Relative stabilities of halogeno-complexes. Part VI. Class (b) character of Pd(diethylenetriamine)2+ and Pd(1,1,7,7-tetraethyldiethylenetriamine)2+ and the stability of their thiocyanato- and isothiocyanato-complexes

Abstract

The relative stabilities of the complexes of Pd dien²⁺, and of Pd Et₄dien²⁺ with X^–, where dien = diethylenetriamine and Et₄dien = 1,1,7,7-tetra-ethyldiethylenetriamine, have been measured in aqueous solution, for X = Cl, Br, I, and SCN. In terms of free energies, the class (b) character of the Pd dien²⁺ ion is moderate and slightly greater than that of the Pd Et₄dien²⁺ ion. In terms of enthalpy changes, the class (b) character of Pd Et₄dien²⁺ is much greater. The difference is explained in terms of opposing steric and electronic effects, the latter being slightly stronger. A combination of kinetic and thermodynamic data suggests that the greater steric hindrance of the sulphur-bonded ligand in Pd Et₄dien SCN⁺ approximately balances the weaker Pd–NCS bonding involved in the isothiocyanato-form of the complex. Steric hindrance in the complex Pd Et₄dienSCN⁺ is estimated to be approximately 6 kcal./mole greater than in the complex Pd Et₄dienl⁺.

Kinetic parameters are reported for the reversible aquation reactions of Pd Et₄dienX⁺, where X = Cl, Br, and I, and approximate values for the stability constants at 24° are given.