Transition binary hydrides

Hydrogen combines with many elements to form binary hydrides MH (or M H ). All the main-group elements except the noble gases and perhaps indium and thallium form hydrides, as do all the lanthanoids and actinoids that have been studied. Hydrides are also formed by the more electropositive transition elements, notably Sc, Y, La, Ac Ti, Zr, Hf and to a lesser... 

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,... 

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. 

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. 

Hydrogen forms three types of binary hydrides. Active metals give ionic hydrides, such as LiH and CaFF nonmetals give covalent hydrides, such as NH3, H2O, and HF and transition metals give metallic, or interstitial, hydrides, such as PdH,.. Interstitial hydrides are often nonstoichiometric compounds. 

Needless to say, when the hydrides have a resemblance in the chemical bonding state, their locations are close to each other in the diagram. For example, any binary hydrides of transition elements appear in the higher AEh region than those of typical elements. This indicates that transition elements could... 

FIGURE 21.3 Binary hydrides of the representative elements. Where hydrogen forms more than ane compaund with the same element, only the formula of the simplest hydride is shown. The properties of many of the transition metal hydrides are not well characterized. 

Yvon, K., Complex transition-metal hydrides, Chimia, 1998, 52(10) p. 613-619. Libowitz, G.G., The Solid State Chemistry of Binary Metal Hydrides, 1965, New York Benjamin Press. 

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

Crystal structures and properties of the binary transition metal hydrides underscore the difficulty of a strict categorization of metal hydrides according to their chemical... 

An important and numerous class of intermetallic compounds AM2 are the cubic Laves phases (MgCu2 type, C15). Many representatives with A or M being one of the transition metals forming stable binary hydrides (see in.B.l) absorb considerable amounts of hydrogen up to compositions AM2H7. Hydrogen occupies tetrahedral interstices of the crystal structure, thereby expanding it. 

TABLE IV Characteristic Features of Binary Main Group and Transition Metal Hydrides ... 

In some cases an intermetallic compound is formed along with the more stable binary hydride instead of the free metal. Buschow, Bouten and Miedema (1) list over 100 intermetallc hydrides that have been prepared and partially characterized. There are several times this many that are known, yet few are really satisfactory for the applications listed above. This paper is limited to transition metal-transition metal type alloys where one metal (A) is the stronger hydrogen-attracting and (B) is the weaker hydrogen-attracting. Examples of A-type metals are early (Illb,... 

The decomposition pathways of binary and ternary metal tetrahydroborates are often highly complex and differ significantly from one system to the next, e.g., LiBHt [45], Mg(BH4)2 [46], and LiZn(BFl4)3 [47], and the formed products can even depend on the details of the desorption conditions. Certain compounds form transition metal hydrides [48], others form transition metal borides [49], di-, deca-, or dodeca-boranes [50], and others again, e.g., Cr, Cd, Mn, and Zn(BH4)2 decompose to the elements [51,52,112]. 

In terms of Miedema s cellular model, the changes in 3d magnetism can be explained as follows (Buschow et al., 1982a) In the ternary hydride the contact area of a given 3d atom comprises 3d-3d contacts as well as a substantial portion of 3d-H contacts. In many respects the situation is similar to the one that would be present in the binary 3d transition metal hydrides, so that an analogous magnetic behaviour can be anticipated. 

The first hydride complex investigated by the neutron diffraction technique was K2[ReH9] containing the terminal M—H linkage. Many examples of complexes containing the terminal hydride ligands are now known for virtually all d-block transition elements. Binary transition-metal hydrides are rather few and the majority are stabilized by carbonyl, phosphine, or other ancillary ligands. 

Hydrogen combines with most of the chemical elements. Here we concentrate on binary hydrides of the typical elements, ignoring those of the transition elements, lanthanides and actinides, which often have metallic properties and so resemble alloys. Binary hydrides are compounds of hydrogen and one other element. A useful classification of the highest hydrides of the typical elements is shown in Figure 5.9. It divides them into three classes salt-like, macromolecular and molecular. 

---