Two diethyl phosphonated phosphine ligands of formula Ph 2 P(CH 2 ) 3 PO 3 Et 2 (ligand L) and Ph 2 P(4-C 6 H 4 PO 3 Et 2 ) (ligand L') were used to prepare different complexes of platinum(II) (1, cis-PtCl 2 L 2 ; 2, trans-PtCl 2 L 2 .H 2 O; 3A and 3B, cis- and trans-PtCl 2 L' 2 ) and palladium(II) (4, [PdCl 2 L] 2 ; 5, trans-PdCl 2 L 2 .H 2 O; 6, trans-PdCl 2 L' 2 .CH 2 Cl 2 ). The single-crystal X-ray structure analyses of complexes 1, 2, 4-6 indicate that complexation involved only the phosphine end, whereas the strong polarization of the P O bond was highlighted by the formation of hydrogen bonds with a water molecule in 2 and 5, and with a dichloromethane molecule in 6, with an exceptionally short C H...O hydrogen bond length (C...O separation 3.094(3) Å).