C4H4, tetrahedran

Fig. 28. Substitution of isolobal and isoelectronic CH and M(CO)3 (M = Co or Ir) fragments in tetrahedral clusters. It is interesting that the final member of the series C4H4 (tetrahedrane) has not been isolated.

The analogies are far from perfect. For example, CH dimerises to give acetylene HC CH but the isolobal Co(CO)3 does not dimerise. However, Co(CO)3 does behave as a trivalent radical. The cluster carbonyl Co4(CO)12 (Fig. 8.8) can be seen as analogous to tetrahedrane C4H4 (note, however, that three of the CO groups are bridging). The compounds Co (CO)3 (CR)4 n = 1,2,3) have been prepared, all containing tetrahedral Co C4 clusters. 

Fig, 2.4. Relief map of ihe electronic charge density in the tetrahedrane molecule, C4H4. The plane shown is a 0-4 symmetry plane and contains two carbon nuclei and their associated protons. The charge density at the central critical point is a local minimum with a value of 0.165 au. The two-dimensional maximum in the foreground is the (2, — 2) maximuln in p in the interatomic surface of the out-of-plane carbon nuclei. The value of p at this point is 0.246 au. A contour map of the charge density in the same plane is shown in Fig. 2.10. 

The neutral chloride B4CLt, with four skeletal bond pairs, and tetrahedrane C4H4 or tetraphosphorus P4 and related molecules, with six skeletal bond pairs, are systems with regular tetrahedral structures. Tetrahedral molecules held together by six skeletal bond pairs can of course be accommodated in the carborane cluster systematics as nido clusters with n atoms and n -1- 2 skeletal bond pairs, if a low connectivity (axial) vertex of the parent trigonal bipyramid is left vacant, instead of the expected high-connectivity (equatorial) vertex. 

Tetrahedrane, 1, C4H4 or (CH)4 is interesting for at least four reasons ... 

The synthesis of the parent tetrahedrane must be considered an enormous challenge, in view of the tremendous but thwarted effort expended by the Maier group to this end, but their great success in obtaining several substituted tetrahedranes indicates the quest is by no means hopeless (e.g. Note 4). It is the kinetic stability of tetrahedrane that concerns ns here, for no one would expect it to be the global minimum on the C4H4 potential energy surface. The indication from computational... 

Extensive STO-3G ab initio calculations, supplemented by semiempirical INDO calculations and experimental data, are available for the C Hg isomers prismane, benzvalene, and bicyclo[2,2,0]hexa-2,5-diene the predicted equilibrium geometries, dipole moments, and vertical ionization potentials are in excellent agreement with experiment. INDO wavefunctions are used to predict C-H and C-C coupling constants. An ab initio calculation in the extended 4-31G basis set has established that tetrahedrane is a local minimum point on the C4H4 potential-energy surface with a barrier of 18 kcal mol for homolytic cleavage of one C—C bond. The heat of formation, AHf, at OK is calculated by two methods to be ca. 130kcalmol, which... 

Two recent theoretical studies, one of ffie C4H4 isomers and the other of tetramethyl- and tetra-t-butyl-substituted cyclobutadiene and tetrahedrane, give further insights into ffie stabilities of tetrahedrane derivatives. 

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