Dihydrido dihydrogen complexes

Scheme 9.7 Formation of flexible CCC pincer Ir(lll) halide complexes and transformation to the dihydrido dihydrogen complex 46. (According to Heinekey and Goldberg [36].)...

From all the above observations, it was concluded that, for diphosphine chelate complexes, the hydrogenation stage occurs after alkene association thus, the unsaturated pathway depicted in Scheme 1.21 was proposed [31 a, c, 74]. The monohydrido-alkyl complex is formed by addition of dihydrogen to the en-amide complex, followed by transfer of a single hydride. Reductive elimination of the product regenerates the active catalysts and restarts the cycle. The monohydrido-alkyl intermediate was also observed and characterized spectroscopically [31c, 75], but the catalyst-substrate-dihydrido complex was not detected. 

Since the transition metal compounds proposed for the catalytic photocleavage of water (2, 3) appear to be inefficient, factors influencing the dihydrogen evolution from cis-dihydrido compounds deserve scrutiny. We know that dihydrogen is readily dissociated thermally from [RhH2(S)2(PPh3)3]+ (S = acetone) (9,10). The complex cannot be formed from water. RhH(PPh)3 lacks a sufficient nucleophilic character to form a cis-dihydro complex by adding water. The question then is how we could release H2 from Complex 2a. When heated to 90°C in dioxane, Complex 2a merely decomposes into an intractable oil irradiation (low-pressure Hg lamp) is also ineffective. 

The two most important mechanisms involve the reaction of an alkene with dihydrido complexes or, the reverse of this, the reaction of dihydrogen with an alkene complex. The third process, known as the alkyl route, is of limited applicability and may give rise to substantial quantities of by-products. 

Several pentacoordinate monohydrido complexes can add solvents are nnsnitable since the hydrido complexes produced an additional, small, nentral ligand to form complexes of the may reduce them with formation of HCl and contaminate type [RhHX2L2L ]. The dihydrido complexes are conveniently the product with monohydrido complexes (see Scheme 16). prepared by the oxidative addition of dihydrogen to rhodium(I) A characteristic stoichiometric reaction of many dihydrides is... 

Photochemical reactions of Ru3(CO)i2 and Os3(CO)i2 with nitrogen hetero-cycles have been studied, with substitution products being formed in most cases. For example, with pyridine the ruthenium compound forms the ortho-metallated complex Ru3H(CO)i i(CsH4N), while the osmium compound gives the simple substitution product Os3(CO)n(py). The novel dihydrido triruthenium cluster H2Ru3(CO)io can be synthesized by photolysis of Ru3(CO)i2 under an atmosphere of dihydrogen. 

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