Hydride 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. 

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. 

For complex hydrides, M1M2H (e.g., NaAlH4, as shown in Figure 12.11), it is known that strong covalent interactions are operating between H and M2 (e.g., M2 = Al) to form complex ions, (M2H ) (e.g., (A1H4) ) and that ionic... 

The nature of the chemical bond between ions is investigated in the perovskite-type hydrides, MMgH3 (M = Na, K, Rb), CaNiH3/ and SrPdH3 by the DV-Xx molecular orbital method. Also, the enthalpy changes in the dehydrogenation reactions are calculated using the pseudopotential method. It is found that the Mg-H bond is rather ionic, but the covalent interaction still remains to some extent. On the other hand, the M-H bond is further ionic. 

In summary, we can say that, because of the unique absence of angular and radial nodes in the H-atom valence shell, the hydride oah orbital is uniquely suited to strong n-a donor-acceptor interactions with Lewis bases. In turn, the unique energetic and angular features of nB-aAH interactions (or equivalently, of B H—A <—> B—H+ A covalent-ionic resonance) can be directly associated with the distinctive structural and spectroscopic properties of B - H—A hydrogen bonding. 

For any perovskite-type hydrides, M1M2H3/ (e.g., KMgH3, as shown in Figure 12.3), it is known that the Ml-H and M2-H interactions are rather ionic in character, even though covalent character still remains to some extent in the M2-H bond because of the shorter M2-H distance than the Ml-H distance. The covalent character further increases in case when M2 is a transition metal. As shown in Figure 12.4, the atomization energy diagram for perovskite-type... 

Hydrides can be categorized in various classes according to the nature of the bond that links the metal and hydrogen. The following categories can usually be found in the literature ionic hydrides (hydrides with group 1 and 2 elements), covalent hydrides, polymeric hydrides, metallic hydrides, borderline hydrides, and transitirai metal complex hydrides. Here the reaction of ionic hydrides with carbon dioxide (CO2) is considered. Transition metal hydrides with the formula L M-H are discussed in Chap. 4, where the interaction of CO2 with analogous E-X systems (where E and X are different elements) is discussed. 

The study of the interaction of carbon dioxide (CO2) with covalent-polar M-H bonds reveals quite a different complexity and intriguing reaction mechanisms with respect to the reaction with ionic metal hydrides, as discussed in Sect. 3.1. As a matter of fact, the interaction of M-H bonds with CO2 has been debated for quite a long time [1], and the insertion of the cumulene into M-H bonds has been shown to follow two routes, as depicted in Scheme 4.1, affording either the M-OCHO group or the M-COOH moiety, both having an applicative interest. 

---