The complexes trans-[PdCl₂L₂][L = Me₂CN–NMePh (L¹); Et₂CN–NMePh (L²); PrⁿMeCN–NMePh (L³); MeHCN–NMePh (L⁴); Me₂CN–NMe₂(L⁵); or MeHCN–NMe₂(L⁶)] have been prepared by reaction of the hydrazone ligands with [PdCl₂(NCPh)₂]. The crystal and molecular structure of trans-[PdCl₂(Me₂CN–NMePh)₂] has been determined from three-dimensional X-ray data. The complex crystallizes in the monoclinic space group P2₁/n with two molecules in a cell of dimensions a= 18.83(3), b= 7.53(2), and c= 7.76(2)Å, β= 89.1(5)°. The palladium atom lies at a centre of symmetry; consequently the four ligating atoms, i.e. the two chloride ions and the two imino-nitrogens of the hydrazones, are located rigorously in a plane passing through the central metal. The co-ordination plane is nearly perpendicular to the plane containing the non-hydrogen atoms of the hydrazone skeleton Me₂CN–N. Significant bond distances are Pd–Cl 2.298(4) and Pd–N 2.047(2)Å. Another isomeric structure of this complex can be obtained by 180° rotation about the Pd–N bond of one of the two hydrazone ligands. This isomer is not centrosymmetric but contains a plane of symmetry which is perpendicular to the co-ordination plane and passes through the Cl–Pd–Cl axis. Both these isomers exist in equilibrium in solution for all the prepared complexes. Values of free energies of activation for the interconversion process have been obtained from temperatures of coalescence of the n.m.r. signals; they vary from 58 to 89 kJ mol^–1 with the nature of the ligand substituents. In particular, complexes of hydrazones derived from aldehydes (L⁴ and L⁶) have ΔG^‡ 10–14 kJ mol^–1 lower than those of analogous hydrazones derived from ketones (L¹–L³ and L⁵); complexes of hydrazones derived from methylphenylhydrazine (L¹–L⁴) have ΔG^‡ 17–21 kJ mol^–1 lower than those of analogous hydrazones derived from dimethylhydrazine (L⁵ and L⁶).