Joint stereochemical and ab initio overview of SnII electron lone pairs (E) and F−(E) triplets effects on the crystal networks, the bonding and the electronic structures in a family of tin fluorides

Abstract

The stereochemistry of 5s² (E) lone pair of divalent Sn (Sn^II designated by M*) and the lone pair triplet around the fluorine ions are examined complementarily with stereo-chemical approach and ab initio quantum investigations focusing on the electron localization and pertaining electronic structure properties, obtained within Density Functional Theory (DFT) and derived Electron Localization Function (ELF) mapping. The review completes a series of former ones focusing on the stereochemical role played by electron lone pairs LP. We start by examining LP-free Sn^IVF₄ then develop on Sn^IIF₂E in its three crystal varieties (α, β, γ). The investigation then extends to study two mixed-valence fluorides: Sn₂^IISn^IVF₆E₂ and Sn^IISn^IVF₆E. The lone pair presence is readily detected in the crystalline network by its sphere of influence characterized by a radius rE, and M*-E directions; all distances are also detailed and assessed. The observations point to significant modifications of the structure which are also analyzed with the electronic density of states DOS projected over the different atomic constituents. Within the selected fluorides details of Sn^II various coordination numbers (CN) generally indicate one-sided coordination; specifically: CN = 4 + 1 SnF₄E triangular bipyramid, CN = 5 + 1 SnF₅E distorted octahedron (square pyramid with E roughly symmetric of its F apex) and CN = 6 octahedron [SnE]F₆. In the latter, the rotation speed of E (which increases with Z number due to relativistic effects) and the size of the F polyhedron make it favorable enough to E rotating around Sn²⁺ with the particularity of its transformation into a large cation [SnE]²⁺ with a size comparable to Ca²⁺, Sr²⁺ or Ba²⁺.