Chemische Wirkungen des 80 mBr-Kernisomeren-Überganges in kristallinen Co-, Rh- und Ir-Pentamminbromokomplexsalzen*

Abstract

The percentage of separable 80gBr--ions, following the 80mBr→80gBr isomeric transition, was determined in labelled pentamminebromo complexes of Co, Rh and Ir at various temperatures. In [Me(NH3)5Br] (NO3)2 crystals the 80gBr--yields are constant (90, 80 and 77% resp.) over a certain range of temperature, but above 50°, 75°, and 110°C for the Co, Rh, and Ir-complex, respectively, the 80gBr--yields decrease lineary with temperature. The annealing of the metastable fragments is rapid and reaches characteristic equilibrium values. The kinetics of this fast reaction was studied and an activation energy of 6,7 kcal/mol was calculated for [Rh (NH3) 5Br] (NO3)2. In contrast to the bromo nitrates, the 80gBr--yields increase with temperature in the Rh- and Irpentamminebromo-halogenid complexes and reach limiting values of ∼66% at 210—250°C. In this case the excitation energy of the isomeric transition seems to effect activated exchange reactions between complex bound bromide and anionic bromide or chloride. In the Co-, Rh- and Ir-pentamminebromocomplexes investivated, the 80gBr--yield is strongly influenced by the chemical nature of the anion and/or the physical properties of the crystals. 12 different [Ir (NH3) 5Br] X2 complexes yield values of 80gBr— between 77% (X= ΝΟ3 —) and 41% (X2 = C2O4 — —) at room temperature. Anions with oxidizing properties show higher values than those with reducing properties. The latter may favour intermolecular charge neutralisation. In an inversion of the above experiments, we found that in anionic 80mBr-labelled Rh-hexamminebromide and also in different Rh- and Ir-pentammine acido bromides, 9 — 18% of the 80gBr daughter atoms were complex bound. These yields are independent of temperature, indicating that substitution reactions of ligand groups occur only by “hot atom” processes. The results are compared with those of the (n,γ) nuclear reaction.