Quantum chemical computations at M06-2X/def2-TZVPD level of theory were employed to compute structural and thermodynamic properties of gaseous donor-acceptor complexes IX·LB [X = I, Cl; LB = trimethylamine, triethylamine, pyridine, 2-aminopyridine, 4,4′-bipyridine, pyrazine, 1,2-bis(4-pyridyl)ethylene,
1,2-bis(4-pyridyl)ethane]. Iodine monochloride is a stronger Lewis acid compared to the molecular iodine. With respect to homogeneous gas phase dissociation into components, the complex of ICl with triethylamine is the most stable and the complex of I2 with pyrazine is the least stable. It is shown that dissociation enthalpies do not correlate with proton affinity of the Lewis base, but there is a good correlation between dissociation enthalpies and I–X bond lengthening upon complex formation. This allows to estimate the stability of the complex based on
its structural data.