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

In this review the preparation and structures of all known salts of the kno wn homopolyatomic cations of the chalcogens and halogens are reviewed. We s how that the structures of these cations, many of which are non-classical a nd cluster-like, arise from positive charge delocalisation, i.e. the reduct ion of Coulomb repulsion by diluting the unfavourable localised charges ove r all the atoms in the ion. The charge delocalisation leads to a combinatio n of intra- (and inter-) cationic pi*-pi*, np(pi)-np(pi), weak np(2)-np(2) (n greater than or equal to 3) and np(2)-->n sigma* interactions. The latte r are important especially for the polymeric tellurium homopolyatomic catio ns and account for most of their intriguing geometries. This thesis is base d 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)), a nd we apply these simple bonding models to qualitatively explain the geomet ries 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 s o minimising the electrostatic Coulomb repulsion. Thus the structures of th ese and related cations have been integrated into the main stream of inorga nic chemistry. In the second part of this article we provide an understandi ng of the thermodynamics governing the syntheses of most of the known chalc ogen and halogen cations. This is based on our new relationship between lat tice enthalpies and thermochemical volumes/radii (for both real and hypothe tical salts), on known experimental gas phase enthalpies of formation, as w ell as high level calculations (in contrast to earlier work, all of these c alculations 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 molecular geometry and not a salt like structure containing the square planar 6 pi aromatic S-4(2+) dication, and account f or 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 the rmodynamic properties is given as well as extensive tabular appendixes of t hermodynamic data of relevant cations and anions (standard enthalpies of fo rmation, fluoride ion affinities, lattice potential enthalpies etc.). (C) 2 000 Elsevier Science S.A. All rights reserved.