Bridging hydrogen atoms

By ab initio MO and density functional theoretical (DPT) calculations it has been shown that the branched isomers of the sulfanes are local minima on the particular potential energy hypersurface. In the case of disulfane the thiosulfoxide isomer H2S=S of Cg symmetry is by 138 kj mol less stable than the chain-like molecule of C2 symmetry at the QCISD(T)/6-31+G // MP2/6-31G level of theory at 0 K [49]. At the MP2/6-311G //MP2/6-3110 level the energy difference is 143 kJ mol" and the activation energy for the isomerization is 210 kJ mol at 0 K [50]. Somewhat smaller values (117/195 kJ mor ) have been calculated with the more elaborate CCSD(T)/ ANO-L method [50]. The high barrier of ca. 80 kJ mol" for the isomerization of the pyramidal H2S=S back to the screw-like disulfane structure means that the thiosulfoxide, once it has been formed, will not decompose in an unimolecular reaction at low temperature, e.g., in a matrix-isolation experiment. The transition state structure is characterized by a hydrogen atom bridging the two sulfur atoms. 

One remarkable derivative of these cube-like clusters was isolated with compound 9, in which one vertex of the Alx cube remained unoccupied [18]. It was obtained by a reaction similar to Eq. (2), however, a slightly better steric shielding was achieved by ethyl groups instead of methyl substituents attached to aluminum [Eq. (5)]. Orange-red crystals of 9 were isolated in 36% yield. It may be described as the product of an incomplete hydroalumination reaction and contains seven aluminum atoms, a hydrogen atom bridging only three metal atoms and two alkenyl groups with C=C double bonds that are located at the open face of the cluster. As... 

The compound Ru4(jt-H)4(CO)12 is obtained as a yellow air-stable powder, which is soluble in most organic solvents, but insoluble in water. The IR spectrum contains v(CO) bands at 2081 (s), 2067 (vs), 2030 (m), 2024(s), and 2009(w)cm-1 (cyclohexane solution) the HNMR spectrum has a resonance at <5 — 17.98 (CDC13 solution). The molecular structure of Ru4(/i-H)4(CO)12 has been determined by X-ray diffraction the four hydrogen atoms bridge the edges of the tetrahedral Ru4 core in a D2d arrangement, while three CO ligands are terminally bonded to each ruthenium.8 The deuterated complex Ru4(/i-D)4(CO)12 can be prepared in the same way if D2 is used in place of H2.6... 

Another important class of bridged silylene complexes that should be handled separately is complexes that have hydrogen atoms bridging the metal-silylene bonds C. This bonding can be regarded as a metal-hydrogen-silicon 3c-2e bond, or the r -coordination of Si-H tr-bonds to metals that has been well established for mononuclear complexes.1011 These C-type complexes are now considered to play important roles in the formation of A- or B-type complexes. 

Cobaloximes with structure 9 have a BF2 bridge rather than the more usual hydrogen atom bridge between the two dioxime moieties. Such a modification leads to better stability of the cobalt11 oximes toward oxidation by oxygen by air. While of little consequence in an academic laboratory, it is impor-... 

The next figure shows examples of hydrogen bonding. The hydrogen atom bridges two, small,... 

Each hydrogen atom not yet accounted for (in this case 2 H atoms, 13 - 11 = 2) contributes one electron to framework bonding regardless of whether these additional hydrogen atoms bridge two boron atoms or participate in a B—H bond as part of... 

Eiiborane is a dimer of borane, BH3. The bonding in diborane is unusual because the hydrogen atoms bridge the two boron atoms with the two monomeric BH3 subunits being bound by two-electron, three-center bonds. Because the boron atom in borane possesses an empty p-orbital, borane is a Lewis acid, and it forms stable complexes upon reaction with tetrahydrofuran (THF) and other ethers, which function as Lewis bases, as illustrated by the formation of a borane-THF complex (Eq. 10.27). 

Figure 1 Structure of the anion [Zn(BioHi2)2] showing the positions of the zinc and boron atoms only each boron has one hydrogen atom attached and each cluster has two hydrogen atoms bridging the...

---