Hydrides anions

A third method for generating ions in mass spectrometers that has been used extensively in physical chemistry is chemical ionization (Cl) [2]. Chemical ionization can involve the transfer of an electron (charge transfer), proton (or otlier positively charged ion) or hydride anion (or other anion). 

Hydrides are compounds that contain hydrogen (qv) in a reduced or electron-rich state. Hydrides may be either simple binary compounds or complex ones. In the former, the negative hydrogen is bonded ionicaHy or covalendy to a metal, or is present as a soHd solution in the metal lattice. In the latter, which comprise a large group of chemical compounds, complex hydridic anions such as BH, A1H, and derivatives of these, exist. 

Nicotinamide is an essential part of two important coenzymes nicotinamide adenine dinucleotide (NAD ) and nicotinamide adenine dinucleotide phosphate (NADP ) (Figure 18.19). The reduced forms of these coenzymes are NADH and NADPH. The nieotinamide eoenzymes (also known as pyridine nucleotides) are electron carriers. They play vital roles in a variety of enzyme-catalyzed oxidation-reduction reactions. (NAD is an electron acceptor in oxidative (catabolic) pathways and NADPH is an electron donor in reductive (biosynthetic) pathways.) These reactions involve direct transfer of hydride anion either to NAD(P) or from NAD(P)H. The enzymes that facilitate such... 

The hypervalent silicon hydride anion, SiHj (cf. SiFs below), has been synthesized as a reactive species in a low-pressure flow reactor ... 

Compare atomic charges for sodium borohydride and lithium aluminum hydride. Which ion contains the most electron-rich hydride The least electron-rich hydride Based on these results alone, which hydride reagent should be the better reducing agent Explain. Obtain atomic charges for free borohydride and aluminum hydride anions. What changes, if any, does the counterion produce ... 

Complexes 79 show several types of chemical reactions (87CCR229). Nucleophilic addition may proceed at the C2 and S atoms. In excess potassium cyanide, 79 (R = R = R" = R = H) forms mainly the allyl sulfide complex 82 (R = H, Nu = CN) (84JA2901). The reaction of sodium methylate, phenyl-, and 2-thienyllithium with 79 (R = R = r" = R = H) follows the same route. The fragment consisting of three coplanar carbon atoms is described as the allyl system over which the Tr-electron density is delocalized. The sulfur atom may participate in delocalization to some extent. Complex 82 (R = H, Nu = CN) may be proto-nated by hydrochloric acid to yield the product where the 2-cyanothiophene has been converted into 2,3-dihydro-2-cyanothiophene. The initial thiophene complex 79 (R = R = r" = R = H) reacts reversibly with tri-n-butylphosphine followed by the formation of 82 [R = H, Nu = P(n-Bu)3]. Less basic phosphines, such as methyldiphenylphosphine, add with much greater difficulty. The reaction of 79 (r2 = r3 = r4 = r5 = h) with the hydride anion [BH4, HFe(CO)4, HW(CO)J] followed by the formation of 82 (R = Nu, H) has also been studied in detail. When the hydride anion originates from HFe(CO)4, the process is complicated by the formation of side products 83 and 84. The 2-methylthiophene complex 79... 

R = Me, = H) adds the hydride anion in a similar fashion, whereas... 

The antibonding LU MO of lithium hydride localizes more on the lithium atom than on the hydrogen atom, so that hydride anion will attack the lithium to form a linear anion. 

Alcohol dehydrogenases are a class of zinc enzymes, which catalyse the oxidation of primary and secondary alcohols to the corresponding aldehyde or ketone by the transfer of a hydride anion to NAD+ with release of a proton ... 

An Alternative Mechanism. Considering the facility of the electron transfer reactions to which a great deal of this symposium has been devoted, we have to worry whether our "proton transfer" reactions may not really be the result of electron transfer in the reverse direction followed by hydrogen transfer. As Bergman (26) has recently reported that another hydride anion may act as a one-electron reducing agent, and as we have evidence implicating 0s(C0) H as an intermediate in a number of... 

The reaction of a metal-dimer with H2 can also be regarded as an oxidative addition reaction. For instance, a dimer of a d7 metal complex reacts with dihydrogen to give two d6 species. In this process dihydrogen also gives formally two hydride anions. A well-known example in the present context is the conversion of dicobaltoctacarbonyl into hydridocobalttetracarbonyl ... 

The determination of electron affinities (EAs) is one of the most serious problems in quantum chemistry. While the Hartree-Fock electron affinity can be easily evaluated, most anions turn out to be unbound at this level of theory. Thus, the correlation effects are extremely crucial in evaluating EAs. At this point, lithium hydride and lithium hydride anion make up a very good benchmark system because they are still small enough yet exhibit features of more complicated systems. Four and five electrons, respectively, give rise to higher-order correlation effects that are not possible in H2. 

Lippard, S.J., Ucko, D.A. 1968. Transition metal borohydride complexes. II. Th reaction of copper(I) compounds with boron hydride anions. Inorg Chem 7 1051-1058. 

In the 1950 s and 1960 s a number of new boron hydrides and boron hydride anions were prepared. Although particular emphasis was placed upon stable higher borane entitles derived from BioHm, significant progress was made in the preparation of new, smaller boron hydrides. Unfortunately, at this period of time, most of these materials could be obtained only in small amounts. (A comprehensive treatment of all boron hydrides prepared through 1978 is given in references (51) and (52). 

The procedures are simple and give essentially stoichiometric reactions which can be carried out in the absence of a solvent. The systematic nature of these syntheses relates to the observation that hydride ion can be abstracted from certain boron hydride anions to give as one of the final products a neutral boron hydride which contains one more boron atom than the anionic starting material. These reactions are described below. 

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