Nucleophilicity of thiols towards planar tetracoordinated platinum(II) complexes

The nucleophilicity of the undissociated thiols is compared to that of thio ethers in the displacement of chloride ion from the Pt-II complexes [Pt(bip y)(NO2)Cl] (bipy = 2,2'-bipyridine) and [Pt(terpy)Cl](+) (terpy = 2,2'-6,2 "-terpyridine). As far as the neutral substrate is concerned the second ord er rate constants for RSH and RSR nucleophiles are equally related to the i nductive and steric effects of the groups linked to the sulphur donor atom. By contrast, only thiols are reactive (under standard experimental conditi ons) towards the cationic substrate, and the difference can be explained, i n this case, by the immediate deprotonation of the substituted product forc ing the reaction to completion. The reverse reaction has been examined in o ne case (PhSH) and the complete protonation of the coordinated thiolate spe cies requires [H+] > 2 mol.dm(-3) In both series of reactions, thiol molecu les also containing a second acidic group (-OH, -CO2H, -NH3+) capable of fa cile interaction via hydrogen bonding with the developing chloride in the t ransition state, exhibit a reactivity larger than that expected on the basi s of electronic and steric effects alone.