The resonance Raman spectra of a mixed-valence copper dimer [Cu-2(L)](3+) encapsulated by the ligand L = N(CH2CH2N(H)CH2CH2N(H)CH2CH2)(3)N are reported. Copper isotope data (Cu-63/65) of solution spectra of [Cu-2(L)](3+) allow identification of vibrational bands at 289 and 172 cm(-1) as predominately Cu-Cu stretching and Cu-Cu-N-eq bending modes. The results indicate that the Cu-Cu bond is sufficiently weak that ligand conformations, influenced by hydrogen bonding, can give rise to distortional isomers of the Cu-Cu bond.