Hydrides atomic configuration

Figure 2.61. Two configurations of Mg-2H periodic structure used in the potential energy calculations of Figs. 2.62-2.63. The situation on top corresponds to the MgHj hydride equilibrium configuration (same as the one shown in Fig. 2.50), while the H atoms in the lower figure have been moved 0.5 ran out of the Mg lattice, to the left along the negative x-axis perpendicular to one of the lattice surface planes (Sorensen, 2004f).

The main dijfference between a linear H3 system and beryllium hydride, BeH2, which is a linear molecule, is that the central atom now contributes two valence electrons and four atomic orbitals the 2s orbital, which is doubly occupied in an isolated ground state Be atom (configuration Is, 2s ), and the three 2p orbitals which are empty in Be but lie not too far above 2s in energy. Graphically, we have ... 

The tributyitin hydride reduction of dihaloaziridines, e.g. (266), represents another example where the ring system has been maintained (79CJC1958). Especially noteworthy is the retained configuration associated with the reaction. This behavior differs from the cyciopropyl analog and was explained on the basis of increased s-character in the exocyciic bond caused by the nitrogen atom. 

Contents Formal Oxidation Numbers. Configurations in Atomic Spectroscopy. Characteristics of Transition Group Ions. Internal Transitions in Partly Filled Shells. Inter-Shell Transitions. Electron Transfer Spectra and Collectively Oxidized Ligands. Oxidation States in Metals and Black Semi-Conductors. Closed-Shell Systems, Hydrides and Back-Bonding. Homopolar Bonds and Catenation. Quanticule Oxidation States. Taxological Quantum Chemistry. 

Evidence for a hydride transfer mechanism (Scheme 29) for the PQQ-dependent enzyme methanol dehydrogenase (MDH) was obtained by a theoretical analysis combined with an improved refinement of a 1.9 A resolution crystal structure of MDH from Methylophilus methylotrophus in the presence of CH3OH <2001PNA432>. The alternative mechanism proceeding via a hemiketal intermediate was discounted when the observed tetrahedral configuration of the C-5 atom of PQQ in that crystal structure was shown to be the C-5-reduced form of the cofactor 198, a precursor to the more common reduced form of PQQ 199. 

Hexagonal AB5 compounds form orthorhombic hydrides. Basal plane expansion, caused by hydrogen occupation of interstitial sites (4), change the compound s structure and can be ordered or disordered. For example, Kuijpers and Loopstra (4) found, by neutron diffraction, that the deuterium atoms in PrCo5D4 were ordered on certain interstitial octahedral and tetrahedral sites. The sites occupied by hydrogen in various AB5 hydrides are not fully understood because of insufficient neutron diffraction data. The available information is considered in the section on configurational entropies. 

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