Nonclassical bonds

From the previous discussion, we can conclude that when the observed value of /(H-Si) is relatively large, say, in the range 70-160 Hz, it is safe to think that the Si H bond is involved in nonclassical bonding with metal. In contrast, any conclusion on the presence or absence of a Si-H interaction on the basis of a small value of silicon-proton coupling constant in the absence of an independent evidence can be erroneous. In this case, the measurement or calculation of the sign of /(Si-H) can help to identify a direct Si H interaction. 

Further oxidation of the nine-atom clusters to formal [Ge9] leads to linear polymers oi[-Ge9-] with two covalent intercluster bonds (Fig. 2i). Trimers [Ge9=Ge9=Ge9] (Fig. 6a) and tetramers [Ge9=Ge9=Ge9=Ge9] (Fig. 6b) occur via nonclassical bond formation between two neighboring atoms of the triangular prism basis planes of the c/oio-shaped clusters, which results in Ge-Ge-Ge bond angles of 90° and in considerably longer Ge-Ge contacts between the cluster units. Quantum-chemical calculations have shown that the exo-bonds participate in a delocalized electronic system that comprises the whole anion [204]. 

Adamantonium Ions. There exists a single theoretical study for ada-mantonium ions.850 Mota and co-workers have found three isomeric structures (490-492) and two van der Waals complexes (1-adamantyl cation+ H2 and 2-adamantyl cation + H2) [MP2(full)/6-31G level]. The C—H bond lengths in the 3c-2e interactions in ion 490 are 1.276 and 1.266 A and in ion 491 are 1.266 and 1.280 A. The C-adamantonium ion (492) has nonequivalent C—H bond distances (1.191 and 1.294 A) and the C—C bond distance is 2.348 A. Interestingly, the 2-adamantyl cation + H2 complex shows a nonclassical bonding nature similar to the... 

The E4 + cations can also be considered as electron-deficient arachno clusters (see Sections). The Sg + and Seg + clusters are related to the neutral E rings by removal of two electrons and formation of transannular E-E bonds. The six-atom Ee + clusters illustrate nicely the contrast between classical and nonclassical bonding situations. The hypothetical electron-precise cluster (3n electrons) would be trigonal prismatic Ee +. Addition of four electrons to give... 

E. Relative Stren0hs of Nonclassical Bonding Interactions in Cp2Zr(R)(L) Complexes... 

Some carboranes have formulae compatible with both classically bonded and nonclassically bonded structures and exhibit valence isomerism. The tet-racarbaborane Me4C4B6Et6 is one such. As prepared by dimerization of the small closo carborane Me2C2B3Et3 [Eq. (3.2)], it has a classical adamantane-type structure, with its four CMe units linked through BEt units (1). However, when heated it isomerizes to the expected nido carborane structure (2) with a skeletal structure like that of decaborane. ... 

From a classical viewpoint, the formation of any complexes of saturated hydrocarbons is still difficult to imagine due to the lack of n- or electrons in their molecules. However, compounds containing hydrogen and other three-center bonds, for example B H B or A1 CH3 Al, are known (see discussions of hydrogen and other nonclassical bonds [4]). Fairly stable complexes of molecular hydrogen have been prepared and thoroughly investigated recently (see, for example, [5]). If these data are taken into account, one can foresee the possibility of the existence of complexes of alkanes. 

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