In this paper, the synthesis and characterization of two new N-alkylaminopyrazole ligands, 1-[2-(ethylamino)ethyl]-3,5-diphenylpyrazole (dpea) and 1-[2-(octylamino)ethyl]-3,5-diphenylpyrazole (dpoa) are reported. The reaction of these ligands with [MCl2(CH3CN)2] (M = PdII, PtII) affords the following square planar complexes: cis-[MCl2(NN')] (M = PdII: NN' = dpea, 1; dpoa, 2; M = PtII: NN'= dpea, 3; dpoa, 4). Reaction of [PdCl 2(CH3CN)2] and dpea or dpoa in 1:2 M:NN' molar ratio, in the presence of NaBF4, yields complexes [Pd(NN') 2](BF4)2 (NN' = dpea, (BF4)2); dpoa, (BF4)2). The solid-state structures of complexes 1, 3, and (BF4)2 have been determined by single-crystal X-ray diffraction methods. In complexes 1 and 3, the dpea ligand is coordinated through the Npz and Namino atoms to the metallic centre, which completes its coordination with two chlorine atoms in a cis disposition. For complex (BF4)2, the crystal structure consists of cations involving a[Pd(Npz)2(Namino) 2]2+ core with a cis disposition of the two dpea ligands in a square-planar geometry and BF4- anions. Theoretical calculations were carried out to optimize the geometries of the cis and trans isomers of the [Pd(dpea)2]2+ cation and of the [Pd(dpea)2](BF4)2 complex. The results show that the trans isomer is the most stable for [Pd(dpea)2] 2+, in contrast with the cis stereochemistry observed in the crystal structure of [Pd(dpea)2](BF4)2. The calculations also predict that in acetonitrile solution, the dissociation of this complex into the corresponding ions is thermodynamically favourable. The cistrans isomerization process of [Pd(dpea)2]2+ in acetonitrile solution has been studied by NMR spectroscopy at different temperatures. These experimental results confirm that the trans isomer is the thermodynamically most stable form of the complexes (BF4) 2 and (BF4)2. © CSIRO 2010.