Hydrogen nonclassical complexes

The concept of hydrogen bonding is constantly evolving from classical hydrogen bonds to nonclassical (or nonconventional) hydrogen-bonded complexes. Here, on the basis of new experimental and theoretical data and new approaches to this problem, the nature of a proton-donor component and a proton-acceptor site is reformulated completely. In addition, experimental criteria that have been used successfully earlier for the detection of hydrogen bonds are also changed. 

Up until now, we have concentrated mainly on complexes with only two hydrogens, but complexes with up to seven are known, and often it is these that show stretched H---H bonds. The factors governing the stabilities of classical versus nonclassical isomers in polyhydride complexes have been extensively studied theoretically by Hall and coworkers, who point out that electron correlation is important in computations but that has been recognized only since 1991.6 133 For example, calculations by Hay in 1992 on ReH7(PH3)2, showed the presence of one H2 ligand with a short dm of 0.80 A, but Cl techniques, which include election correlation, showed that only the classical heptahydride form.134 The actual PPh3 complex... 

Under a hydrogen atmosphere, complex 191 affords 190, which shows a nonclassical interaction between two of the four hydrogen atoms bonded to the osmium atom. In solution, these atoms exchange their positions giving in the H NMR spectra only one resonance, which has T, of 32 ms at -67 KT [77]. 

The complex OsHCl(CO)(P Pr3)2 reacts with HX (X = H, SiEt3, Cl) molecules to give derivatives of the type OsXCl(r)2-H2)(CO)(P Pr3)2 (X = H, SiEt3, Cl), where the hydrogen atoms bonded to the osmium atom undergo nonclassical interaction (Scheme 16). 

The difficulties encountered in firmly establishing the structure of the first di-hydrogen complex still have not been entirely overcome. The H—D coupling constant experiment described above offers a method of distinguishing between the classical and nonclassical forms, but it is not applicable if the system is rapidly fluxional. 

In this case, there is no reason to suppose that hydrogen is coordinated in any way other than the classical dihydride manner (30) see Hydrides Solid State Transition Metal Complexes). However, similar experiments using Cr(CO)5 gave a product Cr(CO)5H2 for which strong circumstantial evidence pointed towards the nonclassical dihydrogen structure (31). Unfortunately, in low-temperature matrices... 

Hydrogen complexes form by reaction of transition metal compounds with molecular hydrogen or by protonation. The hydrogen in a transition metal complex may be bonded in the classical or nonclassical way. The complexes may interconvert, may be deprotonated, or may lose molecular hydrogen, generating vacant coordination sites. Thus, the picture of transition metal hydrogen complexes to-day is one of considerable complexity [1, 20, 24, 32, 33]. 

In spite of the elimination of formic acid in a couple of steps changing the oxidation number of the rhodium metal center from -nl to -i-3 and vice versa, the reaction could take place by an alternative mechanistic pathway via cr-meta-thesis between the coordinated formate unit and the nonclassical bound hydrogen molecule [48, 49]. Initial rate measurements of a complex of the type 13 show that kinetic data are consistent with a mechanism involving a rate-limiting product formation by liberation of formic acid from an intermediate that is formed via two reversible reactions of the actual catalytically active species, first with CO2 and then with H2. The calculations provide a theoretical analysis of the full catalytic cycle of CO2 hydrogenation. From these results s-bond metathesis seems to be an alternative low-energy pathway to a classical oxidative addition/reductive elimination sequence for the reaction of the formate intermediate with dihydrogen [48 a]. 

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