Four-coordinate carbon

As the initial paper (164) was concerned only with boron and carbon, those cases wherein the skeletal atoms involved would be more content in different coordination sites and where, therefore, different deltahedra (with differently coordinated vertices) would be preferred were not considered. However, Wade has deduced that (CO)3CrC6H8 is a nido compound (four-coordinate carbon and nine-coordinate chromium)... 

The carbons (rule 3) within the carboranes are found in four-, five-, and six-coordinate situations, never over 50% above the normal four-coordinate carbon. 

The intrinsic stability of the aromatic n system has two major consequences for the course of reactions involving it directly. First, the aromatic ring is less susceptible to electrophilic, nucleophilic, and free-radical attack compared to molecules containing acyclic conjugated n systems. Thus, reaction conditions are usually more severe than would normally be required for parallel reactions of simple olefins. Second, there is a propensity to eject a substituent from the tetrahedral center of the intermediate in such a way as to reestablish the neutral (An + 2)-electron system. Thus, the reaction is two step, an endothermic first step resulting in a four-coordinate carbon atom and an exothermic second step, mechanistically the reverse of the first, in which a group is ejected. The dominant course is therefore a substitution reaction rather than an addition. 

The crux of organic mechanistic stereochemistry may be the Walden inversion, the inversion of stereochemistry about a four-coordinate carbon atom by nucleophilic attack of, for example, a hydroxide ion on an alkyl halide. Many reactions of inorganic molecules follow the same mechanism. In contrast, the dissociative mechanism of tertiary halides to form tertiary carbocatanion intermediates is essentially unknown among the nonmetallic elements silicon, germanium, phosphorus, etc. The reason for this is the generally lower stability of species with coordination numbers of less than 4, together with an increased stability of five-coordinate intermediates. This difference is attributable to the presence of d orbitals in the heavier elements (Chapter 18). 

Figure 14.3.7 shows some organic molecules containing abnormally short single bonds between two four-coordinate carbon atoms. 

Miietierties has very elegantly demonstrated the interconversion of a carbidic four-coordinated carbon atom into and clusters (39, 41 j. llis... 

Displacement inversion of stereocbemislry about a four-coordinate carbon atom by nucleophilic... 

Crystal structures have been reported for 2,6-dimethoxyphenyllithium, for 2,6-dimethylaminophe-nyllithium and for o-r-butylthiophenyllithium. The crystal structure of the latter compound is characterized diagrammatically as the infinite polymer (121) with relatively planar tetracoor nation at the ipso carbon. In THE solution this polymer dissociates into monomers. Planar four-coordinate carbons are dso observed in the 2,6-dimethoxyphenyl anion (122) as a dimeric unit (123) which forms the basic building block of the solid of this anion. In this solid two of these simple dimers (123) combine to form... 

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... 

Further examples of mixed-metal organometallies containing planar, four-coordinate carbon (marked ) have appeared, building on the initial advances reported last year. 

The Ti -aiyne complex Cp2Zr(PMe3)( n2-C6H4) reacts with two equivalents of AlHBu 2 to form complex 28 containing a planar four coordinate carbon atom. The corresponding reaction with A1Mc3 affords complex... 

Figure 3.2 Example molecules based on a four-coordinate carbon atom trichlorometbane (chloroform) has a C3 axis and dichloromethane has only a C2 axis.

A rich source of hypothetical four-coordinate carbon allotropes is to be found in the known structures of inorganic four-coordinate compounds, especially silica and the silicates and zeolites. These generate perfectly reasonable, but rather open, not dense, carbon networks. 

Complexes like 139 have been prepared and contain planar four-coordinate carbon atoms." " The zwitterionic early-late transition metal bimetallic species 140 has been synthesised and characterised. Little information about the structure, other than connectivity, could be inferred due to the poor nature of the crystallographic data." Heterobimetallic compounds such as 141 have been synthesised and fully characterised. Strong negative solvatachroism was observed along with high first-order hyperpolarisability The compound 142 has been prepared and crystallographically characterised. The role of planar... 

Schroter P, Gessner VH (2012) Tetrahedral versus planar four-coordinate carbon a sulfonyl-substituted methandiide. Chemistry 18 11223-11227. doi 10.1002/chem.201201369... 

Cooper OJ, Wooles AJ, McMaster J, Lewis W, Blake AJ, Liddle ST (2010) A monomeric dilithio methandiide with a distorted trans-planar four-coordinate carbon. Angew Chem Int Ed 49 5570-5573. doi 10.1002/anie.201002483... 

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