The formation, spectra, and properties of a series of (η5-pentamethylcyclopentadienyl)copper compounds have been investigated. Reactions between lithium pentamethylcyclopentadienide and [C1Cu(R3P)]4 (R = C6H5, C2H5) produced [η5C5(CH3)5](PR3)Cu in yields of 43% and 78%, respectively. A reaction between lithium pentamethylcyclopentadienide and copper(I) chloride in ethyl ether solution in the presence of carbon monoxide led to [η5-C5(CH3)5](CO)Cu. This carbonyl derivative was both air- and temperature-sensitive and decomposed in solution over 0 ºC to afford copper metal, carbon monoxide, and decamethyl-1,1′-dihydrofulvalene. A reaction between copper(I) chloride and bis(trimethylsilyl)acetylene produced [η2-(CH3)3SiC==CSi(CH3)3]2Cu2Cl2 in 76% yield. Treatment of the latter dimer with lithium pentamethylcyclopentadienide in ethyl ether solution afforded [η5-C5(CH3)5][η2-(CH3)3SiC=CS í(CH3)3]Cu. The parent compound (η5-C5H5)[η2-(CH3)3SiC=CSi(CH3)3]Cu was also obtained starting with lithium cyclopentadienide. Both products were air- and temperature-sensitive but could be readily converted into the respective triphenylphosphine derivatives. The complexes [η5-C5-(CH3)5][η2-(CH3)3SiC=CS í(CH3)3]Cu and (η2-C5H5)[η2-(CH3)3SiC=CSi(CH3)3]Cu represent the first “mixed-sandwich compounds of copper, as predicted by the effective atomic number (EAN) rule.