Examination of the effect of crystal packing forces on geometric parameters: a combined crystallographic and theoretical study of 2,2'-bipyridyl adducts of R₂SnCl₂

Abstract

Crystal structure determinations on a series of R₂SnCl₂ (bipy) [bipy is 2,2'-bipyridyl] compounds show that these adopt a common structural motif in the solid-state with the Sn-bound organic substituents occupying mutually trans positions in a distorted octahedral geometry. A comparison of the experimental Sn-Cl and Sn-N bond distances with the moderated (i.e. by changing the nature of R) Lewis acidities of the tin atoms shows a general correlation, i.e. longer distances with decreasing Lewis acidity, but with a relatively large spread of values. In order to examine the role of crystal packing effects on the derived geometric parameters, gas-phase structures for these compounds have been modeled employing ab initio molecular orbital theory. These calculations showed that within individual compounds the two Sn-Cl distances are equal as are the two Sn-N distances, in contrast to the solid-state results. Further, a better correlation between the Sn-Cl and Sn-N bond distances and the Lewis acidity of the tin atom was found. These key results are attributed to the influence of crystal packing forces on the geometric parameters about the tin center in the solid-state.