Dihydrogen bonding unconventional

The H H interactions (1.75-1.9 A) here and related systems are referred to as proton-hydride bonding by Morris (60-62) and dihydrogen bonding by Crabtree (63-65), who along with others (66-68) have studied or reviewed such unconventional hydrogen bonds that include M-H H-M, M-H H-X, and X-H a interactions in general (X = C, N, P, 0, etc). These complexes represent intermediates in the heterolytic splitting of H2 and illustrate both the basicity of the M-H... 

Dihydrogen complexes (5) such as [Cr(H2)(CO)5] are covered in a separate article. Dihydrogen bonding, the tendency of an MH bond to act as proton acceptor in an unconventional hydrogen bond with an OH or NH group, is also covered in a separate article, see Hydrogen Bonding). ... 

In this work, the unconventional and weak hydrogen bonds that involve transition elements, namely the dihydrogen bond (symmetric and asymmetric) and the X-H- - -M interactions, will be described and interpreted. More attention will be given to recent results, following an earlier publication [14]. The structures described were taken from the Cambridge Crystallographic Database [15]. 

This is essentially the reverse of the protonation reaction commonly used to synthesize H2 complexes (all the reactions in Scheme 20.7 can be reversible). The intermediate here, M-H H-A, is held together by dihydrogen-bonding, also known as proton-hydride bonding, where the M-H hydrogen is hydridic and the HA hydrogen is protonic [5e, 32]. The unconventional hydrogen bonds here can... 

The chapter describes the spectral (IR, NMR) criteria for the identification of dihydrogen bond formation and competition with H-bonding to other ligands. The spectral and thermodynamic features of these unconventional H-bonds will be compared with those of the classical H-bonds. A discussion of some peculiarities of the proton transfer process and the first examples of full energy profile determination for protonation reactions leading to dihydrogen complexes will be also presented. 

The similarity between the enthalpy values calculated from (7), and (8) and those from (9) proves that the correlation equations (7) and (8) hold true for the unconventional dihydrogen bonds (Table 1). 

The H- H interactions (1.75-1.9 A) in Equation (8) represent unconventional hydrogen bonding often referred to as proton-hydride bonding or dihydrogen bonding and include M-H- H-X, and X-... 

Proton transfer to transition metal and main group element hydrides is a complicated process. It begins with formation of an unconventional hydrogen bond between a metal hydride (MH) and proton donor (HA), which nowadays is widely called a dihydrogen bond MH HA (Scheme 1). The next reaction step is proton transfer itself yielding non-classical di- or polyhydrides. In some cases classical polyhydrides can be formed without observation of ti -H2 intermediates. In subsequent sections we discuss various aspects of spectroscopic studies of hydrogen bond formation and proton transfer paying particular attention to the use of variable temperature IR spectroscopy. 

Planas, J. G., Vinas, C., Teixidor, F. et al. 2005. Self-assembly of mercaptane-metallacarborane complexes by an unconventional cooperative effect A C-H center dot center dot center dot S-H center dot center dot center dot H-B hydrogen/dihydrogen bond interaction. J. Am. Chem. Soc. 127 15976-15982. 

Crabtree. R.H. Siegbahn. P.E.M. Eisenstein, O. Rheingold. A.L. A new intermolecular interaction Unconventional hydrogen bonds with element-hydride bonds as proton acceptor. Acc. Chem. Res. 1996. 29. 348. Morris, R.H. 1995 Alcan award lecture—New intermediates in the homolytic and heterolytic splitting of dihydrogen. Can. J Chem. 1996. 74, 1907. 

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