REACTION BETWEEN 1,10-PHENANTHROLINE AND PLATINUM(2) COMPOUNDS .I. REACTION IN AQUEOUS SOLUTION

1,10-Phenanthroline reacts slowly and quantitatively with K2PtCl4 to give Pt(phen)Cl2. Like mostsubstitution reactions of platinum(II), this one proceeds by a direct path and also by a path that involves intermediate incorporation of a molecule of solvent. The respectiverate constants at 45° in water are 3.74 × 10-3 M-1 sec-1 (ΔH≠ = 11.8 kcal/mol, ΔS≠ = -24 eu/mol) and 11.5 × 10-3 M-1 sec-1 (ΔH≠= 17.3 kcal/mol, ΔS≠ = -16 eu/mol) atan ionic strength of 10.3 × 10-2 Both paths are first order in phenanthroline and first order in platinum(II) concentrations. The reaction rate is sensitive toionic strength and to the basicity of the phenanthroline but insensitive to pH in the range of 2.7-4.9. The slowness of the reaction and theunusual second-order solvent path appear to be the result of the inflexibility of the phenanthroline. The ratio of the rate of its reaction with K2PtCl4 to the rates of the more flexible ligands bipyridyl and ethylenediamine is 1:3:100, which is not explicable in terms of changes in nucleophilicity. The reactions of phenanthroline with cis- and trans-Pt(NH3)2Cl2 are much slower than with Pt(C2H4, and its reaction with Pt(C2H4)Cl3 isvery much faster. The results are discussed in terms of a mechanism that is strongly dependent on dissociation of the platinum(II) complex. © 1969, American Chemical Society. All rights reserved.