Substitution reactions of NN chelating atoms of organoplatinum (II) complexes with phosphorous donor reagents

Substitution reactions of the chelate 2,2′-bipyridine ligand in the organoplatinum(II) complexes of general formula [PtR2(NN)] (NN = 2,2′-bipyridine, and R = CH3, 1, or R = p-CH3C6H4, 1′) by the P donor reagents L, L = P(O–iPr)3 or PPh3 or L2 = dppm, bis(diphenylphosphino)methane, to form the complexes cis-[R2PtL2] were studied. Each of the complexes 1 and 1′ has an MLCT band in the visible region which was used to easily follow kinetics of the related ligand substitution reactions by UV–Vis spectroscopy. Although the complex 1 or 1′ contains two cis Pt–C bonds, the involved substitution reaction followed a normal associative mechanism. The reaction rates were found to be dependent on concentration and nature of the entering group, while independent of the presence of excess free NN leaving group. Reaction of the complex 1 with P(O–iPr)3 proceeded some 20 times faster than similar reaction with the complex 1′. The ΔH‡/ΔS‡ compensation plot gives a straight line suggesting operation of the same mechanism involving all the entering nucleophiles.