Nucleophilic Substitution Reactions at Planar Tetra-coordinate Bis-cationic Platinum(II) Complexes. Kinetics of Displacement of Pyridine from {Pt[2,6-bis(methylthiomethyl)pyridine](py)}2+, {Pt[bis(2-pyridylmethyl)amine](py)}2+ and {Pt[bis(2-pyridylmethyl)sulphide](py)}2+ cations