Metal-metal interactions in linked metallocenes

Metal-metal interactions have been studied in a large number of different bridged metallocene systems. Metal-metal interactions are manifested in a number of ways and to greatly varying extents: in some systems the only small electrochemical effects are detected, while in others complete delocalization between metal centers occurs. Both through-space and through-bond effects are important in the transmission of these metal-metal interactions. Generally stronger metal-metal interactions are favored in systems with closer metal-metal approaches and where metallocenes are held in specific rigid conformations with respect to one another. Metal-metal communication through unsaturated carbon bridges occurs more efficiently than through saturated bridges. Elements from the second and lower rows of the periodic table also allow stronger interactions than comparable carbon bridges. Only small differences between compounds with second, third, fourth, and lower row bridges are observed, presumably owing to compromise between opposing through-bond and through-space effects. The strongest metal-metal interactions of all are observed in pentalene-bridged systems, which essentially consist of two or more metallocenes fused together.