Dynamic NMR experiments on trans-[Pt(Cl)(PHCy2)(2){P(X)Cy-2}](z) where X is a lone pair (1, z = 0), H (2, z = + 1), S (3, z = 0), or 0 (4, z = 0) show that the rotation around the P(X)-Pt bond is hindered for all molecules studied, with Delta(double dagger) ranging from 8.2 to 11.0 kcal/mol. The highest value of the series was calculated for trans-[Pt(Cl)(PHCy2){P(O)Cy-2}] (4) where intramolecular P=(OH)-H-...-P interactions act as a molecular brake at room temperature. Single-crystal X-ray diffraction confirms the presence of both intra and intermolecular P=(OH)-H-... interactions in solid 4. In the case of [Pt(Cl)(PHCy2)(3)]Cl, multinuclear NMR analysis indicates the presence of a P-(HCl-)-Cl-... interaction in aromatic or halogenated solvents which could have also a minor effect on the rotational barrier around the P(X)-Pt bond.