Structural comparison of [PdX₂(P-P)] complexes (X^- = Cl, Br and I; P-P = α,ω-bis(diphenylphosphino)alkanes) and their trends of redox potentials

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, E_1/2(ox), of the shorter Pd-P and longer Pd-X bond lengths in the dppm and dppe complexes are high and reflect strong d_xy (dπ (Pd)) interaction with d_x2-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 d_x2-y2 (dσ^* (Pd)) interaction is largely dependent on X ^-. The reduction potential, E_1/2(red), which reflects the d_x2-y2 (dσ^* (Pd) LUMO energy, and the oxidation potential, E_1/2(ox), which reflects the (d_xy (dπ (Pd) HOMO energy were, determined. The E _1/2(red) and E_1/2(ox) values indicate the σ-bonding properties and π-interaction in the system, respectively. Good linear relationships were observed between the potentials difference [E _1/2(ox) - E_1/2(red)] and the ligand-field transition energy (dπ (d_xy) → dσ^* (d _x2-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.