Cu I complexes of the form [Cu I (PMDETA)(π-M)][BPh 4 ] (where PMDETA=N,N,N′,N″,N″-pentamethyldiethylenetriamine, and M=vinyl monomer) were synthesized and isolated from solution as crystals with methyl acrylate (MA), styrene (Sty), and 1-octene (Oct). The interaction of the CC double bond of the vinyl monomer with Cu I was characterized via FT-IR and 1 H NMR spectroscopy and single crystal X-ray crystallography. A fourth complex with methyl methacrylate (MMA) was synthesized and characterized spectroscopically, but no crystals suitable for X-ray structure analysis could be obtained. In all complexes, PMDETA acts as a tridentate ligand, while the pseudotetrahedral coordination geometry around Cu I is completed by a π-interaction with the CC double bond of M in the presence of a non-coordinating counter-ion. A decrease in CC IR stretching frequencies of Δν(CC)=−110, −80, −109, and −127 cm −1 for complexes with MA, Sty, Oct, and MMA, respectively, was observed upon coordination. No significant change in CC bond length was seen in the crystal structure for complexes with MA and Oct while a slight lengthening was observed for the Sty complex. The upfield shift of the vinyl proton resonances indicated the presence of significant π-back-bonding.