Bis hydride

Thanks to recent advances in soHd-state NMR spectroscopy, the zirconium bis-hydride [(=SiO)2ZrH2] (lb) (20-35%) was found to be generated simultaneously to the major zirconium monohydride [(=SiO)3Zrft] (la) (80-65%) during hydroge-nolysis of the single site sihca or silica-alumina supported [(=SiO)ZrNp3] [14],... 

Mono(hydride) zirconium species grafted on a silica surface show a high reactivity toward alkanes. This system has been studied using modern 2D NMR techniques, such as double quantum (DQ) rotor synchronized 2D H MAS and C- H HETCOR, to investigate the mechanism of formation of a zirconium bis(hydride) species and the simultaneous generation of silicon mono- and bis(hydride) species [111]. Figure 11.4 shows the H MAS NMR spectrum and DQ rotor-synchronized... 

Although group 5 organometallic systems have been found to be of relevance in transition-metal catalyzed hydroboration reactions, structurally authenticated group 5 boryl complexes remain relatively few in number. Smith and co-workers, for example, have probed the mechanisms for the formation of niobium and tantalum mono- and bis(boryls) from propylene complex precursors, with concomitant formation of propyl boronate esters [31,32]. Of particular interest from a structural viewpoint are the relative merits of alternative bonding descriptions for metal(V) boryl bis(hydrides) as borohydride complexes or as mono(hydride) a-borane systems [31-34]. 

When para-H2 is used, the free hydrogen resonance results in a strong emission signal. The observed behaviour can be accounted for by a polarization transfer from para-H2 to the AX spin system of XI and then to the ortho-Hj, which is formed from XI by the dissociative pathway depicted in scheme 10. The otherwise forbidden singlet (para-H2) to triplet (ortho-H2) transition is then made possible through the intermediacy of the bis-hydride derivative.[56]... 

After synthesis of [(=SiO-Zr(Np)3l (1) and species [(=SiO-Ti(Np)3] (2) [30], the efforts were made to synthesize and identify the corresponding surface organometallic hydride which we believed to be the active catalyst for various types of C-H bond activation reaction. 1 and 2 generate tri-podal monohydride (9, 10) as major component when reacted with H2 at 150°C [30, 34]. However, 3 generates bipodal bis-hydride (11) as major component under H2 atmosphere at temperature lower than 100°C (Scheme 5) [35]. Similar hydrides were observed when silica-alumina and alumina-supported Ti(Np)4 and Zr(Np)4 were heated at 150°C in the... 

In aprotic solvents, nitro compounds reacts with [HFe3(CO)n] to give in poor yields an imido complex 6, which, by acidification, leads to the bis-hydride imido derivative 7 (Scheme 10) [64] ... 

When a molecule of H2 approaches a coordinatively unsaturated [45] transition metal center, there are at least four possible outcomes (Scheme 4.3). (i) There is no reaction whatsoever, (ii) An essentially intact H2 molecule is bound to the transition metal center, (iii) The H-H bond is homolytically cleaved, resulting in a bis-hydride complex, (iv) The H-H bond is heterolytically cleaved, resulting in the formation of a metal hydride with loss of H. Note that throughout this chapter, indicates a dihydrogen ligand, (H) ( = 1, 2,...) denotes n classical hydride ligands and H (x=, 2,. ..) indicates an unspecified stractural form. 

HaU and coworkers have utilized ah initio electronic structure calculations to examine the factors that lead to the structures of hydride and // -H2 complexes [48-51]. For 114 polyhydride species, Bayse and HaU found that the most stable structure maximizes use of the n d and (n+1) s orbitals on the metal in the formation of the M-H bonds [5 Ij. lin and HaU examined factors which lead to oxidative addition to form a bis-hydride or association of an intact molecule of H2 to form the corresponding / -H2 complex. They concluded that a bis-hydride is preferred over an / -H2 complex when twice the ionization enthalpy of an electron in the M-H bond is greater than sum of the ionization enthalpies of an electron in the H-H bond and one in the metal d orbital [49]. 

Complexes with H-H distances between 1.0 and 1.5 A are often referred to as elongated or stretched dihydrogen complexes [57]. The properties of these complexes make it difficult to justify their classification as either trae dihydrogen complexes (dH-H<10A) or ds-dihydride complexes (dH-H>l-5A). Other authors further categorize these complexes as true elongated dihydrogen complexes (dn-H < 10-1.3 A) and compressed bis-hydrides (dn-H < 13-1.5 A) [46]. 

HaU and coworkers used density functional theory calculations to assign the inelastic neutron scattering derived vibrational spectrum of the elongated dihydrogen complex, (Tp )Rh(H)2(/ -H2) [66], They conclude that the H-H distance derived from neutron diffraction for the (Tp )Rh(H)2( / -H2) complex may in fact correspond to the average of the H-H distances of the tetrahydride and bis-hydride/// -H2 species. 

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