STRUCTURES OF IONIC DECAMETHYLMETALLOCENES - CRYSTALLOGRAPHIC CHARACTERIZATION OF (ME5C5)2CA AND (ME5C5)2BA AND A COMPARISON WITH RELATED ORGANOLANTHANIDE SPECIES

Distillation of a toluene solution of Cp*2Ca[O(C2H5)2] (Cp* = Me5C5) or vacuum sublimation of Cp*2Ba(THF)2 readily produces the solvent-free metallocenes. Cp*2Ca crystallizes from hexane in the monoclinic space group P21/n, with a = 9.721 (2) Å, b = 12.361 (2) Å, c = 30.849 (10) Å, β = 96.34 (1)°, and Dc = 1.12 g cm-3 for Z = 8. Least-squares refinement on the basis of 3560 observed reflections measured at 116 K led to a final R value of 0.0594. Two independent bent metallocenes appear in the asymmetric cell, with average Ca‒C distances of 2.64 (1) Å [2.64 (2) Å] and ring centroid‒Ca‒ring centroid angles of 147.7° [146.3°]. The smallest intermolecular Ca⋯CH3ʹ contact is 2.98 Å. Cp*2Ba crystallizes from toluene in the space groups P21/n, with a = 18.043 (4) Å, b = 11.830 (2) Å, c = 18.206 (4) Å, β = 105.67 (1)°, and Dc = 1.45 g cm-3 for Z = 8. Least-squares refinement on the basis of 4482 reflections collected at 118 K produced a final R of 0.0295. Two independent metallocenes also appear in the asymmetric unit, with average Ba‒C distances of 2.98 (1) Å [2.99 (1) Å] and ring centroid‒Ba‒ring centroid angles of 130.9° [131.0°]. The shortest intermolecular Ba⋯CH3ʹ distance is 3.349 (5) Å. The metallocene units pack so that, in addition to the two Cp* rings directly bonded to the barium center, the face of a third Cp* ring is in close proximity to the metal. A comparison of the solid-state structures for the ionic metallocenes Cp*2M (M = Ca, Yb, Sm, Eu, Ba) reveals a strong linear correlation between the metal ionic radius and the ring centroid‒M‒ring centroid angles; this correlation is analyzed in terms of intra- and intermolecular steric interactions. Intramolecular methyl‒methyl repulsions may contribute to the regularity in the bending angles in all the decamethylmetallocenes. © 1990, American Chemical Society. All rights reserved.