We prepared hydrazone palladium(II) complexes of [PdCl2(HLn)] and [PdCl(L-n)] (n = 1-3) by the reaction of [PdCl2(cod)] or [PdCl2(PhCN)(2)] and the hydrazone ligands of HLn {N'-(pyridin-2-ylmethylene)picolinohydrazide (HL1), N'-[1-(pyridin-2-yl)ethylidene]picolinohydrazide (HL2), and N'-[(6-methylpyridin-2-yOmethylene]picolinohydrazide (HL3)}. The structures of the complexes were determined by X-ray analysis. The hydrazone ligands had kappa N(pyl), kappa N(imine) and kappa N(amidate) kappa N(py2) bidentate coordination modes in [PdCl2(HLn] (1, n = 1; 2, n = 2) and in [PdCl2(HL3)] (3), respectively. In contrast, tridentate coordination modes of kappa N(pyl)), kappa N(imine), kappa N(py2) and kappa N(py1), kappa N(amidate), kappa N(py2) were observed in [PdCl(L-n)] (4, n = 1; 5, n = 2) and in [PdCl(L-n)] (6, n = 1; 7, n = 2; 8, n = 3). Thermal conversion of complexes 1-3 to complexes 6-8 proceeded in acetonitrile. Complexes 4 and 5 were obtained from complexes 1 and 2, respectively, in a basic acetonitrile solution under dark conditions. Complex 4 reverted immediately to complex 1 in an acidic acetonitrile solution that included hydrochloric acid. However, under room light, in the basic acetonitrile solution that included trimethylamine, complex 4 converted photochemically to complex 6. The thermochromic or vapochromic structure conversion of these complexes also occurred in the solid state. On heating at 180 degrees C, the color of the crystals of complexes 1, 2, and 3 changed from yellow to orange in the solid state. H-1 NMR and/or UV vis absorption spectroscopy confirmed that the orange complexes 6-8 were produced. The reddish-orange crystals of complexes 4 and 5 were exposed to hydrogen chloride vapor to yield the yellow products of complexes 1 and 2, respectively.