New Ni(II), Pd(II) and Pt(II) complexes of 5-methyl-5-(4-nitrophenyle)-hydantoin (L) were synthesized, characterized and their biological activities were evaluated. The resulting spectroscopic and theoretical data offered strong evidences for coordination of hydantoin based ligand from oxygen atom of C@O group to the Ni(II), Pd(II) and Pt(II) metal centers. The theoretical data proposed cis and trans structures for complexes. Moreover, the cytotoxic activity of the compounds was tested against three different cancer cell lines including MCF-7 (breast), A549 (lung) and AGS (gastric) adenocarcinoma cells. Generally, Pt(II) complex possesses a higher cytotoxic effect against the tested cells than the Pd(II) complex. Likewise, antioxidant activity assay exhibited that both of the Pd(II) and Pt(II) complexes had significant potential compared to ascorbic acid and quercetin, as the standards. Also a theoretical study on structure of monomeric complexes of the form of [L ? MCl2 L] where M = Ni(II), Pd(II) and Pt(II) have been investigated at the BP86/def2-SVP level of theory. The bonding situation between the [MCl2] and L fragments in [L ? MCl2 L] complexes, were carried out by natural bond orbital (NBO) and energy-decomposition analysis (EDA), as well as its natural orbitals for chemical valence variation (EDA-NOCV). The results showed that the M O bond interaction energies (DEint) of the L and MCl2 fragments in metal complexes are substantial and conform to the V-shaped trend for the transition metals of the first, second, and third row as Ni > Pt > Pd
