Hydrides localized states

Figure 5-39. Schematic representation of the band structures for various GdXH, com-positions. The bars represent ban

Due to localization of the electron density, Scheme 10, the oxidation state of the metal increases by two units upon passing from a non-classical to a classical coordination, which in principle offers the opportunity to use electrochemistry to discriminate between classical and non-classical hydrides.81... 

Figure 3.5. Continued. The H2-NAD reaction is inhibited neither in air nor in the presence of CO. C,The possible reactions of hydrogen with the Fe-Fe site of active [Fe]-hydrogenases. In the oxidized state, the bimetallic center shows a S = 1/2 EPR signal, presumably due to an Fe -Fe pair (an Fe -Fe pair cannot be excluded). Whether the unpaired spin is localized on iron (Pierik et al. 1998a) or elsewhere (Popescu and Mtlnck 1999) is not known. Hydrogen is presumably reacting at the vacant coordination site on Fe2 (Fig. 3.1C). After the heterolytic splitting, the two reducing equivalents from the hydride are rapidly taken up by the Fe-Fe site (one electron) and the attached proximal cluster (one electron). Subsequently, the electron is transferred from the proximal cluster to the other Fe-S clusters in the enzyme. Under equilibrium conditions, the proximal cluster in the active enzyme appears to be always in the oxidized [4Fe-4S] state (Popescu and Mtlnck 1999). Protons are not shown.

As a general rule, it can be stated that all elements with electronegativity in the range 1.35-1.82 do not form stable hydrides [34]. Exemptions are vanadium (1.45) and chromium (1.56), which form hydrides, andmolybdenum (1.30) and technetium (1.36), where hydride formation would be expected. The adsorption enthalpy can be estimated from the local environment of the hydrogen atom on the interstitial site. 

Actually, the energies of vibrational states of molecules can be described by both models, where the NM model expresses well low vibrational states and the LM model modes that are localized in a particular group of atoms. In most cases, overtones of hydride stretching vibrations, that is, X—(X = C, N or O) are better described by the LM model due to their relatively small couplings to other bonds. 

Fig. 10.17. Transition states of the selectivity-determining step of a stereogenic addition of a hydride donor to an a-chiral carbonyl compound (the energy profile would be allowed to contain additional local energy maxima provided that they do not have a higher energy than the two highest maxima shown in the figure).

Hydride is transferred intramolecularly in (67 equation 29) in a degenerate process. Clearly the six-membered transition state characteristic of the MPV reaction cannot be achieved here because the orientation of hydroxy and carbonyl is incompatible with simultaneous coordination to a metal ion. The rates depend on the cation if the alkoxide is a solvent-separated ion pair the hydride-donating potential in the intramolecular reaction is increased, consistent with the greater charge localized on alkoxide oxygen. 

On the other hand, there are good reasons for believing that this is not always the case and with, for example, antimony and bismuth, the heavier members of the group, there is evidence for the presence of an inert pair of s electrons. Because the angles between the substituents of a neutral antimony(III) or bismuth(III) compound are close to 90°, it is possible to consider that, rather than using sp hydrid orbitals, the substituents are attached via pure p orbitals, with the lone pair of electrons remaining localized in the appropriate s orbital. The concept of an inert pair of electrons has some validity and it allows rationalization of much of the chemistry of these elements in the + 3 oxidation state. It is, however, difficult to provide direct experimental evidence for the effect, but it is difficult otherwise to rationalize the almost ideal octahedral structure of [SbClg]. ... 

Analysis of the electron localization function for geometries of the same reaction path provides an alternative, but compatible, perspective on the evolution of electronic structure. The novel asynaptic basin of the DRA, which represents a pair of electrons that is delocalized over the periphery of the ammonium cation core, is transformed into a conventional, monosynaptic basin that is associated with the departing hydrogen at the geometries near the transition state. After the transition state, the NH system may be described as a hydride-ammonia complex. 

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