Structures and stability of I₂ and ICl complexes with pyridine: Ab initio and DFT study

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.