Hydrogen three-center

In the case of hydrogen molecule, the term (ri l/ r + R ri2), which involves three centers, does not show up in the calculation. We will not discuss this integral in the present work. 

Fig. 5. Modes of M—H—B bonding where M—H—B represents a three-center hydrogen bridge bond for (a), (b), (c) tetrahydroborates and for (d), (e), (f)...

The valence theory (4) includes both types of three-center bonds shown as well as normal two-center, B—B and B—H, bonds. For example, one resonance stmcture of pentaborane(9) is given in projection in Figure 6. An octet of electrons about each boron atom is attained only if three-center bonds are used in addition to two-center bonds. In many cases involving boron hydrides the valence stmcture can be deduced. First, the total number of orbitals and valence electrons available for bonding are determined. Next, the B—H and B—H—B bonds are accounted for. Finally, the remaining orbitals and valence electrons are used in framework bonding. Alternative placements of hydrogen atoms require different valence stmctures. 

In certain cases. X-ray crystallography has shown that a single H—A can form simultaneous hydrogen bonds with two B atoms bifurcated or three-center hydrogen bonds). An example is an adduct (1) formed from pentane-2,4-dione (in its... 

Figure 6.38. Potential energy diagram for the hydrogenation of ethylene to the ethyl (C2H5) intermediate on a palladium(m) surface. The zero of energy has been set at that of an adsorbed H atom, (a) Situation at low coverage ethylene adsorbed in the relatively stable di-cr bonded mode, in which the two carbon atoms bind to two metal atoms. In the three-centered transition state, hydrogen and carbon bind to the same metal atom, which leads to a considerable increase in the energy...

However, our discussion so far applies to low coverages, as is usually the case in kinetic modeling. With highly covered surfaces another mechanism prevails, which offers an alternative to the energetically unfavorable three-centered transition state of Fig. 6.39(a). At higher coverages, ethylene and hydrogen are forced closer and... 

Figure 8.7 Diborane, BaH. (a) Contour map of pb in the plane of the terminal hydrogens, (b) Contour map of pb in the plane of the bridging hydrogens, (c) Calculated geometry, (d) Experimental geometry. (e) Interatomic H-H distances, (f) Ionic model, (g) Resonance structures, (h) Protonated doublebond model, (i) VSEPR domain model showing the two three-center, two-electron bridging domains, (j) Hybrid orbital model.

In each of the B-H-B bridges, only two electrons bond the three atoms together by having the orbitals on the boron atoms simultaneously overlap the hydrogen Is orbital. A bond of this type is known as a two-electron three-center bond. In terms of molecular orbitals, the bonding can be described as the combination of two boron orbitals and one hydrogen orbital to produce three molecular orbitals, of which only the one of lowest energy is populated ... 

Bonding of this type and other boron hydrides that have three-center two-electron bonds with hydrogen bridges is discussed in Chapter 13. 

The combination of boron and hydrogen orbitals in the three-center bond can be shown in a molecular orbital diagram as in Figure 13.2. Using this approach to bonding, the structures of some of the... 

Hydrogen bonds may be considered special types of 3c/4e interactions, closely related to other forms of hypervalency in main-group (Section 3.5) and d-block (Section 4.6) compounds. However, the fundamental nB— oah interaction of B - HA hydrogen bonding displays unusual characteristics compared with other three-center MO phenomena, due mainly to the unique properties of the H atom, whose valence shell contains only the isotropic Is orbital for construction of ctah and ctah NBOs. 

Each B-H-B three-center two-electron bridge bond corresponds to a filled three-center localized bonding orbital requiring the hydrogen orbital and one hybrid orbital from each boron atom. 

Complex 1 1 is considered the only complex present, but the hydrogen bond may be either two (76) or three center (77). Nitroenamines are more prone to complex with 4-fluorophenol than the nitroanilines and they form the strongest hydrogen bond presently known for nitro-bases. In particular, l-piperidino-2-nitroethylene (78) and 1-dimethylamino-2-nitroethylene (79) (both in E form) present a hydrogen bond basicity comparable to that of tributylamine. 

---