Solid-phase ligand substitution processes by outer-sphere anions in isomeric mixed-ligand hexamine chromium(III) complexes

The mixed-ligand complex compoundscis- andtrans-[Cr(en)₂(NH₂CH₃)₂]Br(B₁₀H₁₀) were synthetized. It was demonstrated by IR spectroscopy that the hydrogen atoms of the anions B₁₀H ₁₀ ^2? carrying the negative charge interact with the proton of the amino group in the coordinated amine, forming the bond system N-H-H-B. The complex salts obtained have high densities (d ²⁰=1.55 g/cm³ and 1.47 g/cm³ for thecis andtrans isomer, respectively) and high thermal stability 230–250 °C. At 250–270 °C both thecis and thetrans compound dissociate, with simultaneous substitution of two methylamine molecules in the coordination sphere of the chromium(III) ion by the anions Br^? and B₁₀H ₁₀ ^2? . The process is described by the topochemical equationf(α)=(1?α)^2/3 (reaction on the interface of the phases), and is characterized by high values of the kinetic parameters: E_a=510–524 kJ/mol, logA=49.9–50.2. We found that the value ofE _a for the amine substitution process in thecis compound, determined by evolved gas analysis under non-isothermal conditions, decreases by 220 kJ/mol when the heating rate is increased from 5.0 to 7.5 deg/min. This finding can be explained in that when the heating rate is increased, the intervals in which thecis- trans isomerization and ligand substitution reactions proceed come closer to one another, and finally overlap; the activation energy of the isomerization process then compensates part of the energy required for activation of the ligand substitution process.