DIVERSITY IN THE ELECTRONIC-STRUCTURES OF METALLOCARBOHEDRENES - AB-INITIO STUDY OF M(8)C(12) (M=TI, V, ZR, NB) AND TI(4)M'(4)C(12) (M'=V, ZR)

Al electron ab initio configuration interaction (CI) calculations have been carried out on M(8)C(12) (M = Ti, Zr, V, Nb) and mixed-metal M(4 )M'C-4(12) (M = Ti, M)= V, Zr) metallocarbohedrene clusters (met-cars) . The structure of a tetracapped tetrahedron of metal atoms with T-d s ymmetry has been assumed for all clusters. The geometries of the groun d state and of certain excited states have been optimized at the open- shell Hartree-Fock or the limited Cf (spin coupling) level of calculat ion, assuming the constraints of the T-d paint group. Two competing st ates very close in energy have been characterized for Zr8C12. The prob able ground state is a ''localized'' state with four antiferromagnetic ally coupled d electrons accommodated in the small tetrahedron of meta l atoms. Another low-energy state, with four d electrons ''delocalized '' on the whole metal framework, was characterized at a slightly diffe rent optimal geometry. That state is closely related to the one advoca ted by Lin and Hall (J. Am. Chem. Sec. 1993, 115, 11165) from orbital energy considerations. After correlation of the 20 metal electrons, th e localized state was found to be more stable by 20 kcal . mol(-1). Th e ground state of all other considered met-cars is unambiguously local ized. The ground state of homometallic met-cars is characterized by th e presence of four unpaired electrons either localized on the small te trahedron of metal atoms and displaying antiferromagnetic coupling (Ti , Zr) or localized on the large, capping tetrahedron and ferromagnetic ally coupled (V, Nb). The 24 electron, mixed-metal met-car V4T4C12, St ill hypothetical, is expected to display a really diamagnetic, closed- shell ground state. The stability of the mixed-metal clusters is depen dent on the site assigned to one or the other metal: Ti(4)ZT(4)C(12) i s expected to exist as a small tetrahedron of Ti atoms capped by zirco nium, and the inner sites of V4Ti4C12 should be occupied by vanadium. The two isomers of V4Ti4C12 compatible with T-d symmetry display strik ingly different equilibrium geometries and metal-dicarbon electronic i nteractions.