The xenon-chlorine conundrum: van der Waals complex or linear molecule?

We present an approximate molecular orbital analysis of the potential energy surface for XeCl2, supported by qualitative perturbation theoretic analysis of the bonding in these species. Three minima are located on the XeCl2 surface: the observed Xe⋯Cl2 van der Walls complex, the linear Cl-Xe-Cl molecule, and between them an uexpected strongly bent ClXeXl intermediate. The latter is characterized by a qualitatively different electronic configuration and is likely to have barriers between it and the two other potential minima. Possible experimental methods to observe both the intermediate geometry and the linear molecule are discussed. The reaction pathway for the conversion of the van der Waals isomer of XeCl2 to the more stable linear isoner passes through the newly predicted bent isomer and thus proceeds through two curve-crossing regions. This explains why the van der Waals isomer can be observed in the gas phase.