Stable Dihydrogen Complexes

Several synthetic routes to H2 complexes are available. The simplest method is reaction of H2 gas with a coordinatively unsaturated complex such as W(CO)3(PR3)2, the original method, as described in Chapter 2. This is analogous to the well-studied reversible addition of H2 to Vaska s complex, IrCl(COXPPh3)2, except that for the latter OA to the dihydride occurs. A second, very common method of preparation is protonation of metal hydride complexes  

In the late 1950s certain transition metal complexes were recognized to be basic and subject to protonation with acids. This method is an offshoot whereby protonation occurs at the basic hydride ligand and has been widely applicable because it does not require an unsaturated precursor that often either does not exist or is difficult to synthesize. Neutral polyhydride complexes L MHX are often easy targets for protonation to cationic hydrido-H2 complexes, [L M(H2)HX 1]+, which are frequently more robust than complexes prepared from H2 gas. 

Many variations of half-sandwich -type complexes are known for Cp, Tp, and triazacyclononane (Cn) ligands, including the entire series for Run. Because the Cn 

Only the last two types have been isolated as solids, and the X-ray structure of RuH2(H2)2(PCy3)2 shows unstretched cis H2 ligands as in structure 2 below (X = H). The novel, X-ray-characterized 16e species RuHX(H2)(PCy3)2 (X = Cl, I) add a second H2 ligand in equilibrium fashion [Eq. (3.2), observable only in solution].65 

A complex with two different rj2-coordinated a bonds is known, RuH2( /2-H2)-0f2-SiHPh3)(PCy3)2,3, wherein the bulky phosphines are in an unusual cis configuration (P-Ru-P = 109.7°) apparently because favorable attractive interactions between the Si atom and the hydride ligands (see Chapter 4) overcome steric repulsion between the phosphines.64 

---

The chemistry of transition metal dihydrogen complexes, such as formed in Eq. (1) is in the focus of very intensive studies [13, 14] and such complexes were shown to be involved in the mechanism of several hyrogenation processes [15]. Despite this fact, their role in aqueous-phase hydrogenations is largely unexplored, although examples of water-soluble (or water-stable) dihydrogen complexes are known [16, 17]. Reaction (1) does not involve the change of the oxidation state of... 

PALLADIUM (DIHYDROGEN)- THE FIRST PREDICTION OF A STABLE DIHYDROGEN COMPLEX... 

The study of species in which ethylene is coordinated to transition metal centres holds great interest in areas of catalytic and polymerization chemistry (7). The bonding of the ethylene ligand to the metal centre in such species has been compared to that of the dihydrogen complexes described above (14,15,22). Photolysis of chromium hexacarbonyl, Cr(CO)6, in conventional solvents in the presence of dissolved ethylene gas is known to lead initially to a highly labile species in which one CO ligand is replaced by ethylene. Further photolysis leads to a more stable compound which contains two ethylene ligands trans to each other across the metal centre (25), equation 3. The conventional synthesis is experimentally difficult the two photochemical... 

---