An Extraordinarily Distorted cyclo-Tetrastibine. Crystallographic Structure of a New Polymorph of cyclo-(σ-C5Me5Sb)4

A new crystalline polymorph of cyclo-(Cp*Sb)4 has been crystallographically characterized that contains the mostly acutely angled four-membered, homoatomic ring system known (Sb−Sb−Sb = 76.1(1)°). In the previously reported form, the Cp* and Sb4 ring centroids (viewed along the C−Sb bond) are anti; in the new form the Cp* ring rotates over 100° to a more crowded gauche form. The new form melts at a temperature 72 °C higher than the one previously reported and is about 15% more dense. While the Sb−Sb ring distances are normal (2.836(1) A), the transannular Sb···Sb distances are the shortest known for antimony (3.518(1) A) and apparently provide a stabilization that compensates for the extreme ring distortion.