Mononuclear Metal Hydrides

Schwarz s group has employed in-source CID to generate a range of metal hydrides for reactivity studies [78-80]. For example, the influence of additional ligands (L = CO, H2O, HO) on the reactivity of nickel hydride cluster cations, NiH toward methane has been explored (Eq. (6.40)) [78]. All systems react 3vith methane via a metathesis reaction (Eq. (6.40)), albeit at a slower rate than NiH+. A related 

2 Unmasking Reactive Metallic Intermediates via Collision-Induced Dissociation 211 [MojOgCOH)]-- CH3OH 

Ag4H+ + 3CH2 = CHCH2Br Ag(C3H5)2 -I-AgjBr3 -I-CH2 = CHCH3 

6 Gas Phase Ligand Fragmentation to Unmask Reactive Metallic Species 

---

Carbonyl hydrides and carbonylate anions are obtained by reducing neutral carbonyls, as mentioned above, and in addition to mononuclear metal anions, anionic species of very high nuclearity have been obtained, often by thermolysis. These are especially numerous for Rh and in certain Rh, Rh and Rhi5 anions have structures conveniently visualized either as polyhedra encapsulating further metal atoms, or alternatively as arrays of metal atoms forming portions of hexagonal close packed or body... 

The simplest supported catalysts are mononuclear metal complexes, exemplified by industrial supported metallocene catalysts, used (with promoters) for alkene polymerization these are the so-called single-site catalysts that are finding wide industrial applications (Kristen, 1999 Kaminsky, 1999 Roscoe et al., 1998). The most common supports are metal oxides and zeolites. The metals in these complexes range from oxophilic (e.g., Zr and Ta) to noble (e.g., Rh). Supported metal complexes are stabilized by ligands—in addition to those provided by the support—such as hydride (H), hydrocarbons, and carbonyl (CO). In a typical supported metal complex, the metal is present in a positive oxidation state. Although some such complexes are relatively stable, most are, befitting their roles as catalysts, highly reactive and air- and moisture-sensitive. 

The compositions and ligand geometries of most mononuclear T-metal hydride complexes can be rationalized in terms of conventional electron counts based on full charge... 

Methods of preparing selected hydrido complexes were given in equations 23.2 and 23.46-23.50. Selected properties of the mononuclear complexes HMn(CO)5, H2pe(CO)4 and HCo(CO)4 are given in Table 23.6. Metal hydrides play an important role in organometallic chemistry, and scheme 23.57 illustrates some ligand transformations involving M—H bonds HCo(CO)4 is an industrial catalyst (see Section 26.4). 

Compounds with M-H-M bridges can often be formed by oxidative addition to preformed Cluster compounds (equation 14). A general strategy for synthesizing hydride clusters from mononuclear precursors is to generate a coordinatively unsaturated see Coordinative Saturation Unsaturatiori) fragment L M in the presence of a metal hydride, say The metal hydride tends to bind to the... 

The Bronsted acidity of transition metal hydrides is a well known phenomenon which has been extensively studied in both mononuclear and polynuclear compounds. [50] The Bronsted acidity is a likely property of the dinuclear products resulting from H2 additions, especially for cationic species, since they often contain metal centers in relatively high formal oxidation states. In this respect, it... 

The vast majority of known polynuclear metal hydride complexes contains bridging hydride ligands. Obviously, the possibility of this bonding mode constitutes the main peculiarity of polynuclear compounds in comparison to mononuclear hydrides. Some representative examples of dinuclear complexes containing up to four hydride bridges are shown in Scheme 18.[54-56]... 

Of the simple mononuclear metal carbonyls, only Fe(CO)s appears to be readily attacked by a weak base with subsequent formation of metal hydride bonds. We reasoned that if the metal carbonyl system... 

Table 6.2 Mononuclear transition metal-hydrogen complexes in solid state metal hydrides as of May 2007 ou = average due to ligand disorder...

---