Mixed-ligand palladium(II) chloride complexes bearing the nucleobases, adenine (Ad), cytosine (Cyt), and guanine (Gua), have been synthesized and characterized by UV-vis spectrophotometric methods, magnetic susceptibility, molar conductivity, elemental analysis, FTIR, and 1H-NMR. The complexes were found to have the composition cis-[PdCl2(Gua)(Cyt)], cis-[PdCl2(Ad)(Cyt)], and cis-[PdCl2(Ad)(Gua)]. A four-coordinated square-planar geometry is proposed for these Pd(II) complexes based on magnetic evidence and electronic spectra. The complexes as well as the free nucleobase ligands were tested for their in vitro cytotoxicity on human promyelocytic leukemia (HL60) and human histiocytic leukemia (U937) cell lines. cis-[PdCl2(Ad)(Gua)] showed IC50 values of 11.29？±？2.91 and 8.31？±？1.44？μM against HL60 and U937, respectively, which was higher than that of the positive control (curcumin) against U937. The complexes also showed significant antioxidant activity when tested against 2,2-diphenyl-1-picrylhydrazylradical (DPPH). 1. Introduction The versatility of nucleobases has made them suitable ligands for the synthesis of numerous transition metal complexes [1–5]. In particular, palladium complexes of nucleobases and their derivatives have been of current interest since these complexes form faster than their platinum analogues and produce analogous products in solution [6, 7]. The mode of bonding and structure of these and related ligands have been exhaustively studied, both spectroscopically and crystallographically [8–11] and have been shown to be monodentate via the N3 of the pyrimidine and N7 of the purine ligand (as shown in Figure 2). In view of the strong electron withdrawing ability of these metal ions, there is a drift of electron density onto the metal centre and in effect ligand hydrogens are easily lost in the presence of a free radical. This makes these metal complexes act as better antioxidants as compared to the free ligand [12, 13]. Mixed-ligand complexes play a key role in biological chemistry [14] because mixed chelation occurs commonly in biological fluids as millions of potential ligands are likely to compete for metal ions in vivo [15]. Because of this profound role, mixed-ligand complexes have been extensively studied for their thermodynamic [16–18] as well as kinetic stability [19–23]. These complexes have received tremendous attention in the search for novel drugs against drug resistant diseases [24–27] with cisplatin being a classic example [28]. Metal based antioxidants have received recent attention for their capacity to protect
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