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 5s 2 (E) lone pair of divalent Sn (Sn II designated by M*) and the lone pair triplet around the luorine 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 IV F 4 then develop on Sn II F 2 E in its three crystal varieties (α, β, γ). The investigation then extends to study two mixed-valence luorides: Sn 2 II Sn IV F 6 E 2 and Sn II Sn IV F 6 E. The lone pair presence is readily detected in the crystalline network by its sphere of inluence characterized by a radius rE, and M*-E directions; all distances are also detailed and assessed. The observations point to signiicant modiications of the structure which are also analyzed with the electronic density of states DOS projected over the diferent atomic constituents. Within the selected luorides details of Sn II various coordination numbers (CN) generally indicate onesided coordination; speciically: CN = 4 + 1 SnF 4 E triangular bipyramid, CN = 5 + 1 SnF 5 E distorted octahedron (square pyramid with E roughly symmetric of its F apex) and CN = 6 octahedron [SnE]F 6 . In the latter, the rotation speed of E (which increases with Z number due to relativistic efects) and the size of the F polyhedron make it favorable enough to E rotating around Sn 2+ with the particularity of its transformation into a large cation [SnE] 2+ with a size comparable to Ca 2+ , Sr 2+ or Ba 2+ .

Localization and steric effect of the lone electron pair of the tellurium Te4+cation and other cations of thep-block elements. A systematic study

“…We now observe clear trends: the RDs decrease down a group and increase from left to right in a period. These results confirm the previous work (Galy et al, 1975) and generalize it to a large number of compounds. The decrease of the relative M*-LP distance down a group means that, for a given cation charge, the smaller the core radius, the larger the M*-LP distance.…”

“…Later, an ab initio study (Suehara et al, 2004b) performed on a TeO 4 H 4 molecule, with geometry close to the TeO 4 disphenoid found in -TeO 2 , also revealed an electronic density maximum at $0.6 Å from the core. The studies of Galy et al (1975) and Christy & Mills (2013) Local environments of Te 4+ cations found in TeO 2 -based crystal structures: (a) usual environments and (b) atypical environments. Te 4+ cores are shown in black and O 2À anions in red; blue arrows represent the Te 4+ LP and the longest Te-O bonds are drawn with dashed lines.…”

“…The first modeling method was introduced by Galy et al (1975). It was based on geometrical concepts considering that LP cations are located at the center of a regular polyhedron (tetrahedron, triangular bipyramid or octahedron) made of a lone pair and surrounding anions.…”

“…1, entails that LPs and anions are modeled by spheres of equal size. In this pioneering work, Galy et al (1975) systematically calculated the M*-LP distances found in 48 non-equivalent environments of 40 crystal structures containing 12 LP cation types and determined trends relating the average M*-LP distances to the positions of the elements in the periodic table. While powerful, this method assumes a regular polyhedron around the LP cation and works poorly with structures whose anionic packing is not compact enough, e.g.…”

“…ELF maps can be used to localize LPs by identifying high ELF values in the LP cation neighborhood. The ELF method has been applied, for instance, in the case of compounds containing Te 4+ (Hamani, 2010), Tl + to Po 4+ (Matar & Galy, 2015;Galy & Matar, 2016), N 3+ to Bi 3+ (Galy et al, 2017;Galy & Matar, 2018;Galy & Vignoles, 2020) and Sn 2+ cations (Galy & Matar, 2019). However, this procedure is highly demanding and generally requires lengthy computer calculations.…”