[Cu 4 @E 18 ] 4- (E = Sn, Pb): Fused Derivatives of Endohedral Stannaspherene and Plumbaspherene.

We report the synthesis of two new cluster anions, [Cu ₄ @E ₁₈ ] ^4- (E = Sn, Pb), in which a Cu ₄ subunit is incorporated into a continuous E ₁₈ 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@Sn ₁₂ ] ^{q -} and [M@Pb ₁₂ ] ^{q -} , the Matryoshka bronze [Sn@Cu ₁₂ @Sn ₂₀ ] ^12- , and also [Pd ₂ @E' ₁₈ ] ^4- (E' = Ge, Sn).