Equilibrium with dihydride

Calculated from and bracketed by the empirical relationships, dm -- In equilibrium with dihydride tautomer in solution. 

The dihydride complexes [RhH2SxLn]+ are in equilibrium with monohydride... 

Some years later, para-enriched dihydrogen experiments indicated the existence of a non-detectable amount of a dihydride in equilibrium with [RhP2S2]+ species [69]. 

The acidification of H2 may also be involved in hydrogenase action, where H2 is beheved to bind to an Fe(II) center. Isotope exchange between H2 and D2O is catalyzed by the enzyme see Nickel Enzymes Cofactors Nickel Models of Protein Active Sites Iron-Sulfur Proteins). Similar isotope exchange can also occur in H2 complexes. Oxidative addition to give a classical dihydride is also a common reaction. [W(H2)(CO)3(PCy3)2] is in equilibrium with about 20% of the dihydride in solution. This can lead to subsequent hydrogenolysis of M-C bonds as in the case of a cyclometallated phenylpyridine complex of Ir(III). ... 

The nonclassical dihydrogen complex W(CO)3(PPr 3)2(H2) is in a slow and reversible equilibrium with the classical dihydride complex W(CO)3(PPr 3)2(H)2 (eq. (2)) which is seven-coordinate and stereochemically nonrigid. The activation energy for this oxidative addition is 16 kcal/mol [1, 22-24]. 

To effectively model the asymmetric hydrogenation reaction, we must look at the mechanism carefully. The first step involves the displacement of solvent and the coordination of the enamide to produce the two diastereomers (Fig. 3) (17-20). It appears as though the enamide-coordinated diastereomers are in rapid equilibrium with each other through the solvento species (Fig. 4). This square planar rhodium(I) cation is then attacked by dihydrogen to form an octahedral rhodium(III) complex (Fig. 4). Hydrogen then inserts into the Rh-C bonds, and the product is reductively eliminated (Fig. 4). From a molecular mechanics standpoint we have three entities to model the square planar rhodium(I) solvento species and the two intermediates (square pyramidal dihydrogen complex and the octahedral dihydride). 

In 1983, the first H2 complex, W(CO)3(P Pr3)2(i/2-H2) (1), not only was isolated and characterized but later shown to exist in dynamic equilibrium with a dihydride form (2) in solution. The tautomeric relationship shown in Eq. (1.5) is extremely... 

Parahydrogen induced polarization ( )62 is useful in the detection of short-lived reaction intermediates in hydrogenation processes and is potentially useful in studying reactions of H2 ligands. Apparently because of the rapid equilibrium with the dihydride tautomer in Eq. (5.2) the effect is not seen for pare-H2 addition to W(CO)3(PR3)2 or Mo(CO)(diphosphine)2 systems.10 However, it provides crucial evidence in experiments demonstrating that i/2-H2 can react directly with substrates bound to the same metal center without first undergoing OA (Section 9.5.1). The Mo-norbomadiene (NBD) complex in Eq. (5.10) forms an unstable H2 complex on photolysis, which is proposed to directly transfer H2 to NBD to form norbomene (NBN).63 In situ NMR studies of the photocatalyzed... 

ReH4(CO)(PMe2Ph)3]+ is the only example of a pentagonal bipyramidal dihydrogen-dihydride complex (5) in equilibrium with a dodecahedral tetrahydride complex (6).13 In this case 5 is at a lower concentration than 6, but [5] increases at... 

Related chemistry has been seen with Tp PtPhH2. Protonation of one arm of the Tp ligand induces reversible reductive coupling of phenyl and hydride to give an t7 -C6H6 complex (Equation (13)). This species was found to be in rapid equilibrium with the Pt(iv) phenyl dihydride complex. Addition of 8(0565)3 to the methyl dihydride induces loss of methane and activation of benzene solvent. ... 

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