Hard metal hydrides

The interstitial carbides are compounds formed by the direct reaction of a d-block metal and carbon at temperatures above 2000°C. In these compounds, the C atoms occupy the gaps between the metal atoms, as do the H atoms in metallic hydrides (see Fig. 14.9). Here, however, the C atoms pin the metal atoms together into a rigid structure, resulting in very hard substances with melting points often well above 3000°C. Tungsten carbide, WC, is used for the cutting surfaces of drills, and iron carbide, FesC, is an important component of steel. 

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,... 

Complexes between organometallic or metal hydride complexes and hard bases have attracted attention recently.1 In many cases the reaction solvent plays the role of a Lev is base such complexes can be catalytically active by solvent ligand dissociation.2,3 We describe here the complexes [IrH2S2(PPh3)2][BF4], where S = Me2CO,4,5 H20,5 or S2 = o-... 

For some others (not all of them metal hydrides) see Hutchins Kandasamy J. Am. Chem. Soc. 1973, 95. 6131 Risbood Ruthven J. Org. Chem. 1979, 44, 3969 Babler Invergo Tetrahedron Lett. 1981, 22, 621 Fleet Harding Tetrahedron Lett. 1981,22, 675 Yamaguchi Kabuto Yasuhara Chem. Lett. 1981, 461 Kim Kang Yang Tetrahedron Lett. 1984, 25, 2985 Kamitori Hojo Masuda Yamamoto Chem. Lett. 1985, 253 Borbaruah Barua Sharma Tetrahedron Lett. 1987, 28, 5741. 

The reactions of type II proceed by transmetallation of the complex 5. The transmetallation of 5 with hard carbon nucleophiles M R (M = main group metals) such as Grignard reagents and metal hydrides MH generates 8. Subsequent reductive elimination gives rise to an allene derivative as the final product. Coupling reactions of terminal alkynes in the presence of Cul belong to Type II. 

Another instructive scenario may be found when considering the metalation of arenes. There are two distinct mechanisms for the metalation of aromatic C-H bonds - electrophilic substitution and concerted oxidative addition (Box2). The classical arene mercuration, known for more than a century, serves to illustrate the electrophilic pathway whereas the metal hydride-catalyzed deuterium labeling of arenes document the concerted oxidative addition mechanism [8, 17]. These two processes differ both in kinetic behavior and regioselectivity and thus we may appreciate the need to differentiate these two types of process. However, the choice of C-H bond activation to designate only one, the oxidative addition pathway, creates a similar linguistic paradox. Indeed, it is hard to argue that the C-H bond in the cationic cr-complex is not activated. 

The errors seen in the above examples are of course the result of the necessary incompleteness of orbital and configiaration basis sets. The power of these expansion approaches is that if one works hard enough (uses a sufficiently complete/ or at least appropriate/ basis) one should get the right answer. The recent extensive transition metal hydride studies indicate the possibilities (25-30). Nevertheless/ heavy atom electron correlation involving d and even f subshells is such an enormous paroblem that every alternative should be explored. 

The reactions of Type II proceed by transmetallation of the complex 8, Hard carbon nucleophiles MR (M = Main group metal), such as Grignard reagents and metal hydrides... 

There is ample evidence to support this prediction, since it simply says that hard ligands will favor a higher positive oxidation state for the central metal. We saw an example of this in Chapter 1, when the acidity of transition metal hydrides was discussed. It is also an example of the HSAB Principle, or the symbyiotic effect. 

Nitrides of the J-block metals are hard, inert solids which resemble metals in appearance, and have high melting points and electrical conductivities (see Box 14.5). They can be prepared from the metal or metal hydride with N2 or NH3 at high temperatures. Most possess structures in which the nitrogen atoms occupy octahedral holes in a close-packed metal lattice. Full occupancy of these holes leads to the stoichiometry MN (e.g. TiN, ZrN, HfN, VN, NbN) cubic close-packing of the metal atoms and an NaCl lattice for the nitride MN is favoured for metals in the earhest groups of the J-block. 

The problem is apparently due to some residual aluminum that is hard to remove. If, however, the reduction is carried out in a iV-methylmorpholine solution, followed by addition of potassium tartrate, a pure product can be isolated. A -Methylmorpholine is a good solvent for reductions of various macromolecules with metal hydrides.In addition, the solvent permits the use of strong NaOH solutions to hydrolyze the addition complexes that form. Other polymers that can be reduced in it are those bearing nitrile, amide, imide, lactam, and oxime pendant groups. Reduction of polymethacrylonitrile, however, yields a product with only 70% of primary amine groups. Complete reductions of pendant carbonyl groups with LiAlH4 in solvents other than A -methyl-morpholine, however, were reported. Thus, a copolymer of methyl vinyl ketone with styrene was fully reduced in tetrahydrofuran. ... 

Unlike many, if not most, transition metal hydrides which are relatively unstable and sensitive to air and moisture, the manganese hydride described here is completely stable in air. It serves as the starting material for the synthesis of a variety of derivatives in which the hydrogen atom is replaced by other atoms (or groups of atoms) directly bonded to manganese. Earlier methods of preparation involved the displacement of CO by bis(diphenyl-phosphino)ethane from the difficult to prepare and hard to handle HMn(CO)5 as well as from the decarboxylation of a postulated intermediate (diphos)Mn(CO)3COOH. Our preparation is a modification of a previously described procedure in which 1-propanol was used as reagent and as solvent. In addition to a much shorter reaction time, this procedure results in a pure, almost colorless product uncontaminated by an unknown dimer which frequently accompanies the hydride. ... 

The application of powder metallurgical techniques using mixtures of the elements or the metal hydride and graphite have resulted in relatively pure carbides of various compositions. This is a particularly useful method when defective compounds in Groups 5 and 6 are needed. The powdered metals or hydrides in Group 4 and the actinide series are sufficiently reactive to air that the introduction of oxygen is hard to avoid. 

---