Synthesis and Structure of Heavy Alkali Metal Pentalenides

The solid‐state structures of the first rubidium and caesium pentalenides [Rb(THF)] 2 [Ph 4 Pn] and [Cs(THF)] 2 [Ph 4 Pn] have been determined by single crystal X‐ray diffraction. Both were found to be polymeric in the solid state through interactions of the cations with the phenyl substituents, in contrast to their lighter group 1 congeners which are monomeric for lithium and sodium, and THF‐bridged for potassium. Both [Rb(THF)] 2 [Ph 4 Pn] and [Cs(THF)] 2 [Ph 4 Pn] displayed increased η 8 coordination, demonstrating a shift towards higher hapticities down the group as previously predicted computationally for the parent M 2 [Pn] complexes (M=group 1 metal). The solid‐state structures of the polydentate donor adducts [M(DME) x ] 2 [Ph 4 Pn] (M=Li, x=1; M=Na, x=2) and [M(Me 6 TREN)] 2 [Ph 4 Pn] (M=K, Rb, Cs) were all monomeric and displayed increased metal‐carbon distances and decreased ring slippage values relative to the THF adducts.