X-ray crystallographic analyses revealed that the Pd–P and Pd–X bond lengths of [PdX2(P–P)] complexes (X−=Cl, Br and I and P–P=Ph2P(CH2)nPPh2; n=1: dppm, 2: dppe, 3: dppp, 4: dppb) are: Pd–P (dppm, dppe)<Pd–P (dppp, dppb) and Pd–X (dppm, dppe)>Pd–X (dppp, dppb). The oxidation potentials values, E½(ox), of the shorter Pd–P and longer Pd–X bond lengths in the dppm and dppe complexes are high and reflect strong dxy (dπ (Pd)) interaction with dx2-y2 (dσ∗ (Pd)) and that this interaction is less dependent on X−. The longer Pd–P and shorter Pd–X bonds for the dppp and dppb complexes reflect that the dx2-y2 (dσ∗ (Pd)) interaction is largely dependent on X−. The reduction potential, E½(red), which reflects the dx2-y2 (dσ∗ (Pd) LUMO energy, and the oxidation potential, E½(ox), which reflects the (dxy (dπ (Pd) HOMO energy were, determined. The E½(red) and E½(ox) values indicate the σ-bonding properties and π-interaction in the system, respectively. Good linear relationships were observed between the potentials difference [E½(ox)−E½(red)] and the ligand-field transition energy (dπ (dxy)→dσ∗ (dx2-y2)) in the series of dihalogenopalladium(II) complexes. The linear relationship shows that the energies of the d–d transitions and ΔE(Redox) of the dppm and dppe complexes are less dependent on X− whilst those of the dppp and dppb are largely X− dependent.