Abstract The syntheses and characterizations by X-ray crystallography, EPR, and electronic absorption spectroscopy are reported for [Cu(aneN 3) 2][CF 3SO 3] 2·H 2O ( 1) [Cu(aneN 2O) 2][BF 4] 2 ( 2), and [Cu(aneN 2O) 2][NO 3] 2 ( 3). The dynamic Jahn–Teller distortion present in the Cu(aneN 3) 2 2+ cation is removed upon replacement of one nitrogen donor with oxygen. The pseudo-octahedral structures of 2 and 3, determined crystallographically, exhibit a strong tetragonal distortion with the two oxygen atoms in the elongated axial positions. In contrast, crystallographic characterizations of a number of Cu(aneN 3) 2 2+ salts reveal that the octahedral-like coordination about the Cu(II) is dependent on crystal packing forces, and the degree of tetragonal distortion varies with the identity of the counter ion. A room temperature powder EPR spectrum of [Cu(aneN 2O) 2]Br 2 exhibits a static Jahn–Teller distortion while that of [Cu(aneN 3) 2][CF 3SO 3] 2 is dynamically averaged. Solution EPR and electronic absorption spectra of [Cu(aneN 2O) 2][CF 3SO 3] 2 indicate a strong interaction between the equatorial nitrogen atoms and the Cu(II) as evidenced by the observation of nitrogen hyperfine splitting in g x and a shift of the ligand-field absorption to higher energy. The absence of a dynamic Jahn–Teller effect in the presence of the aneN 2O ligand correlates with a lowered reactivity for phosphodiester hydrolysis, suggesting that ligand fluxionality may be important in accessing the transition state in copper-promoted hydrolysis.