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

A review, with over 209 refs., is given on the prepn. and structures of all known salts of the known homopolyat. cations of the chalcogens and halogens. The structures of these cations, many of which are nonclassical and cluster-like, arise from pos. charge delocalization, i.e. the redn. of Coulomb repulsion by dilg. the unfavorable localized charges over all the atoms in the ion. The charge delocalization leads to a combination of intra- (and inter-) cationic p*-p*, npp-npp, weak np2-np2 (n >= 3) and np2 ns* interactions. The latter are important esp. for the polymeric tellurium homopolyat. cations and account for most of their intriguing geometries. This thesis is based on the results of quant. theor. studies on the simpler cations (I42+, I3+, I5+, M42+, M82+ and M42+ (M = S, Se, Te)), and the authors apply these simple bonding models to qual. explain the geometries of all the remaining cations. The geometries of the more cluster like cations can also be rationalized by the Wade-Mingos rules, consistent with the pos. charged atoms approx. adopting positions on a sphere so minimizing the electrostatic Coulomb repulsion. Thus the structures of these and related cations were integrated into the main stream of inorg. chem. In the 2nd part of this article the authors provide an understanding of the thermodn. governing the syntheses of most of the known chalcogen and halogen cations. This is based on the authors' new relation between lattice enthalpies and thermochem. vols./radii (for both real and hypothetical salts), on known exptl. gas phase enthalpies of formation, as well as high level calcns. (in contrast to earlier work, all of these calcns. now reproduce the exptl. geometries, vibrational spectra and energetics of the cations in question, e.g. M82+, M42+, I42+). The authors now can quant. understand why S42+(AsF6-)2 is formed and not 2S2+(AsF6-); why S4Cl2 adopts a chain like mol. geometry and not a salt like structure contg. the square planar 6p arom. S42+ dication, and account for all the features in the structure of S82+. The authors lay the foundation for establishing whether or not as yet unknown homopolyat. cation salts can be prepd. in the solid state. A short overview of methods to est. thermodn. properties is given as well as extensive tabular appendixes of thermodn. data of relevant cations and anions (std. enthalpies of formation, fluoride ion affinities, lattice potential enthalpies etc.). [on SciFinder (R)]