Binary metal hydride

The lattice gas approach is valid within certain limits for typical metallic hydrides, binaries as well as ternaries. Deviation from this idealized picture indicates that metallic hydrides are not pure host-guest systems, but real chemical compounds. An important difference between the model of hydrogen as a lattice gas, liquid, or solid and real metal hydrides lies in the nature of the phase transitions. Whereas the crystallization of a material is a first-order transition according to Landau s theory, an order-disorder transition in a hydride can be of first or second order. The structural relationships between ordered and disordered phases of metal hydrides have been proven in many cases by crystallographic group-subgroup relationships, which suggests the possibility of second-order (continuous) phase transitions. However, in many cases hints for a transition of first order were found due to a surface contamination of the sample that kinetically hinders the transition to proceed. 

Switendick was the first to apply modem electronic band theory to metal hydrides [5]. He compared the measured density of electronic states with theoretical results derived from energy band calculations in binary and pseudo-binary systems. Recently, the band structures of intermetallic hydrides including LaNi5Ht and FeTiH v have been summarized in a review article by Gupta and Schlapbach [6], All exhibit certain common features upon the absorption of hydrogen and formation of a distinct hydride phase. They are ... 

G. G. Libowitz, The Solid State Chemistry of Binary Metal Hydrides, W. A. Benjamin, New York, 1965. 

No binary hydrides have been characterized, but reactions of the metal powders with alkali metal hydrides in a hydrogen atmosphere lead to Li3RhH4 (planar RhH4 ) and M3MH6 (M = Li, Na M = Rh, Ir) with octahedral MHj [34],... 

The reactivity of these metal hydride-metal carbonyl reactions can be correlated with the nature of the reactants in a manner consistent with the proposed mechanism nucleophilic attack by hydride on coordinated CO. Thus reactions involving the highly nucleophilic group IV hydride, Cp gZrHg, are much faster than those of group V metal hydrides. On the other hand, the relatively electrophilic neutral binary metal carbonyls all react with Cp2NbH under mild conditions (20-50° C), whereas more electron-rich complexes such as cyclopentadienylmetal carbonyls (Cp2NbH(C0), CpV(CO) ) or anionic carbonyls (V(CO)g ) show no reaction under these conditions. 

The development of G. N. Lewis s octet rule for the s/p-block elements was strongly influenced by the stoichiometric ratios of atoms found in the common compounds and elemental forms (CH4, CCI4, CO2, CI2, etc.). Let us therefore begin analogously by examining the formulas of the common neutral binary chloride, oxide, and alkyl compounds of transition metals. (Here we substitute alkyl groups for hydrogen because only a small number of binary metal hydrides have been well characterized.)... 

Because of their low intrinsic electronegativities, neutral late transition metals (bearing an abundance of lone pairs) can serve as good donors in nM— ctah interactions of the form (5.69a). Furthermore, transition-metal-hydride bonds (Section 4.4.1) often display sufficient covalency or polar-covalency (particularly in transition-metal cations) to serve as good acceptors in ns— ctmh interactions of the form (5.69b). In the present section we shall briefly examine the simple example of platinum dihydride (PtH2) as a water-mimic in binary H-bonded complexes with H20,... 

Hydrides of variable composition are not only formed with pure metals as solvents. A large number of the binary metal hydrides are non-stoichiometric compounds. Non-stoichiometric compounds are in general common for d,f and some p block metals in combination with soft anions such as sulfur, selenium and hydrogen, and also for somewhat harder anions like oxygen. Hard anions such as the halides, sulfates and nitrides form few non-stoichiometric compounds. Two factors are important the crystal structures must allow changes in composition, and the transition metal must have accessible oxidation states. These factors are partly related. FeO,... 

The binary hydrides of the transition metals are predominantly metallic in character and are usually referred to as metallic hydrides. They are good conductors of electricity and have a metallic or graphite-like appearance. 

The stability of metal hydrides is presented in the form of van t Hoff plots. The most stable binary hydrides have enthalpies of formation of AHf= —226kJ mol H2, for example, H0H2. The least stable hydrides are FeHo.s, NiFlo.s and M0H0.5, with enthalpies of formation of AHf = + 20, + 20 and + 92 kj mol Fl2, respectively [42]. 

This volume summarizes recent results of some of the leading investigators in trahsition metal hydride research. Readers interested in more extensive background material are urged to consult some of the many excellent books on the subject, such as Transition Metal Hydrides edited by E. L. Muetterties (Marcel Dekker, Inc., New York, 1971), which covers covalent metal hydride complexes, and Metal Hydrides edited by W. M. Mueller, J. P. Blackledge, and G. G. Libowitz (Academic, New York, 1968), which comprehensively covers work in binary and ternary metal hydrides. Also available in the covalent metal hydride area are excellent reviews by Ginsberg [Transition Metal Chemistry (1965) 1,112], and Kaesz and Saillant [Chemical Reviews (1972) 72, 231]. In this book we have not tried to be comprehensive rather, our purpose is to update recent developments in both major areas of metal hydride research. 

This chapter presents results of NMR studies of several heavy metal hydrides, including both a summary of completed work by Lau et al. (i) on a binary hydride, Th4His, and preliminary results on several carbonyl hydrides, H2Os3(CO)io, H4Os4(CO)i2, and H4Ru4(CO)i2. The binary hydride, Th4Hi5, has attracted interest recently with the discovery of its super-conducting properties (2) and the carbonyl hydrides are metal cluster hydrides (3) which are of interest as models in the study of catalysis (4). 

Figure 1. Occurrence of binary transition and rare earth metal hydrides...

CH2=CH2 + H20 - CH3CH2OH. hydride A binary compound of a metal or metalloid with. hydrogen the term is often extended to include all binary compounds of hydrogen. A saline or saltlike hydride is a compound of hydrogen and a strongly electropositive metal a molecular hydride is a compound of hydrogen and a nonmetal a metallic hydride is a compound of certain d-block metals and hydrogen. 

In earlier chapters, we saw examples of how the metallic or nonmetallic character of an element affects its chemistry. Metals tend to form ionic compounds with nonmetals, whereas nonmetals tend to form covalent, molecular compounds with one another. Thus, binary metallic hydrides, such as NaH and CaH2, are ionic solids with high melting points, and binary nonmetallic hydrides, such as CH4, NH3, H20, and HF, are covalent, molecular compounds that exist at room temperature as gases or volatile liquids (Section 14.5). 

The existence of H atoms of a higher coordination number (four, five or six) is an intriguing problem. It was not until very recently that examples of such species were unequivocally established in molecular complexes246,247). In the solid state literature, however, high-coordination H atoms are well-known entities there are many examples of binary metal hydrides in which H atoms are known to occupy tetrahedral or octahedral sites in a metal lattice8). 

Hydrogen combines with many metals to form binary hydrides MHX. The hydride ion H has two electrons with the noble gas configuration of He. Binary metal hydrides have the following characteristics ... 

Binary compounds with less electronegative elements include hydrides, nitrides, sulphides and phosphides. They are decomposed by water and can provide convenient routes for the preparation of non-metal hydrides. The anions may be polyatomic or polymerised, as with CaC2, which contains C22- and reacts with water to give acetylene (ethyne) C2H2. 

We have carried out "direct" crystallization of aluminum hydride in one stage using interaction of binary metal hydride with aluminum chloride in the medium of ether-toluene at 60-100°C and using solvent distillation. In the reaction of LiH with AICI3, we achieved output of pure crystal A1H3 of hexagonal modification, which was close to quantitative. 

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