Carbon atom hypercoordinate

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. 

Hypercarbon compounds contain one or more hypercoordinated carbon atoms bound not only by 2e-2c but also 2e-3c (or >3c) bonds. 

The CH cation 1, protonated methane, is the parent of hypercoordinated carbocations containing a five coordinated carbon atom. It is elusive in solution and has not been observed by NMR spectroscopy but gas-phase infrared investigations have shown its fluxional structure which has been proven by ab initio molecular dynamic simulation.18... 

The results indicate that the formation of long-lived trimethyl substituted silyl cations, in the presence of aromatic solvents, as claimed by Lambert et al.95 is not feasible under these conditions. Persistent silicenium ions require sterically more shielding substituents at silicon or hypercoordinative stabilization.96 98 13C and 29Si NMR chemical shifts were calculated for a series of disilylated arenium ions 85 using density functional theory (DFT). The calculations predict consistently the unsaturated carbon atoms to be too deshielded by 8-15 ppm. Applying an empirical correction, the deviation between experiment and theory was reduced to -0.4 to 9 ppm, and the 13C NMR chemical shift of the highly diagnostic cipso is reproduced by the calculations (Ad = -3.8 to 2.7 ppm).99... 

A5 C(C1) = —29.8, (A5 C(C1) = —54.5) are observed and a relatively large /(C2H) coupling constant of 165.9 Hz is detected. This counter-intuitive low-frequency shift of the C NMR resonance of Cl and C2 as well as the large scalar CH coupling constant was rationalized for similar bishomoaromatic carbon cations like the 7-norbornenyl cation, 79, by the hypercoordinated nature of the vinylic C atoms and was put forward as spectroscopic evidence for bishomoaromaticity. " ... 

Decamethylsilicocene (82) can be regarded as an electron-rich silicon(II) compound containing a hypercoordinated silicon atom. The chemistry of 82 is determined by (a) the nucleophilicity of the silicon lone-pair (cr-donor function towards electrophiles, oxidative-addition processes) and (b) the weakness of the silicon-(cyclopentadienyl)carbon jr-bond rearrangement, Si—C bond cleavage). In the following section, the chem-... 

The much more highly charged silicon atom can interact far more readily with nucleophiles. Silyl cations may even be complexed simultaneously and symmetrically by two electron pair donors (hypercoordination), in contrast to carbocations. With ammonia, the methyl cation gives the very stable protonated methyl amine, H3C-NH3 a second ammonia molecule is only weakly bound to this complex. If both NH3 groups are forced to be equidistant from carbon, a Sn2 transition state results, 20 kcal mol" higher in energy than the minimum. 

Tithiated dicarbaborane clusters yield (organo)gold-substituted clusters on reaction with T AuX or R2AUX compounds, also with hypercoordinate carbon atoms (equations 36-38)." " ... 

Compound (15) also exhibits a 2-centre, le bond between P atoms and again the bond is long (about 2.76 A), while the unusual biradicaloid (16) is thermally stable up to 150°C. The constraints placed upon the phosphorus atoms in compound (17) result in hypercoordination of the phosphorus adjacent to the oxygen, while (18) constitutes the first cationic phosphorus-carbon cluster, namely nido-[3,5- Bu2-1,2,4-C2P3]. All such observations show great promise for future development. 

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. 

To show how the study of hypercarbon compounds helps us to understand the mechanisms of many organic reactions, reactions in which carbon atoms become temporarily hypercoordinated in intermediates or transition states even though the reagents and products contain only normally coordinated carbon atoms. 

The carbon atoms of most binary metal carbides have hypercoordinated environments like those shown in Figure 1.5. In particular, octahedral carbon coordination is common in the interstitial carbides formed by many transition metals, materials of variable composition in which carbon atoms... 

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... 

Hie involvement of hypercoordinated carbon species in Sn2 reactions was commented on in Section 1.4 (Fig. 1.10). Compounds have been synthesized that keep the displaced and displacing atoms close to the carbon atom undergoing nucleophilic substitution, in order that the relative energies of the classically bonded reagent or product and the hypercoordinated transition state can be both more readily assessed and modihed (see Chapter 6). 

The many reactions that involve insertion of alkenes or alkynes into metal-carbon or metal-hydrogen bonds provide further examples of hypercoordination of carbon atoms during reactions. For example, an alkene may coordinate to the coordinatively unsaturated metal atom of a metal hydride complex prior to inserting into the metal-hydrogen bond [Eq. (1.9)] ... 

Exclusive hypercoordination of all of the metal-attached carbon atoms is... 

Although their hydrogen atoms were not located, their relatively short metal-metal distances and acute M-C-M angles at the hypercoordinated carbon atoms show the metal-carbon bonding to resemble that in A Mce discussed previously. This resemblance to the aluminum system is underlined by the structure of the mixed metal methyl Mg(AlMe4)2 (28), also established by an X-ray study. ... 

In all of these systems, the metal-carbon distances involving hypercoordinated carbon atoms are significantly longer than those involving the four-coordinate carbon atoms of the terminal alkyl groups (monomeric BMe2 has a Be-C distance of 1.70 A as shown by an electron diffraction study of the vapor, while two-center Mg-C bonds are typically about 2.16-2.17 A in... 

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. 

Rather less symmetrical tetrameric (LiEt)4 molecules have been found (by X-ray diffraction ) in crystalline ethyUithium, again held together by hypercoordinate carbon atoms forming four-center bonds to three neighboring metal atoms located 2.19-2.47 A distant. The Li—Li distances range from 2.42 to 2.63 A and the Li-C-Li angles range from 66° to 67°. 

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. 

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