A series of Pt(IV), Pt(II), and Pd(II) complexes (PtCl₄L₂ (1), PtCl₂L₂ (2), PdCl₂L₂ (3), and Pd₂Cl₄L₂ (4)) with 5-methyl-5-(3-pyridyl)-2,4-imidazolidenedione ligand (L) have been synthesized by the reaction of metal chlorides (K₂[PtCl₆], K₂[PtCl₄], K₂[PdCl₄], and PdCl₂) with L in 1:2 (1-3) and 1:1 (4) molar ratios. The binding manner of L, and the composition and geometry of the metal complexes were examined by elemental analysis, IR, ¹H, and ¹³C NMR spectroscopies. Theoretical calculations invoking geometry optimization of different isomers, performed using density functional theory, suggested that in both gas and solution phases the trans isomers are more stable than the cis ones. The experimental results and calculated molecular parameters, bond distances and angles, revealed slightly distorted octahedral (1) and square-planar (2–4) geometry around the metallic center through the pyridine-type nitrogen (N_py) and chlorine atoms. In 4, the binuclear complex, each palladium atom is coordinated by one nitrogen and three chlorine atoms (one as terminal and two as bridging ligands). Antibacterial activity of L and the corresponding complexes was investigated against six species of microorganisms. Testing was performed by disk diffusion method and minimum inhibitory concentrations (MIC) have been determined. The results showed that the title compounds have the capacity of inhibiting the metabolic growth of bacteria to different extents. In general, the binuclear palladium(II) complex was the most active one.