Hypercoordinate bridging

The structure of the 2-norbornyl cation has been a focal point of controversy in physical organic chemistry. Experimental NMR spectroscopy and computational methods have been the decisive tools, favoring the hypercoordinated symmetric bridged structure 30, a protonated nortricyclane.79 The tricoordinated 2-norbornyl cation 31 is not a local minimum (MP2/6-31G(d)) on the energy surface.80... 

Similarly to the 2-norbornyl cation, comparison of calculated (IGLO/DZ//MP2/ 6-31G(d)) and experimental 13C NMR chemical shifts allowed to differentiate between the hypercoordinated 70 and the trivalent form 71 of the bicyclo[2.1.1.]hexyl cation.85 The experimental (157.8 ppm) and calculated (158.5 ppm) values for Cl and C2 (averaged signal) are reported to be nearly identical for the symmetrically bridged... 

The hydrogen-bridged carbocation (B) contains a two-coordinate hydrogen atom. Hypercoordination— which includes two-coordination for hydrogen and at least five-coordination for carbon—is generally... 

To illustrate the wide and developing scope of hypercarbon chemistry by illustrating the variety of compounds now known to contain hypercarbon atoms (carbocations,organometallics, carboranes, metal-carbon cluster compounds," and metal carbides ).They include bridged metal alkyls such as alkyl-lithium reagents (LiR) in which the hypercoordinated nature of the metal-attached carbon atoms, and the roles that the metal atoms play in their chemistry, are often overlooked. 

For most of the systems discussed so far, hypercoordinated carbon atoms have featured in the most stable forms of the compounds in question. For example, the bridged metal alkyl structures found by X-ray studies on crystalline samples of such substances as (AlMe3)2 ° or (l. iMc)/ persist in solutions of... 

Although the coordination numbers are unexceptional, and strictly do not justify treatment of these systems as examples of hypercoordinate carbon, we shall see that the bonding of their carbon atoms is very similar to that of the hypercoordinate atoms in associated dialkyls, in that three carbon valences are essentially occupied in bonds within the bridging ligand, while the remaining valency is used to form a three-center metal-carbon-metal bond. 

As is the case with alkyl bridges between aluminum atoms, these bridges between beryllium and magnesium atoms are relatively weak, and the metal orbitals are put to better use by addition of Lewis bases (L), which cleave the polymer chains, forming MR2L2 monomeric molecules, in which carbon atoms are no longer hypercoordinated [Eq. (2.8)]. In weakly basic solvents dimers (30) that retain alkyl bridges (and so hypercoordinate carbon atoms) may be formed. 

Again, as in (LiMe)4 (34), the hypercoordinate carbon atom forms three normal two-center bonds within the alkyl group and one multicenter bond to the bridged metal atoms. The molecules of benzene of crystallization are located over the equilateral triangular faces of the Lig antiprism. 

Though technically not hypercoordinate in that they are only four coordinate, the bridging carbon atoms of structure 39 resemble those of 38 in that they use three of their four valences to bond to the neighboring carbon atoms in the ligand, employing the fourth to bond to the two bridged metal atoms (3c-2e). 

For example, the 2-dimethylaminomethyl-5-methylphenyl copper tetramer [CuC6H3(2-CH2NMe2)(5-Me)]4 contains p2-ligands of the type shown in structure 45a and a butterfly-shaped arrangement of its four metal atoms, whereas the ps-ligand environment is found in 2-dimethylaminophenyl copper compounds 45b. In both types of compounds, pairs of copper atoms are bridged by (hypercoordinate) carbon atoms of the type already noted in Al2Me4Ph2... 

In compound 76 and in dimesitylmanganese, which crystallizes as the trimer [Mn(mesityl)2]3 (77) " the degree of association is limited by the bulk of the substituents. All of these systems show the characteristic features of 3c-2c Mn-C-Mn bridge bonding greater Mn-C interatomic distances to the bridging (hypercoordinated) carbon atoms than to their terminal counterparts sensitivity of the metal-carbon distance to the metal coordination number and acute Mn-C-Mn bond angles at the hypercoordinated carbon atoms. 

For many years, a lively controversy centered over the actual existence of nonclassical carbocalions. " The focus of argument was whether nonclassical cations, such as the norbornyl cation, are bona fide delocalized bridged intermediates or merely transition states of rapidly equilibrating carbenium ions. Considerable experimental and theoretical effort has been directed toward resolving this problem. Finally, unequivocal experimental evidence, notably from solution and solid-state C NMR spectroscopy and electron spectroscopy for chemical analysis (ESCA), and even X-ray crystallography, has been obtained supporting the nonclassical carbocation structures that are now recognized as hypercoordinate ions. In the context of hypercarbon compounds, these ions will be reviewed. 

Diese discussed results, together with theoretical calculations where the classical form of 2-norbomyl cation is not even an energy minimum, clearly proves the symmetrically (or very close to symmetrical) bridged structure of the 2-norbornyl cation (126) involving hypercoordinate carbons. 

BRIDGING HYPERCOORDINATE SPECIES WITH DONOR ATOM PARTICIPATION... 

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