Covalent metal hydride complexes

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. 

Compounds tend to be covalent. Metals form complex ions and their oxides are only weakly basic. Mercury forms no hydride. 

In this article, the term metal hydrides refers almost exclusively to molecular species that is, to covalent transition metal hydride complexes. We will not review the rather extensive literature on solid-state metal hydrides (e.g., UH3, TI14H15,... 

Of the monomeric metal carbonyl hydrides [HMn(CO)s,H2Fe(CO)4,HCo(CO)4], only HMn(CO)s has been analyzed crystallographically 10 4S). The lack of results for H2Fe(CO)4 and HCo(CO)4 is probably related to the fact that the compounds are thermally unstable. The neutron diffraction analysis of HMn(CO)s by La Placa and co-workers45) produced a Mn—H distance of 1.60(2) A. This result, together with the earlier measurement of the Re—H distance of 1.68(1)A in K2ReH9, provided conclusive proof that M—H distances in metal hydride complexes are normal (i.e., are consistent with the known covalent radii of the elements), and not anomalously short as suggested by some researchers (this controversy is reviewed in Refs. 1—3). 

The hydride ion H as a ligand can donate a pair of electrons to a metal to form a metal hydride. The crystal structures of many metal hydride complexes, such as Mg2NiH4, Mg2FeH6, and K2ReH9, have been determined. In these compounds, the M-H bonds are covalent a coordinate bonds. 

The transition metal hydrides considered here are those discrete complexes which contain one or more hydrogen atoms bonded directly to a transition metal by an essentially covalent, two-electron bond. For many years, the only known complexes in this class were the unstable and reactive carbonyl hydrides of iron and cobalt and, by virtue of their novelty, the hydride ligand acquired a unique status in the minds of many chemists. Today that status is no longer generally acceptable in view of the number and diversity of presently known hydride complexes, many of which possess... 

Many metals react with hydrogen to form hydrides. Depending on the nature of the hydrogen bond, hydrides are classified into four principal categories covalent, ionic, complex and metallic. 

The hydride complex contains phosphine as the ligand (Figure 8.1) (see Bau et al., 1978). There are seven Rc-H bonds and two P —> Re bonds. The seven metal-hydrogen bonds are covalent bonds formed by pairing of the lone electrons from seven hydrogen atoms... 

The M —H distances are approximately equal to the sum of covalent radii as shown by neutron and X-ray studies. In order to find out the precise structure of metal hydrides, both neutron and X-ray studies are needed. The distances obtained from NMR studies in the solid state based on the second moment calculated from the line shape are too small. This results from the inaccuracy of the Van Vleck equation. The Mn-H distance in [MnH(CO)5] calculated from the second moment of the Van Vleck equation is 128 pm. The longer distance (144 pm) was calculated from NMR data based on the modified Van Vleck equation. " A similar distance was calculated from electron diffraction studies in the gas phase. All these distances are lower than the sum of covalent radii which equals 157 pm. The Mn-H distance in [MnH(CO)5] obtained from neutron diffraction studies equals 160.1 pm. Similar distances were found for other hydride complexes. 

HYDROGEN reacts with most of the metals to form chemical compounds— metal hydrides. Their properties, crystal structures and chemical bonding cover a wide range, including features of typical ionic, covalent, metallic and complex compounds. Metal hydrides are particularly useful for reversible storage of hydrogen in applications as an energy carrier. 

Hydrides can be categorized in various classes according to the nature of the bond that links the metal and hydrogen. The following categories can usually be found in the literature ionic hydrides (hydrides with group 1 and 2 elements), covalent hydrides, polymeric hydrides, metallic hydrides, borderline hydrides, and transitirai metal complex hydrides. Here the reaction of ionic hydrides with carbon dioxide (CO2) is considered. Transition metal hydrides with the formula L M-H are discussed in Chap. 4, where the interaction of CO2 with analogous E-X systems (where E and X are different elements) is discussed. 

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