Thermal cis—trans isomerization of bis(diamine) cobalt(III) and chromium(III) complexes in a solid phase

Thermal cis—trans isomerization of the simple bis(diamine) complexes MX₂(aa or bb)₂]X · HX · n H₂O and the mixed bis(diamine) complexes MX₂(aa)(bb)]X · HX · n H₂O was investigated in a solid phase, where M = Co(III) or Cr(III) ion, X = Cl or Br, aa and bb are one of the diamines selected from ethylenediamine (en), d, l-1,2-propane-diamine (pn), d,l-2,3-butanediamine (dl-bn), d,l-1,2-cyclohexanediamine (chxn), 1,3-propanediamine (ln) and d,l-2,4-pentanediamine (ptn), and n = 0–2. The information obtained may be summarized as follows. (1) The features of isomerization are considerably dependent upon the kind of metal ions, halide ions and diamines contained in the complexes. (2) Trans-cis isomerization was identified in the simple bis(diamine) complexes containing en, pn, dl-bn or chxn which can form five-membered chelate rings with metal ions, whereas cis-trans isomerization was detected in the simple bis(diamine) complexes containing tn or ptn which forms six-membered rings; all the mixed bis(diamine) complexes isomerize from trans to cis even when they have a combination of five- and six-membered chelate rings. (3) The cobalt(III) complexes isomerize in a temperature range of dehydration and/or dehydrohalogenation with activation energies of about 100 kJ mole^?1, whereas the chromium(III) complexes usually isomerize in the anhydrous state and the activation energies amount to as much as 150–190 kJ mole^?1. (4) “Aquation-anation” and “bond rupture” were proposed for the isomerization of the cobalt(III) and the chromium(III) complexes, respectively.