[Cu4@E18]4- (E = Sn, Pb): Fused Derivatives of Endohedral Stannaspherene and Plumbaspherene.

We report the synthesis of two new cluster anions, [Cu4@E18]4- (E = Sn, Pb), in which a Cu4 subunit is incorporated into a continuous E18 tetrel cage. Both anions are characterized by X-ray crystallography and mass spectrometry, complemented by quantum-chemical calculations that highlight the relationships to known Zintl clusters, including the stannaspherenes and plumbaspherenes [M@Sn12]q- and [M@Pb12]q-, the Matryoshka bronze [Sn@Cu12@Sn20]12-, and also [Pd2@E'18]4- (E' = Ge, Sn).