The quest for Jahn-Teller distortion

According to the Htickel model proposed by Ceulemans and Fowler, the two extra n electrons of the neutral TigC are accommodated in a triply degenerate orbital with t symmetry. Such a situation is susceptible to geometric instability of the Jahn-Teller type. Rather than considering a small deformation toward the trigonal symmetry predicted from the epikemel principle to be the most stable, 

Ceulemans and Fowler propose a quite different conformation for the met-car cage, namely an anti-aromatic C12 ring bicapped by electropositive metal tetrahedra (Fig. 12). 

The energy of this Dsd structure was later computed by Lin and Hall and compares quite well at the ab initio Hartree-Fock level with that of the dodecahedron for Y8C12 (-20.1 kcal mol ) and for Nb8Ci2 (+3.6 kcal moL ). 

The problem of degenerate low-spin states for neutral Ti8Ci2 inducing an important Jahn-Teller distortion was also encountered at the Hartree-Fock level by Rohmer et Those calculations were performed assuming an outer valence 

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