The effect of donor atom and ligand size on the molecular and solid state structure of selected tin (IV) compounds

The arrangement of bonded and nonbonded atoms surrounding the central tin atom in several methyltin halide and pseudohalide compounds has been calculated using a stereochemical model that takes into account the spatial requirements of atoms. Inherent in the analytical procedure is a parameter which provides a measure of the validity of the calculations when applied to a particular compound. The technique calculates an optimum ligand arrangement based upon a balancing of nonbonded intramolecular interactions; the procedure is then completed by calculating the distances of the Sn…X intermolecular interactions that occur when the approach of the associating atom is restricted only by the van der Waals contacts of the ligands on the central tin atom. In general, the calculations reproduce or account for the structural features of the methyltin compounds of known structure. The calculations show that the (CH₃)₃SnX, (CH₃)₂SnX₂, and CH₃SnX₃ molecules have ligand arrangements that may accommodate, 1, 2, or 3 strong intermolecular interactions, respectively. This agrees with the known tendency of (CH₃)₃SnX compounds to possess five-coordination and (CH₃)₂SnX₂ compounds to exhibit six-coordination. The calculated strengths of the intermolecular interactions vary in the order Sn…F > Sn…Cl > Sn…Br > Sn…I. Also, the strength of a particular type of Sn…X intermolecular interaction in a series of (CH₃)_nSnX_4?n compounds varies in the order (CH₃)₃SnX > (CH₃)₂SnX₂ > CH₃SnX₃. These calculations correlate with the observed variations in the discretely molecular or polymeric character of the solid states.