Anionopentaaminecobalt(III) complexes with polyamine ligands. XIX. Computer simulated intermediates for dissociative aquation reactions

Abstract

Strain energy minimization calculations have been performed for several octahedral chloropenta-amine cobalt(III) complexes with polyamine ligands. Similar calculations on the five-coordinate residue obtained by removal of the chloro ligand allow an estimation of the geometry and energy of a potential intermediate in a chloride release reaction proceeding via a dissociative mechanism. In all cases the five-coordinate residue is less strained than the six-coordinate octahedron and the non-replaced ligands play an essential role in determining the resulting distortion. Thus for the series mer-CoCl(NN)(dien)²⁺ (NN = en, (NH₃)₂,tn), the total strain energy difference between the minimized ground state and the minimized five-coordinate residue is 12.2, 18.3 and 25.8 kJ mol^-1, respectively. This order is identical (where data are available) to that of the rate constants (and activation energies) for thermal aquation, Hg²⁺ assisted aquation and mer → fac-dien isomerization in this series. Similar calculations have been performed for a series of trans-CoCl₂(N₄)⁺ systems and again the energy differences are in the approximate order of the rates of thermal aquation. In the case of N₄ = (NH₃)₂, the energy difference between trans-and cis-CoCl(NH₃)₄(OH₂)²⁺ products is about 1 kJ mol^-1, the trans-isomer being the more stable.