Alkali ionic-covalent interactions

The recently available spectroscopic data and the RKR potentials of the alkali hydrides allow us to determine the "experimental" values of the parameters relevant to the transition probability of the charge transfer processes. In the Landau-Zener model these parameters are the energy gap between the and X S adiabatic potentials at the avoided crossing distance and the coupling matrix elements. In this paper the coupling matrix elements are evaluated in a two-state ionic-covalent interaction model. The systema-tic trends found in the alkali hydride series for their X e potentials are presented. This leads to a simple model for the ionic potentials. 

The qualitative features are explained nicely by the theory of non-adiabatic transitions due to Berry (3). Unfortunately the spectra cannot be resolved and identified to yield quantitative data relevant to the ionic-covalent interaction. For alkali halides these interactions are more amenable to atomic beam scattering experiments (4). In contrast the optical spectra of... 

In this study we used the scaled theoretical potential curves of Olson and Liu (21), Stevens, Karo and Hiskes ( ), and Laskowski and Stall cop (2T) to make a short extrapolation of the X E" RKR potentials into the avoided crossing region. Experimental measurements to obtain strictly experimental RKR potentials for this region are in progress. New optical measurements on the dipole moments (, ), the transition moments (38> nd radiative lifetimes (, ) of the alkali hydrides are also becoming available. This type of information will soon provide additional details about the ionic-covalent interactions in these molecules. 

A slight but systematic decrease in the wave number of the complexes bond vibrations, observed when moving from sodium to cesium, corresponds to the increase in the covalency of the inner-sphere bonds. Taking into account that the ionic radii of rubidium and cesium are greater than that of fluorine, it can be assumed that the covalent bond share results not only from the polarization of the complex ion but from that of the outer-sphere cation as well. This mechanism could explain the main differences between fluoride ions and oxides. For instance, melts of alkali metal nitrates display a similar influence of the alkali metal on the vibration frequency, but covalent interactions are affected mostly by the polarization of nitrate ions in the field of the outer-sphere alkali metal cations [359]. 

We also assume that the dodecahedral ion A behaves as a perfectly ionic species in the crystal structure, whose only role is that of charge compensation, i.e. that there is no A-0 covalent interaction. For alkali or alkali-earth cations the above assumption is justified. 

The traditional classification of chemical bonds into ionic, covalent, donor-acceptor, metallic and van der Waals corresponds to extreme types, but a real bond is always a combination of some, or even all of these types (Fig. 2.1). Purely covalent bonding can be found only in elemental substances or in homonuclear bonds in symmetric molecules, which comprise a tiny fraction of the substances known. Purely ionic bonds do not exist at all (although alkali metal halides come close) because some degree of covalence is always present. Nevertheless, to understand real chemical bonds it is necessary to begin with the ideal types. In this Section we will consider mainly the experimental characteristics of different chemical bonds and only briefly the theoretical aspects of interatomic interactions. 

The packing density depends on the ion radius and on the character of interatomic interaction (with ionic-covalent character) which is reflected in the different character of the relation between the packing density and cation radius. These relations are obviously different in the case of MO and M2O3 oxides of A- and / electron metals and MO2 oxides of/-electron metals compared to alkali metal oxides and metal oxides with framework structure with dominating covalent interaction. 

Based on the concept of mixed-framework lattices, we have reported a novel class of hybrid solids that were discovered via salt-inclusion synthesis [4—7]. These new compounds exhibit composite frameworks of covalent and ionic lattices made of transition-metal oxides and alkali and alkaline-earth metal halides, respectively [4]. It has been demonstrated that the covalent frameworks can be tailored by changing the size and concentration of the incorporated salt. The interaction at the interface of these two chemically dissimilar lattices varies depending upon the relative strength of covalent vs. ionic interaction of the corresponding components. In some cases, the weak interaction facilitates an easy... 

Interpretating Alkali Iodide Data. The alkali iodide data given above show that the idealized model of ionic crystals is inadequate since 8 9 constant and hp 9 0. To interpret the data one must consider the effects on the iodine 5p population by covalency, deformation of the charge cloud by electrostatic interaction, and deformation by overlap. 

The simple theory of electronegativity fails in this discussion because it is based merely on electron transfer energies and determines only the approximate number of electrons transferred, and it does not consider the interactions which take place after transfer. Several calculations in the alkali halides of the cohesive energy (24), the elastic constants (24), the equilibrium spacing (24), and the NMR chemical shift 17, 18, 22) and its pressure dependence (15) have assumed complete ionicity. Because these calculations based on complete ionicity agree remarkably well with the experimental data, we are not surprised that the electronegativity concept of covalency fails completely for the alkali iodide isomer shifts. 

In general, species containing transition metals and metalloids such as As, Sb, Se and Sn are thermodynamically more stable than those of the alkali and alkaline earth metals. Transition metals and metalloids form an integral part and are linked to the organic constituents by covalent bonds. In contrast alkali and alkaline earth metals are attached loosely by predominantly ionic bonds. Readers interested in the fundamentals of metal-protein interactions are referred to books... 

Whereas ionization and dissociation are clearly defined processes on the left side of Scheme 35, the situation is more complicated for carbanionic systems (Scheme 35, right). Organic alkali metal compounds, for example, which often exist as aggregates, are often described as covalent species with a certain percentage of ionic character [140-142]. If the formal carbanion is a resonance-stabilized species (e.g, diphenylmethyl lithium or sodium), the species with the closest interaction between the organic fragment and the metal is usually called a contact ion pair. In... 

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