Recent advances in the understanding of the syntheses, structures, bonding and energetics of the homopolyatomic cations of Groups 16 and 17

Abstract

In this review the preparation and structures of all known salts of the known homopolyatomic cations of the chalcogens and halogens are reviewed. We show that the structures of these cations, many of which are non-classical and cluster-like, arise from positive charge delocalisation, i.e. the reduction of Coulomb repulsion by diluting the unfavourable localised charges over all the atoms in the ion. The charge delocalisation leads to a combination of intra- (and inter-) cationic π*–π*, np π– np π, weak np 2– np 2 ( n≥3) and np 2→ nσ* interactions. The latter are important especially for the polymeric tellurium homopolyatomic cations and account for most of their intriguing geometries. This thesis is based on the results of quantitative theoretical studies on the simpler cations (I 4 2+, I 3 +, I 5 +, M 4 2+, M 8 2+ and M 4 2+ (M=S, Se, Te)), and we apply these simple bonding models to qualitatively explain the geometries of all the remaining cations. The geometries of the more cluster like cations can also be rationalised by the Wade–Mingos rules, consistent with the positively charged atoms approximately adopting positions on a sphere so minimising the electrostatic Coulomb repulsion. Thus the structures of these and related cations have been integrated into the main stream of inorganic chemistry. In the second part of this article we provide an understanding of the thermodynamics governing the syntheses of most of the known chalcogen and halogen cations. This is based on our new relationship between lattice enthalpies and thermochemical volumes/radii (for both real and hypothetical salts), on known experimental gas phase enthalpies of formation, as well as high level calculations (in contrast to earlier work, all of these calculations now reproduce the experimental geometries, vibrational spectra and energetics of the cations in question, e.g. M 8 2+, M 4 2+, I 4 2+). We now can quantitatively understand why S 4 2+(AsF 6 −) 2 is formed and not 2S 2 +(AsF 6 −); why S 4Cl 2 adopts a chain like molecular geometry and not a salt like structure containing the square planar 6π aromatic S 4 2+ dication, and account for all the features in the structure of S 8 2+. We lay the foundation for establishing whether or not as yet unknown homopolyatomic cation salts can be prepared in the solid state. A short overview of methods to estimate thermodynamic properties is given as well as extensive tabular appendixes of thermodynamic data of relevant cations and anions (standard enthalpies of formation, fluoride ion affinities, lattice potential enthalpies etc.).