Dihydrogen bonds model

Since N-H and O-H bonds are obviously more polarized than C-H bonds, the corresponding dihydrogen bonds can be expected to be stronger. Nevertheless, examples of intramolecular bonding N-H H-B and O-H H-B are still not numerous. One can be found in recent computational modeling of oligomers... 

Figure 5.18 IR spectra in the NH stretching region of cluster 1 with the dihydrogen bond and a model system (HN=CPh2)Os3(CO)n. (Reproduced with permission from ref. 28.)...

Figure 10.7 Arrhenius plot for solid-state proton transfer in dihydrogen-bonded complex LiBFLj triethanolamine. The data are obtained in the framework of the Avrami-Erofeev model. (Reproduced with permission from ref. 14.)...

The metal-dihydrogen bonding is similar to the Dewar-Chatt-Duncanson model for metal-olefin bonding [29-31]. There is a donor bond from the electron pair in the bonding orbital of the H2 ligand into an empty metal orbital (Figure 1, left-hand side). 

THEORETICAL STUDIES OFM(C0)3(PR3)2(H2) AND BONDING MODEL FOR DIHYDROGEN COORDINATION... 

Hydrosilylation reactions are formal additions of Si-H units across multiple bonds. They are fundamental reactions in organosilicon chemistry. Despite early reports of Marinetti and Ricard on Pd-catalysed hydrosilylation of alkenes with phosphetanes (up to 65% ee with styrene) and of Zhang and co-workers on Ru-catalysed hydrosilylation of ketones (up to 54% ee with acetophenone), most of the work on enantioselective hydrosilylation with P-stereogenic ligands has been carried out with Rh(I) complexes and prochiral ketones as substrates. Initially, silyl ethers are formed but they are usually cleaved under acidic conditions affording alcohols. As a result, hydrosilylations of ketones are formally identical to hydrogenations but do not involve the manipulation of dihydrogen. The model substrate for enantioselective hydrosilylation is acetophenone (Scheme 7.17). 

The interactions of the Ir-H... H-X type (X=0, N) were studied eomputa-tionally in models of a neutral organometallie eomplex and two eationie derivatives. From an AIM analysis it follows that a HB between the hydride and the protonic hydrogen is found only in the neutral eomplex. Furthermore the only HBs appeared to be formed between hydrogen atoms and fluorine atoms (of the anion ). The authors pointed out that a short H... H distanee in Ir-H... H-X arrangement is not in itself a diagnostic for a dihydrogen bond. 

Complexes with H—bonds are often called nonclassical hydrides. By the bonding model of Fig. 1.96, we expect that more ir-basic metals will tend to split the H2 and form a dassical dihydiide 3.36, while less tr-basic metals will tend to form the dihydrogen complex, 3.37. Morris has shown how increasing the electron density at the metal favors 337 by looking at the IR... 

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