Abstract : The versatile coordination behavior of 4(N, N')-diethylaminosalicylaldehyde-4(N)-substituted thiosemicarbazones (H2L1-4) was examined in 1 : 1 stoichiometric reactions with [PdCl2(AsPh3)(2)]. All the new complexes (1-4) were characterized by analytical, IR, absorption and H-1 NMR spectroscopic studies. The complexes [Pd(DeaSal-tsc)(AsPh3)] (1), [Pd(H-DeaSal-mtsc)(AsPh3)] (2) and [Pd(H-DeaSal-etsc)(AsPh3)] (3) were structurally characterized by single crystal X-ray diffraction studies. Crystallographic results showed that the ligand H2L1 is coordinated as mono negative tridentate ONS donor ligand in the complex 1 by forming six and five member rings. Whereas, the ligands H2L2 and H2L3 bound to palladium in 2 and 3 as mono negative bidentate NS donor by forming a five member chelate ring by leaving the third potential donor atom, phenolic oxygen remain intact without being participated in bonding. The non-participation of the phenolic oxygen may be due to the intra molecular hydrogen bonding between phenolic hydrogen and N2 nitrogen. The interaction of the palladium(II) precursor [PdCl2(AsPh3)(2)], ligands ((H2L1-H2L4)) and their corresponding complexes (1-4) with calf-thymus DNA (CT DNA) has been explored by absorption and emission titration methods. Based on the observations, an intercalative binding mode of DNA has been proposed. The protein binding abilities of the new complexes along with their precursor were monitored by quenching of tryptophan and tyrosine residues using BSA as model protein. From the studies, it was found that the new palladium metallacycles exhibited better affinity than their precursors.