Nonclassical pyramidal structure

Ab initio calculations for a model dication (CH)g also support the nonclassical pyramidal structure Of late Hogeveen has corroborated the nonclassical structure of dication 506 with both Saunders isotopic perturbation method and the comparison — after Schleyer et al. — of the sum of all chemical carbon shifts in the NMR spectrum of the ion with that for the neutral hydrocarbons 507 and 508 the calculated differences were 54.6 and 195.7 ppm per charge unit, respectively, typical for nonclassical ions. 

Nonclassical type structures of borides involving different basic units of B atoms, e.g., the B5 pentagonal pyramid in MgB4, the B octahedron in Sm2B3 and finally the B 2 octahedra in Mg2Bi4, MgAlBi4 and NaxBogBi4, are considered here. 

The described dications represent nonclassical ions of a special kind — pyramidal mono- and dications the latter can be regarded as a link to the elementoorganic chemistry e.g., boranes, by the peculiarities of their stereochemistry and of their bond nature. Thus, the pyramidal structure of pentaborane 509 is similar to that of monocation (CH)j 510 with a pentacoordinated carbon while the pyramidal dication (CH) 511 is similar to carborane 512. By the peculiarities of their stereochemistry pyramidal cations are similar to metalloorganic sandwich compounds, e. g., ferrocene 513 the dication 511 is regarded as a semi-sandwich . 

We expect the preferred structure of CHjLi will be the one in which planar CHj radical, with maximally strong C—H overlap bonds, makes an interstitial bond with Li (G). This is much better than having a pyramidal CHj radical, with weakened C—H overlap bonds (due to the greater 2p AO character of the carbon valence AO s form an interstitial bond with Li (F). Schleyer and coworkers have performed calculations which indicate that CHjLi prefers a Dj, geometry rather than the nonclassical geometry preferred by CHjH The antisymmetric interstitial bond ol Dj, LH3Li2 is shown in bond diagralnmatic form in Fig. 41a. 

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