Hypercoordinated carbon

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

Schmidbaur, H., Gabba, F.P., Schier, A. and Riede, J. (1995) Hypercoordinate Carbon in Protonated Tetraauriomethane Molecules. Organometallics, 14, 4969 971. 

Tetragoldmethane complexes are obtained from tetra-(boryl)methane compounds upon reaction with gold-halide complexes in the presence of an ionic fluoride (equation 35)." Tetragoldmethane species, such as (1), can only be isolated with bulky tertiary phosphines, L, which shield the molecule from further attack by [TAu]+ nucleophiles. With smaller ligands T, penta- and hexaauration occurs, leading to hypercoordinate carbon compounds. " " " ... 

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

Distorted Square-pyramidal Hypercoordinated Carbon Compounds... 

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). 

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. 

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. 

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. 

The metal-carbon cluster systems we have considered so far in the present chapter, like the carboranes considered in the previous chapter, have contained one or more skeletal carbon atoms occupying vertex sites on the cluster deltahedron or deltahedral fragment. We now turn to some molecular cluster systems in which hypercoordinated carbon atoms occupy core sites in the middle of metal polyhedra. Most are metal carbonyl carbide clusters of typical formulae Mj (CO)yC. Their carbide carbon atoms are incorporated within polyhedra, which in turn are surrounded by y carbonyl ligands. Such compounds, for which few controlled syntheses are available, have been found primarily among the products of thermal decomposition of polynuclear metal carbonyls Mj (CO)j, their carbide carbon atoms result from disproportionation reactions of carbonyl ligands (2 CO CO2 + C). 

The second class of carbocations contains one or more hypercoordinate carbon atoms. These hypercarbons are coordinated to five or more atoms within reasonable bonding distance. Hypercoordinate or nonclassical carbocations cannot be described by using 2c-2e single bonds alone, but necessitate the involvement of three- (or multi-) center, two-electron bonds, e.g., 3c-2e bonds. Each hypercarbon in the cation is always associated with eight electrons, although the ion, overall, is electron deficient. The methonium ion (Cd I5 ) may be considered the parent of the hypercoordinate carbocations (Fig. 5.1). 

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. 

In the complexes of transition metals, where the metal is coordinatively unsaturated (i.e., it has access to less than 18 electrons in its coordination shell), the metal becomes electron deficient. In the absence of better n-donor and K-donor systems, such coordinatively unsaturated metals can draw electrons from neighboring a bonds to satisfy the electron deficiency of the metal. In fact, many such stable C-H a-bond-inserted complexes containing hypercoordinate carbons are known (also see Chapter There is even a... 

Carbon-Hydrogen Bond Insertion In the early 1960s the activation of alkanes by metal systems was realized from the related development of oxidative addition reactions " " in which low-valent metal complexes inserted into carbon-heteroatom, silicon-hydrogen, and hydrogen-hydrogen bonds. The direct oxidative addition of metals into C-H bonds was found in the cyclometallation reaction [Eq. (6.61)].The reverse process of oxidative addition is called reductive elimination, which involves the same hypercoordinate carbon species. 

Computational studies of the Rh-P,N system have shown that the C-C activation product is the most stable (A < -12kcalmol relative to the C-H activation product) and its formation is fast and irreversible. C-H activation is fast and reversible. When choosing structure 132 as the entry channel, structure 133 is the common intermediate for both C-H and C-C activations and structure 134 is a possible transition state, all having hypercoordinate carbon atoms. 

There were numerous mechanisms proposed for the Ziegler-Natta polymerization. Various valencies have been suggested for the involved titanium (the original catalyst system) ranging form four to two. A popular early concept first suggested by Natta " involved the active catalytic species 161 with hypercoordinate carbon, which is formed between surface T/ ions and the cocatalyst (alkylaluminum). 

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