Carbon atom pentacoordinated

It can be considered to be constituted by two Co6 prisms, each encapsulating a carbon atom, having a common apex (Col) but rotated with respect to each other. The whole assembly is bound, on opposite sides, by a pentacoordinate cobalt atom (Col2 and Col3).ld Clearly, the structural complexity results in inequivalent bond distances. 

Benzylic N-protection (entry 1) is often unsatisfactory since competitive reaction at the exocyclic methylene can occur [84JCS(PI)481 85S302]. As with bis(l-pyrazolyl)methane (Section I1,C,1)(83T4133), the nature of the isolated product is determined by the type of electrophile used, with benzyl halides and, to a certain extent, iodomethane reacting at the A-benzyl carbon atom, whereas most other electrophiles give rise to the normal C-2 alkylated products (90JHC673). This result was interpreted in terms of steric rather than thermodynamic factors, since the two species that reacted at the benzylic carbon required a bulkier pentacoordinate transition state, whereas all of the others were able to react via a lower coordinate species (90JHC673). 

According to most models proposed for polymerisation with vanadium-based homogeneous Ziegier-Natta catalysts such as VCI4—A1R2C1, the active site may involve pentacoordinated three-valent vanadium species with three Cl atoms, the secondary carbon atom of the last monomer unit inserted (predominantly by 2,1 enchainment of the a-olefin) and the coordinating olefin [323-327]. However, a model of the active site that assumes a hexacoordinated V(III) species with four Cl atoms, the carbon atom of the last chain unit and the coordinating olefin has also been proposed [328],... 

In the model proposed for the active centre of the propylene syndiospecific polymerisation, the V(III) atom is pentacoordinated [327]. Its ligands include three chlorine atoms (two of which are bridge-bonded to the aluminium atom), the chiral carbon atom of the last monomer unit of the growing chain and the coordinated propylene molecule. Prior to its coordination and after its insertion, the vanadium atom is tetracoordinated. In the alternative similar model, two chlorine atoms are substituted by a bidentate dionate, and the chlorine atom is bridge-bonded to the aluminium atom in dimeric A1R2C1 [2]. 

The results of studies concerning reactions between nucleophiles and trimethyl phosphate and trimethyl phosphite should be mentioned briefly. In both esters the nucleophiles may attack the phosphorus centre to react in this way via pentacoordinate and tetrahedral type species, respectively, to products. Various nucleophiles, such as F-, CD30 , DNO , OH and NH2 have been allowed to react with trimethyl phosphate, but only OH and NH have been found to attack the phosphorus centre to a minor extent. The most prominent process is nucleophilic attack on one of the carbon atoms to generate by an SN2 reaction the phosphate diester anion as... 

The MINDO/3 calculations [215] on (C4H9)+ considered the 2-butyl cation decomposing to the 1-methylallyl cation and hydrogen. The transition state contained a pentacoordinated carbon atom and the loss of H2 from the transition state formally resembled a 1,1 elimination. The large proportion of the reverse critical energy observed to be released as translation was attributed to repulsive forces and tightness of the transition state. 

The Ge atom in 154 is pentacoordinated due to intramolecular N —Ge coordination and it has a distorted TBP arrangement at the germanium center, with three equatorial carbon atoms and axial chlorine and nitrogen. The axial bond lengths (Table 36) are significantly longer than the sum of the covalent radii for these elements (Table 1). 

In most stannocanes the pentacoordination state of the tin atom is retained in solution. This was confirmed by the high-field shifts of the signals of these compounds in the Sn NMR spectra compared to model tetracoordinated compounds. This is confirmed by the increase in the coupling constants 7( Sn—for the equatorially bonded carbon atoms in a TBP arrangement as in 4,6-dicarbastannocanes and also by the coupling constants 7( Sn—D) (D = (Tables 60 and 61). NMR studies not only answer... 

Overall, the gaseous Sn2 reaction shown in Fig. 5-4 consists of a collision step, which generates reactant ion-dipole complexes (b), followed by a chemical activation step forming activated complexes (c) that contain pentacoordinated carbon atoms. Once the transition state has been surmounted, product ion-dipole complexes (d) are formed, the dissociation of which yields the products (e). 

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