Bond dihydrogen complex

We have shown above that when H2 interacts with nontransition metal atoms or diatoms such as Be, B, Be, Li or Be, volcanic ground potential energy surfaces (PES) are generated while an exceptional bond takes place. Rather than an ordinaiy dihydride with the simultaneous scission of the H2 bond, dihydrogen complexes are formed, e.g. Be [51. 

One family of porphyrin complexes that will be treated in the review, even though they do not contain metal-carbon bonds, are metalloporphyrin hydride and dihydrogen complexes. As in classical organometallic chemistry, hydride complexes play key roles in some reactions involving porphyrins, and the discovery of dihydrogen complexes and their relationship to metal hydrides has been an important advance in the last decade. 

Another example of selective C=C bond hydrogenation has arisen from mechanistic studies on an iron m-hydride dihydrogen complex, [Fe(PP3)(FI)(H2)](BF4) [PP3 = P(CH2CH2PPh2)3], a catalyst inactive with alkene substrates. Scheme 6 shows that no decoordination of dihydrogen is required in any step of the cycle and that the vacant site is created by unfastening of one of the P-donor atoms (species (16)).50 Extensive studies on catalytic alkene hydrogenation by analogous tripodal (polyphosphine) Rh, Os, and Ir complexes have been carried by Bianchini and co-workers.51,52... 

A ruthenium dihydrogen complex G or a ruthenacycle D, which was proposed as a potential intermediate, catalyzed the insertion of ethylene into sp2-C-H bonds, with TONs reaching 19 after 48 h of reaction and under very mild conditions (room temperature as opposed to the usual 135 °C) (Equation (96)).91,91a91c... 

Dihydrogen Complexes and Non-Classical Hydrogen Bonding in Catalysis... 

Schemes 3.2 and 3.3 show intermediates containing dihydrogen ligands with the H-H bond intact. It has only been appreciated since the discovery of the first dihydrogen complexes by Kubas and coworkers in 1984 [14] that such complexes are key intermediates in catalytic cycles [11, 13, 14].

The fact that metal hydrides can be acidic may seem paradoxical in view of the nomenclature that insists that all complexes with a M-H bond be referred to as hydrides regardless of whether their reactivity is hydridic or not. Not only can some metal hydrides donate a proton, but some can be remarkably acidic. Some cationic dihydrogen complexes are sufficiently acidic to protonate Et20 [8], and some dicationic ruthenium complexes have an acidity comparable to or exceeding that of HOTf [9],... 

TABLE 3.1. Structural and Electronic Properties of H-H and M-H Bonds in the W and Ir Dihydrogen Complexes Shown in Eigure 3.5... 

Figure 3.17 Electron density at the bond critical point (in an) versus intermolecular distance (in A) obtained for hydrogen-bonded (HB) and dihydrogen-bonded (DHB) complexes. Solid circles and triangles correspond to B3LYP calculations. Open circles and triangles represent MP2 calculations. The solid and dashed lines are fittings to the exponential function for B3LYP and MP2, respectively. (Reproduced with permission from ref. 31.)...

The effects, caused by methods of calculations, on energies of dihydrogen bonding in complexes of Li and Na corrected for BSSE are shown in Table 6.1. It is clearly seen that only the MP2/6-31G level gives remarkable deviations. On the other hand, it has been found that the MP2/6-311-H-G method is sufficient to describe dihydrogen-bonded complexes. 

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