Penta-coordinated carbon

Tetra- and penta-coordinate carbon-substituted heavy ketones stabilized by the coordination of nitrogen atoms... 

A novel iron carbonyl carbide [Fe5(CO)15C] (41), obtained in small yields from the reaction of dodecacarbonyl iron with pent-l-yne, has been shown to contain a lone penta-coordinated carbon atom surrounded by five iron... 

First, as discussed earlier in connection with the aluminum halide catalyzed rearrangements of hydrocarbons (Section II. A. 2), intermolecular hydride transfer reactions appear to be fairly unselective processes. Apparently, charge development in the transition states of these reactions is minimized a penta-coordinate carbon intermediate may be involved. As a result, the strong preference for the bridgehead positions exhibited by most ionic substitution reactions is partially overcome. 

The study of (C3H7)+ losing H2 found the transition state to be cyclic, (CH2CH2CH3)+, with both of the hydrogen atoms to be eliminated attached to the same (penta-coordinated) carbon atom [744]. The loss of H2 from the transition state resembled a 1, 1 elimination this decomposition has been observed to release most of its reverse critical energy as translational energy. This paper also contains a careful examination of the isotope effects /Hj //Hd/ d2 on metastable ion abundances. 

The carbonium ion is a nonclassical molecule, because it contains a penta-coordinated carbon atom. It is very unstable and is therefore difficult to form. It is formed at temperatures of about 500°C. Once formed it readily decomposes to a carberuum ion by H2 formation or cracking (Fig. 4.58). 

In all these cases, the carbonium ion mechanism (direct H/D exchange via penta-coordinated carbon) can be rejected primarily on the basis of 1) lower activation energies observed, and 2) the observation of rearrangement products due to carbenium ion intermediates. 

G. Olah proposed to name the classical carbocations carbenium ions (cf. and those with penta-coordinated carbon atoms carbonium ions . In keeping with his recommendations the authors d ing with the above ion groups in their English-language publications now use the term arenium ions more often than the one used earlier ( arenonium ions ). 

The interpretation of a transition state with a penta-coordinated carbon atom is very difficult. It was supposed [7] that the orbital of the free electron pair in the hydroxyl ion and the backlap of the C—Br antibonding orbital might be overlapped. The HO—C bond could thus be formed before the C—Br bond was dissolved. Such an overlap is probably difficult for the C—Cl bond. CCI4, CHCI3, CHjClj and CH3CI can only be solvolysed by prolonged contact with water at 180-225 °C or a concentrated alcoholic alkali-hydroxide solution at 100°C. 

Penta- (or higher) coordinate ( nonclassical carbonium ions contain five or (higher) coordinate carbon atoms. They cannot be described by two-electron two-center single bonds alone but also neces-... 

Whether the formation of alkene 3 proceeds directly from alkoxide 4 or via a penta-coordinated silicon-species 6, is not rigorously known. In certain cases—e.g. for /3-hydroxydisilanes (R = SiMes) that were investigated by Hrudlik et al —the experimental findings suggest that formation of the carbon-carbon bond is synchronous to formation of the silicon-oxygen bond ... 

First, we must say a word about the naming of A. For many years these species were called carbonium ions , though it was suggested as long ago as 1902 that this was inappropriate because -onium usually refers to a covalency higher than that of the neutral atom. Nevertheless, the name carbonium ion was well established and created few problems until some years ago, when George Olah and his co-workers found evidence for another type of intermediate in which there is a positive charge at a carbon atom, but in which the formal covalency of the carbon atom is 5 rather than 3. The simplest example is the methanonium ion CHj (see p. 770). Olah proposed that the name carbonium ion be henceforth reserved for penta-coordinated positive... 

The bonding of the three allylic carbon atoms to the metal in these compounds may be regarded in terms of a bidentate ligand. In the dimeric allyl-metal halides, this is consistent with the normal tetra-coordination, and in the cyclopentadienyl-allyl compounds it is interesting to note that the metal atoms are effectively penta-coordinated. Diamagnetism is preserved throughout. 

The chemical shifts of meta and para (but not ortho carbon atoms in monosubstituted benzenes correlate well with the inductive and resonance constants of the substituents " . Examination of and a° constants obtained from NMR data (Table 14) shows that for the groups containing a penta-coordinate silicon atom the cr-donor component is enhanced and the rc-acceptor one is lowered. Similar conclusions have been reached by other measurements e.g., the in-... 

Significant theoretical and experimental efforts to get a planar arrangement of atoms belonging to representative elements, which are included in simple and elaborate molecular entities, appeared in the last decade. Along this line, an interesting structural pattern 16a-r has been presented by the group of Li, where planar central silicon and carbon atoms with tetra-, penta-, hexa-, and heptacoordination numbers were found for the former and tetra- and penta-coordination numbers for the latter <2004JA16227>. 

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