Stereoisomers of palladium(II) complexes with the dipyridyl ligands 2,3-di(2-pyridyl)pyrazine (dpp), 2,3-di(2-pyridyl)quinoxaline (dpq) and 6,7-dimethyl-2,3-di(2-pyridyl)quinoxaline (ddpq). Crystal structure of [Pd(dpq)2](PF6)2