Thermodynamics of the decomposition processes of donor-acceptor complexes MX₃·en·MX₃ and MX₃·en

The structural and thermodynamic properties of the donor-acceptor (DA) complexes of Group 13 metal halides (MX₃) with ethylenediamine and their decomposition products have been studied theoretically at the B3LYP/LANL2DZ(d,p) level of theory. Gasphase dissociation into various components and HX elimination reactions are considered. Both processes are endothermic but favored by entropy. Complexes of 2:1 composition are predicted to be stable in the gas phase up to 640-1000 K. It is found that complexation with the second acceptor molecule lowers the HX elimination enthalpy; in turn, HX elimination increases DA bonding with a second MX₃ molecule. Exceptionally high values of the dissociation enthalpies (310-390 kJ mol ^-1) and HX elimination reactions (360-420 kJ mol^-1) of the amido compounds MX₂NHC₂H₄NH₂ and MX₂NHC₂H₄NHMX₂ make them important intermediates in the decomposition processes. Dissociation reactions of the complexes are more favorable than HX elimination reactions; however, the subsequent oligomerization and cyclization processes of coordinationally unsaturated amido and imido compounds may facilitate HX elimination. Since HI elimination reactions are predicted to be the least endothermic, and aluminum-containing compounds have the strongest M-N dissociation enthalpies, it is expected that compounds based on aluminum iodide are promising objects for experimental studies.