Anionic hydride complexes

The list of ligands covered by Ref. 1 (vol. 2) starts with the exotic mercury ligand (Dean, p. 1). Silicon and a series of metals (Ge, Ti, Pb) (Harrison, p. 15) also reveal ligand properties. Hydrogen and a variety of hydride anion complexes (Crabtree, p. 689), as well as the complexes formed by anions with a carbon-donor center (cyanides, fulminates, etc.) (Sharpe, p. 25) are briefly discussed. 

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. 

During the course of these mechanistic studies a wide range of possible applications of this reaction have been revealed. When the reduction is carried out with lithium aluminum deuteride and the anion complex decomposed with water, a monodeuterio compound (95) is obtained in which 70% of the deuterium is in the 3a-position. Reduction with lithium aluminum hydride followed by hydrolysis with deuterium oxide yields mainly (70 %) the 3j5-di-epimer (96), while for the preparation of dideuterio compounds (94) both steps have to be carried out with deuterated reagents. ... 

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... 

The formation of these compounds has been rationalized according to Scheme 6. The reaction of Os (E )-CH=C 11 Ph C1 (C())( P Pr3)2 with n-BuLi involves replacement of the chloride anion by a butyl group to afford the intermediate Os (/i> CH=CHPh ( -Bu)(CO)(P Pr3)2, which by subsequent hydrogen (3 elimination gives OsH ( >CI I=CHPh (CO)( P Pr3)2. The intramolecular reductive elimination of styrene from this compound followed by the C—H activation of the o-aryl proton leads to the hydride-aryl species via the styrene-osmium(O) intermediate Os r 2-CH2=CHPh (CO)(P Pr3)2. In spite of the fact that the hydride-aryl complex is the only species detected in solution, the formation of OsH ( )-CH=CHPh L(CO)(P Pr3)2 and 0s ( )-CH=CHPh (K2-02CH)(C0)(P,Pr3)2 suggests that in solution the hydride-aryl complex is in equilibrium with undetectable concentrations of OsH ( )-CH=CHPh (CO)(P,Pr3)2. This implies that the olehn-osmium(O) intermediate is easily accessible and can give rise to activation reactions at both the olefinic and the ortho phenyl C—H bonds of the... 

The reaction of [Ni(ethene)3] with a hydride donor such as trialkyl(hydrido)-aluminate results in the formation of the dinuclear anionic complex [ Ni(eth-ene)2[2l 11 [22]. The nickel(O) centers in this complex are in a trigonal planar environment of two ethene molecules and a bridging hydride ion, with the ethene carbons in the plane of coordination. The two planes of coordination within the dinuclear complex are almost perpendicular to each other, and the Ni-H-Ni unit is significantly bent, with an angle of 125° and a Ni-Ni distance of 2.6 A [22],... 

The tetraorganostannanes, ITSn, react with butyllithium to form, in solution, the anions R4SnBu Li+. On the other hand, butyllithium reacts with tin hydrides to form the anions Bu SnH3 -Li+, and there are as yet no reports of species such as Bu3SnH2-Li+, though five-coordinate halide hydride ate complexes of the type Bu2SnIBrH- are known in solution.1... 

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 ... 

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. 

A number of complex metal hydrides such as lithium aluminium hydride (LiAlH4, abbreviated to LAH) and sodium borohydride (NaBHj) are able to deliver hydride in such a manner that it appears to act as a nucleophile. We shall look at the nature of these reagents later under the reactions of carbonyl compounds (see Section 7.5), where we shall see that the complex metal hydride never actually produces hydride as a nucleophile, but the aluminium hydride anion has the ability to effect transfer of hydride. Hydride itself, e.g. from sodium hydride, never acts as a nucleophile owing to its small size and high charge density it always acts as a base. Nevertheless, for the purposes of understanding the transformations. 

Whilst the complex metal hydride is conveniently regarded as a source of hydride, it never actually produces hydride as a nucleophile, and it is the aluminium hydride anion that is responsible for... 

The mechanism of reduction by boranes and alanes differs somewhat from that of complex hydrides. The main difference is in the entirely different chemical nature of the two types. Whereas complex hydride anions are strong nucleophiles which attack the places of lowest electron density, boranes and alanes are electrophiles and combine with that part of the organic molecule which has a free electron pair [119]. By a hydride transfer alkoxyboranes or... 

The structure of the cyclic ketone is of utmost importance. Reduction of cyclic ketone by complex hydrides is started by a nucleophilic attack at the carbonyl function by a complex hydride anion. The approach of the nucleophile takes place from the less crowded side of the molecule (steric approach or steric strain control) leading usually to the less stable alcohol. In ketones with no steric hindrance (no substituents flanking the carbonyl group or bound in position 3 of the ring) usually the more stable (equatorial) hydroxyl is generated (product development or product stability control) [850, 851, 852, 555]. The contribution of the latter effect to the stereochemical outcome of... 

The condensation of aldehyde 2 with cyclopentadiene yields the fulvene 20 which can be transformed to the respective cyclopentadienyl anion 21 by addition of a hydride anion (Scheme 1.5.8). The anion 21 opens up the possibility of the formation of sandwich and half-sandwich type of complexes with suitable metal fragments. 

The hydrido complexes are diamagnetic, and are square planar or five-coordinate. Their stability, in general, increases with the number of coordinated phosphines. In the complex [Ni(BH4)(H)(PCy3)2] (153) the nickel atom is coordinated in the equatorial positions by two hydrogens of the borohydride and by one hydride anion. 

Another possibility is to change the steric factors. Thus, Shaw et al. (5) have shown that, with respect to transition metal complexes, there is a relationship between the tendency toward hydride formation and the bulkiness of the phosphine ligands so that larger phosphines increase the tendency toward the formation of hydride. This may be explained by the fact that the hydride anion is the smallest conceivable ligand, and thus for purely steric reasons hydride formation is facilitated when the other ligands are very large. 

A nucleophilic mechanism can be applied in reductions with complex hydrides of highly fluori-nated aliphatic and alicyclic fluoroalkenes with electron-deficient C = C bonds the hydride anion adds as a strong nucleophilic agent to the more electrophilic carbon atom the intermediate anion can then lose a fluoride ion either from the original C = C bond, or from the allylic position finishing an SN2 displacement of the fluorine. Thus, the reductions of vinylic C-F bonds with hydrides proceed by a nucleophilic addition-elimination mechanism. Displacement of fluorine in highly fluorinated aromatic compounds proceeds by the same mechanism ... 

In the first, Fe(CO)5 is converted into the carbonyl hydride anion by reductive decarboxylation following initial OH" attack on a carbonyl carbon (55). The carbonyl group is thus oxidized to carbonate in basic medium. The second reaction, (3) (56), is one of E. O. Fischer s celebrated carbene forming reactions (57). Phenyl lithium reacts with Cr(CO)6 leading to an anionic acyl complex. A subsequent alkylation step using Me30+ yields the methoxy-phenylcarbene complex Cr(CO)5(CPh(OMe)). 

The key of alkane transformation was assigned to the formation of CX3+-type cations that are electrophilic enough (probably due to a second complexation of A1X3), to abstract a hydride anion from linear and cycloalkanes. When these cations are generated in superacidic media, a protosolvation induces a superelectrophilic character, which was supported by Olah on the basis of high-level ab initio calculations 65 The generation of these cations was also used by various teams66,67 to initiate selective low temperature alkane activation. 

Arenes usually undergo electrophilic substitution, and are inert to nucleophilic attack. However, nucleophile attack on arenes occurs by complex formation. Fast nucleophilic substitution with carbanions with pKa values >22 has been extensively studied [44]. The nucleophiles attack the coordinated benzene ring from the exo side, and the intermediate i/2-cvclohexadienyl anion complex 171 is generated. Three further transformations of this intermediate are possible. When Cr(0) is oxidized with iodine, decomplexation of 171 and elimination of hydride occur to give the substituted benzene 172. Protonation with strong acids, such as trifluoroacetic acid, followed by oxidation of Cr(0) gives rise to the substituted 1,3-cyclohexadiene 173. The 5,6-trans-disubstituted 1,3-cyclohexadiene 174 is formed by the reaction of an electrophile. 

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