Hydrides structural transformations

Transition Metal Hydrides , ed. E. L. Muetterties, Dekker, New York, 1971. Crystal Structure Transformations in Binary Halides , C. N, R, Rao and... 

Table 10.1 lists the pressures of phase transitions in MX compounds from the ZnS structure type (Ac = 4) to the NaCl or NiAs (Ac = 6), and into the CsCl (Ac = 8) types. This table shows that the pressures of phase transitions (Ttr) fall with the increase of ionic radii of both the cations (which agrees with the ionic model) and anions (which contradicts it). This contradiction disappears, if we take into account the polar character of M-X bonding the increase of the bond covalency in the successions MF MI or MO -> MTe rises r(M+) and reduces r(X ), thus increasing the kr = r+lr- ratio and diminishes Ftr- In alkali hydrides [18, 19] the pressure of the structural transformation from the NaCl into the CsCl type decreases with the increase of the cation size, viz. 29.3 GPa in NaH, 4.0 in KH, 2.7 in RbH and 1.2 in CsH. In CsH one more phase transition was discovered at 17.5 GPa [70] whereupon the CsCl structure converts into the CrB type this phase is stable up to 253 GPa and its compressibility at the this pressure is the highest for the MX compounds. Wo = 0.26. 

Praseodymium forms, like La and Ce, P-phase hydrides up to x=l, and this permits one to observe the approach towards a potential M-S transition. The resistivities of three x-rich PrH2+x specimens (Burger et al. 1988) shown in fig. 38 indicate a strongly hysteretic structural transformation at 200 T 250 K for x = 0J6 which is probably a precursor for a M-S transition. The p-isothermals seen in fig. 39 show an ordering plateau for 0.1 X 0.5 at T 200 K and probably another one for x > 0.6. 

The isomerization of an allylic amine to an enamine by means of a formal 1,3-hydrogen shift constitutes a relatively small structural change. However, this transformation could be extremely valuable if it could be rendered stereoselective. In important early studies, Otsuka and Tani showed that a chiral cobalt catalyst, prepared in situ from a Co(ii) salt, a chiral phosphine, and diisobutylaluminum hydride (Dibal-H), can bring about the conversion of certain pro-chiral olefins to chiral, isomeric olefins by double bond migra-... 

The screened proton model of nickel or palladium hydrides and Switendick s concept of the electronic structure do not constitute a single approach sufficient to explain the observed facts. In this review, however, such a model will be used as the basis for further discussions. It allows for the explanation and general interpretation of the observed change of catalytic activity of the metals, when transformed into their respective hydrides. 

The change in the electronic structure of a bulk metal catalyst, in consequence of its transformation into the hydride, influences respectively the metal surface atoms (ions) or, strictly speaking, their d orbitals. Recent achievements and the present knowledge of the subject only permit us so far to formulate such general conclusions. 

Rh(OEP)H reacts with CNR (R = Me, n-Bu,) to give the adduct Rh(OEP)-(H)CNR (which has no parallel in CO chemistry) which then slowly transforms to the formimidoyl insertion product, Rh(OEP)C(H)=NR. The dimer Rh(OEP))2 reacts with CNAr (Ar = 2.6-Cf,H3Mc2) in aqueous benzene to give the carbamoyl product. Rh(OEP)C(0)NHAr (characterized by an X-ray crystal structure) together with the hydride, which it.self reacts further with the isocyanide. This is suggc.sted to form via a cationic carbene intermediate, formed by attack of HiO on coordinated CNAr in concert with disproportionation to Rh(III) and Rh(l). 

The seco amide alkaloids have been subjected to various transformations, mainly for structure elucidation purposes. When treated with lithium aluminium hydride, arnottianamide (206) was converted to the tertiary amine, deoxyarnottianamide (224), which on methylation with the Rodionow reagent gave deoxy-O-methylarnottianamide (225) (172,175). Arnottianamide (206) could be O-acetylated (174) as well as O-methylated with diazomethane in HMPA (172). Isoarnottianamide (208) was O-methylated to trimethoxy derivative 226, which under Bischler-Napieralski conditions recyclized to the benzophenantridine alkaloid, chelilutine (227) (176) (Scheme 33). 

Molecular hydrogen is rather unreactive at ambient conditions, but many transition and lanthanide metal ions are able to bind and therefore activate H2, which results in transformation into H (hydride) 11 (hydrogen radical) or H+ (proton), and subsequent transfer of these forms of hydrogen to the substrate.7,8 In this context, not only metal hydride but also dihydrogen complexes of transition metal ions, play a key role,9 10 especially since the first structural characterization of one of these species in 1984 by Kubas.11... 

The observed methane generation points to a plausible I —> III or II - III transformation, but it does not distinguish which of the structures (II or III) is the metathesis-active carbene. This matter is mechanistically significant with regard to the chain termination process. Type III may terminate by a bimolecular dimerization sequence as in Eq. (11), or it may convert to a 7r-olefin complex via an uncommon 1,2-hydride shift ... 

The Rh dimer after H2 adsorption exhibited similar EXAFS oscillation and Fourier transform to those for the fresh imprinted catalyst Detailed analysis of the EXAFS data confirmed retention of the local conformation of the Rh dimer with a Rh-Rh bond (CN = 1.3 0.4), two Rh-O bonds (CN = 1.7 0.5) and a Rh-P bond (CN = 1.2 0.2). No formation of Rh metallic particles was observed. However the Rh-Rh bond contracted from 0.268 0.001 to 0.265 0.001 nm with the hydride dimer, indicating stabilizahon of the dimer structure by electronic rearrangement due to monohydride coordination on both Rh atoms in the dimer. After reaction of the Rh-dimer hydride species with 3-methyl-2-pentene, the shrunken Rh-Rh bond of the monohydride species expanded again to recover the... 

Aluminum methoxide Al(OMe)3 is a solid which sublimes at 240 °C in vacuum. Aluminum isopropoxide melts in the range 120-140 °C to a viscous liquid which readily supercools. When first prepared, spectroscopic and X-ray evidence indicates a trimeric structure which slowly transforms to a tetramer in which the central Al is octahedrally coordinated and the three peripheral units are tetrahedral.162,153 Intramolecular exchange of terminal and bridging groups, which is rapid in the trimeric form, becomes very slow in the tetramer. There is MS and other evidence that the tetramer maintains its identity in the vapour phase.164 Al[OCH(CF3)2]3 is more volatile than Al[OCH(Me)2]3 and the vapour consists of monomers.165 Aluminum alkoxides, particularly Al(OPr )3, have useful catalytic applications in the synthetic chemistry of aldehydes, ketones and acetals, e.g. in the Tishchenko reaction of aldehydes, in Meerwein-Pondorf-Verley reduction and in Oppenauer oxidation. The mechanism is believed to involve hydride transfer between RjHCO ligands and coordinated R2C=0— A1 groups on the same Al atom.1... 

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