Hydrogen complex transition metal hydride

There exist two broad families of complex transition metal hydrides. The first contains hydrogen bonded to transition elements only and has the general composition... 

The gravimetric hydrogen content of complex transition metal hydrides can reach values of more than 5 wt.% (Mg2FeH6 = 5.5 wt%), but most of these systems have a... 

The complex transition metal hydrides known so far show too low hydrogen contents and can therefore not be considered for hydrogen storage. 

A second major route to metal-metal complexes, related to the salt-elimination method described above, is elimination of neutral molecules with concurrent formation of metal-metal bonded complexes. Transition metal hydrides readily undergo these dinuclear reductive elimination reactions. The oxidative addition/reductive elimination see Oxidative Addition and Reductive Elimination) reaction of molecular hydrogen is a key reaction in this area (equation 47). 

Fig. 6.18 Metal—hydrogen complexes and cation environments in complex transition metal hydrides. Small filled circles hydrogen large open circles Mg + and Ba +.

Table 6.4 Hydrogen storage properties of selected complex transition metal hydrides (found and/or characterized in Geneva)...

The characteristic part of a complex transition metal hydride is the anionic homoleptic transition metal (M) hydride complex [MmH/,]""", which is balanced by the cation A+ or A + (A = Li-Cs, Mg-Ba, Eu, Yb). The formation of an extended solid is a consequence of the attractive electrostatic Coulomb interaction between cations and complex anions, whereas within the complex, hydrogen... 

Two closely related reactions, (a) and (b), illustrated by Eq. (12) (Rj = HPhj, Etj, Phj, CI3, CljPh) and (13), of silicon hydrides with transition metal complexes generate compounds with Si—M bonds with elimination of hydrogen (a) cleavage of metal-metal bonds and (b) reaction with transition metal hydrides. Reactions discussed in this section are relevant to... 

An obvious limitation to the hydrogen-elimination method, especially for early transition metal elements, is the availability of transition metal hydrides (this applies also to the HCl elimination) or binuclear complexes. 

In 1931, Hieber and Leutert reported Fe(CO)4(H)2 not only as the first iron hydride complex but also as the first transition-metal hydride complex (FeH2 was reported in 1929 from FeCl2 and PhMgBr under a hydrogen atmosphere. However, it exists only in a gas phase) [2, 3]. The complex synthesized from Fe(CO)5 and OH (Scheme 1) is isolable only at low temperature and decomposes at room temperature into Fe(CO)5, Fe(CO)3, and H2. 

Usui, Y, Hrrano, M., Fukuoka, A. and Komiya, S. (1997) Hydrogen abstraction from transition metal hydrides by gold alkoxides giving gold-containing heterodinuclear complexes. Chemistry Letters, 26, 981. 

We do not know exactly where the hydrogen binds at the active site. We would not expect it to be detectable by X-ray diffraction, even at 0.1 nm resolution. EPR (Van der Zwaan et al. 1985), ENDOR (Fan et al. 1991b) and electron spin-echo envelope modulation (ESEEM) (Chapman et al. 1988) spectroscopy have detected hyperfine interactions with exchangeable hydrous in the NiC state of the [NiFe] hydrogenase, but have not so far located the hydron. It could bind to one or both metal ions, either as a hydride or H2 complex. Transition-metal chemistry provides many examples of hydrides and H2 complexes (see, for example. Bender et al. 1997). These are mostly with higher-mass elements such as osmium or ruthenium, but iron can form them too. In order to stabilize the compounds, carbonyl and phosphine ligands are commonly used (Section 6). 

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