N → Sn coordination in the complexes of tin halides with pyridine: A comparison between Sn(II) and Sn(IV)

The experimentally well-known complexation of tin(II) and tin(IV) halides with pyridine (py) leads to structures showing N -> Sn coordination. In the present work, the complexes SnX(n.)mpy (where X = F, Cl, Br, I; n = 2, 4; m = 1, 2) possessing this kind of coordination were studied using a computational quantum chemical approach. Various aspects in the theoretical picture of these complexes were examined to find similarities and differences in their N -> Sn coordination. The aspects included, among others, the physical nature of intermolecular interactions, and their role in establishing the structure and energetic stabilization of the complexes. In this context, the effect of tin valency was inspected in great detail. As proven by several theoretical methods, a largely ionic character with a certain covalent component can be attributed to the studied N -> Sn coordination, irrespective of tin valency. All complexes are destabilized by py-py and three-body interactions, but the Sn(II) complexes experience it to a greater extent. Marked differences are observed in the structural behavior of N -> Sn and SnXn during complex formation. This affects the energetics of complexation and, in consequence, the penta-coordinated Sn(IV) center shows a higher propensity to expand its coordination number, compared with the tri-coordinated Sn(II) center. The present study supplements the experimental characterization of SnXn.mpy and, in general, it sheds light on the coordination of heteroaromatic nitrogen to tin. The survey of the Cambridge Structural Database revealed that such coordination occurred in a number of crystal structures.