ION-PAIR MECHANISM IN SQUARE-PLANAR SUBSTITUTION - REACTIVITY OF CATIONIC PLATINUM(II) COMPLEXES WITH NEGATIVELY CHARGED NUCLEOPHILES IN SOLVENTS OF HIGH, MEDIUM AND LOW POLARITY

The rates of displacement of dimethyl sulfoxide from the cation [Pt(ph en)(CH3)(Me(2)SO)](+) by a series of uncharged and negatively charged nucleophiles have been measured in a methanol/water (19:1 vol./vol.) m ixture. The starting complex and the reaction products were characteri zed either as solids or in solution by their IR and H-1 NMR spectra. T he substitution reactions take place by way of a direct bimolecular at tack of the ligand on the substrate. The sequence of reactivity observ ed is as expected on the basis of a nucleophilicity scale relevant for + 1 charged substrates ([Pt(en)(NH3)Cl](+) used as standard). The dif ference of reactivity between the first (t-BuNH(2)) and the last (SeCN -) members of the series spans five orders of magnitude. The value mea sured for the nucleophilic discrimination (1.55) is the highest found so far for cationic substrates. This is a result of the easy transfer of some of the electron density brought in by the incoming ligand into the ancillary ligands. When the reaction is carried out in a series o f protic and dipolar aprotic solvents, using chloride ion as nucleophi le, the rate of formation of [Pt(phen)(CH3)Cl] is dominated by the ext ent of solvation of Cl-, as measured by its values of the Gibbs molar energy of transfer Delta(t)G(0). Conductivity measurements at 25 degre es C in dichloromethane were fitted to the Fuoss equation and the valu es of the dissociation constants K-d for the ion pairs were calculated as follows: 2.27 x 10(-5) M for Bu(4)NCl, 2.75 X 10(-5) M for Bu(4)NS CN and 17.05 X 10(-5) M for [Pt(phen)(CH3)(Me(2)SO)]PF6. The pseudo-fi rst-order rate constants k(obs) for the reactions with Bu(4)NCl, Bu(4) NBr, Bu(4)NSCN and Bu(4)NI showed a curvilinear dependence on the conc entration of the salt which levels off very soon (at concentrations hi gher than 0.005 M the kinetics are zero order in [Bu(4)NX]). On additi on of the inert electrolyte Bu(4)NPF(6) the rates slow down and the ki netics follow the rate law k(obs) = kK(ip)[Bu(4)NX]/([Bu(4)NPF(6)] + K -ip[Bu(4)NX]). These findings fit well with a reaction scheme which in volves a pre-equilibrium K-ip between ion pairs, followed by unimolecu lar substitution within the contact ion pair [Pt(phen)(CH3)(Me(2)SO)X] (ip). Values of the equilibrium constants K-ip for ion-pair exchange a nd of the internal substitution rates k were derived. The latter showe d that the discrimination in reactivity between Cl-, Br-, SCN- and I- is greatly reduced with respect to aqueous solutions. The reason behin d this may be desolvation of the ions coupled to the fact that a conta ct ion pair is already at a certain distance along the reaction coordi nate in the direction of the transition state. Applications of the spe cial salt effect and of ion pairing to synthesis are discussed.