Non-classical bonds

Two structures are possible for the interaction of aromatic hydrocarbons with acids.270 In the a-structures a covalent bond is established between the acidic reagent and a particular carbon atom of the benzene ring. The a-structures are essentially classical carbonium ions. In the -structures a non-classical bond is established, not to any particular atom, but to the -electron cloud in general. It is quite likely that both types of structure are represented by actual examples. Thus m-xylene interacts more strongly with hydrogen chloride than does o-xylene, but the difference between the two hydrocarbons is much more pronounced when their interactions with a boron trifluoride-hydrogen fluoride mixture are compared. This is readily understandable... 

Although this non-classical bonding situation is highly unusual, catalyst 55 represents an extremely interesting catalyst class as it exhibits high selectivities whilst being extremely easy to prepare (Scheme 23) [178],... 

Homoatomic sulfur cation are a fascinating class of compounds that maximize non-classical bonding in their solid state structures. Synthesis as well as quantum chemical calculations of these species are difficult but yet possible tasks and their structures and bonding interactions are now well understood. Structure and bonding of the in the solid state characterized sulfur... 

Non-classically bonded compounds. There are many compounds in which metal-to-carbon bonding cannot be explained as either ionic or covalent in the simple sense of a 2c-2e M—C bond or bonds. The largest and most important class of these non-classical molecules comprises those formed primarily by the transition elements in which unsaturated groups are attached to metal atoms by interaction of the n electrons with metal orbitals. These include simple metal—alkene complexes of the types (10-XX) and (10-XXI) as well as those in which cyclopentadienyl and benzene rings are... 

In the heterolytic splitting of dihydrogen, the acidic dihydrogen complex [M](ti2-H2) transfers a proton to the base (eq 2-4 of the introduction). The proton can then remain associated with the hydride in a 1.7-2.0 A contact in the ion-pair -[MJH—HB+. This may be an intramolecular interaction or an intermolecular interaction. In the following sections, our work on intramolecular hydridic-pro-tonic bonds will be reviewed first, followed by that on such intermolecular non-classical bonds. 

Furthermore, natural bond orbital (NBO) analysis of the first-order density has also been used to quantify aromaticity [73,74]. More recently Boldyrev and Zubarev [75] developed the adaptive natural density partitioning (AdNDP) algorithm attempting to combine the ideas of Lewis theory and aromaticity. The results obtained by the application of the AdNDP algorithm to the systems with non-classical bonding can be readily interpreted from the point of view of aromatic-ity/antiaromaticity concepts. 

The term homoaromaticity, introduced by Winstein et al. in 1959, describes a type of aromaticity in which conjugation is interrupted by an sp -hybridized carbon atom [1]. Since then, the concept of homoaromaticity has attracted much attention both theoretically and experimentally, focusing on the non-classical bonding mode and new chemical reactions. Although many cationic species and a few anionic species have been confirmed to be homoaromatic, whether or not neutral species can be homoaromatic is still a matter of debate. Thus, the establishment of experimental models for potential neutral homoaromatic molecules has long been an exciting pursuit in synthetic and theoretical chemistry. 

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