The preparation of multiple-decker sandwich clusters V n (C6H6) n +1 and their large size dependence of the ionization energies have recently been reported by Kaya and his co-workers (J. Phys. Chem. 1995, 99, 3053). In the present paper, the bonding scheme between benzene and metal atoms (Ti, V, and Cr) was investigated by using Mayer's bond order analysis with ab initio MO calculations, and it was attributed mainly to the delocalization of metal dδ electrons via the LUMOs of the benzene molecules. Moreover, the lowest ionization of most multiple-decker sandwich clusters was found to occur from the upper end of the dδ orbitals, and the large size dependence of the ionization energies was also related to the significant one-dimensional delocalization of these dδ electrons. The proposed Hückel type treatment for these frontier orbitals explains the above properties very simply and suggests also the large size dependence of the photoabsorption band positions and even the thermodynamical stability of the one-dimensional polymer materials denoted by [M(C6H6)]∞. Besides, the ionization energies of these polymeric species are estimated to be 2.68, 3.15, and 4.28 eV for M = Ti, V, and Cr, respectively.

Transition-metal benzene clusters, M n (benzene) m (M = Ti, V, and Cr), were synthesized by the reaction of laser-vaporized metal atoms with benzene vapor. All the clusters exhibit magic number behavior at m = n + 1, which is rationalized by the structure of a multiple-decker sandwich, but V atoms can efficiently take the sandwich structure (up to n = 5) in particular. This metal specificity of the V atoms and their growth mechanism were examined by quantum chemical calculations, the full valence configurational interaction (FVCI) method with configuration-averaged SCF orbitals. The calculation results imply that (1) total spin conservation in growth process plays an important role and (2) the production in the sandwich clusters particularly favors a process through lower spin states. The combination between experimental and theoretical investigations leads us to a better comprehension of both the bonding scheme in the sandwich clusters and the growth mechanism, and accordingly, a more efficient production method is proposed generally for the transition-metal sandwich complexes.

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The collectivity of the electronic motion in finite systems is studied by using both the linear response density functional theory (LRDFT) and the collectivity index defined by the transition density matrix. We demonstrate a collectivity analysis on the size-dependent peaks of electronic excitations of small sodium clusters (rings and linear chains). We find the excitation-mode dependence of the collectivity and large collectivities for the higher-energy plasmonic excitations. The collectivity analysis also clarifies the existence of the nondipolar collective motion at the energies very close to the higher-energy plasmonic excitations. The importance of the nondipolar motion is pointed out in light of nano-optics.