Kinetics, stoichiometric model reactions, and stereochemical studies have been employed to investigate the mechanism of the zirconocene-catalyzed cyclization of nonconjugated dienes with organomagnesium reagents. Transmetalation of a zirconacyclopentane intermediate with magnesium alkyls has been implicated as a key step in the catalytic reaction. A zirconacyclopentane derived from 9,9-diallylfluorene was prepared and structurally characterized. Zirconium metallacyclopentanes transmetalate with dialkylmagnesium reagents having 8-hydrogens to form bis(magnesiomethyl)-substituted carbocycles and products resulting from the elimination of alkane from dialkylzirconocene. Metallacycles of this type also act as catalyst precursors for the catalytic cyclomagnesiation of 1,7-octadiene. The kinetics of the catalytic cyclomagnesiation of 9,9-diallylfluorene have been examined. The reaction obeys the following rate law : rate = k(obs)[Zr][Bu(2)Mg] when [Bu(2)Mg] <0.35 M, and rate = k(obs)[Zr] when [Bu(2)Mg] >0.35 M. The stereochemistry of the products depends upon the substrate, the magnesium reagent, the magnesium reagent concentration, and the temperature of the cyclization. The diastereoselectivity of the reaction depends on the relative rate of metallacycle formation, isomerization, and transmetalation. An understanding of these relative rates allows for the rational control of the diastereoselectivity to reflect either the kinetic selectivity of metallacycle formation or the thermodynamic stabilites of the diastereomeric metallacycles.