Why do vanadium atoms form multiple-decker sandwich clusters with benzene molecules efficiently?

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.