Theoretical investigation of thermodynamic stability and bonding features of possible isomers of the molecular and ionic complexes of pyridine with molecular iodine and iodine monochloride IX (X = I,Cl) is presented. M06-2X DFT functional is found to provide bond distances and dissociation energies which are close to those obtained at high-level ab initio CCSD(T)/aug-cc-pvtz//CCSD/aug-cc-pvtz benchmark computations for the most stable isomers, formed via donation of a lone pair of nitrogen atom of pyridine to the iodine atom. These isomers are by 23–33 kJ mol-1 (in case of I2) and by 39–56 kJ mol-1 (in case of ICl) more stable than other molecular complexes.
T-shaped π-σ* bonded isomers turn out to be energetically comparable with
van der Waals bound compounds. Among the ionic isomers, structures featuring
[IPy2]+ cation with I3- or ICl2- counterions are more stable. Oligomerization favors ionic isomers starting from the tetrameric clusters of the composition (IX)4Py4.