)palladate(II), Ag2[Pd(C2O4)2], can be photochemically reduced in the presence of poly(N-vinylpyrrolidone) to produce bimetallic silver–palladium colloids.
This reaction has been used as a route to Ag–Pd nanoparticles and is one of the better-studied applications of a bis(oxalato)palladate salt.
References