The congeners of benzene and their isomers

In a recent comprehensive theoretical study of all the possible combinations of silaben-zenes, C Si6-nH6 (n = 1-6), Baldridge, Uzan and Martin380 found (using basis sets of 

TABLE 35. Relative energies (kcal mol ) of MeHe isomers  

M I 6h M6H6 (79) 3d M6H6 (80)c Metallaprismane 81 Metalla-Dewar-benzene (82) Metalla-benzvalene (83) 

It is interesting to note that the energy of the metalla-Dewar-benzene (82) and the metallabenzvalene (83) relative to 79 changes relatively little as a function of M and they are only moderately (by up to 11 kcalmol-1) more stable than 79 196. 

The fact that many metallaprismanes24,240,382 and metalla-Dewar-benzenes240 have been synthesized, but that not even a single M6R6 metalla-analog of benzene has been isolated, is consistent with theoretical predictions regarding their relative stabilities. 

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