The interaction of imidazole with a [Cu(acac)(2)] complex was studied using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), hyperfine sublevel correlation spectroscopy (HYSCORE), and density functional theory (DFT). At low Im ratios (Cu:Im 1:10), a 5-coordinate [Cu(acac)(2)Im(n=1)] monoadduct is formed in frozen solution with the spin Hamiltonian parameters g(1) = 2.063, g(2) = 2.063, g(3) = 2.307, A(1) = 26, A(2) = 15, and A(3) = 472 MHz with Im coordinating along the axial direction. At higher Im concentrations (Cu:Im 1:50), a 6 coordinate [Cu(acac)(2)Im(n=2)] bis-adduct is formed with the spin Hamiltonian parameters g(1) = 2.059, g(2) = 2.059, g(3) = 2.288, A(1) = 30, A2 = 30, and A3 = 498 MHz with a poorly resolved N-14 superhyperfine pattern. The isotropic EPR spectra revealed a distribution of species ([Cu(acac)(2)], [Cu(acac)(2)Im(n=1)], and [Cu(acac)(2)Im(n=2)]) at Cu:Im ratios of 1:0, 1:10, and 1:50. The superhyperfine pattern originates from two strongly coordinating N-3 imino nitrogens of the Im ring. Angular selective 14N ENDOR analysis revealed the (N)A tensor of [34.8, 43.5, 34.0] MHz, with e(2)qQ/h = 2.2 MHz and eta = 0.2 for N-3. The hyperfine and quadrupole values for the remote N-1 amine nitrogens (from HYSCORE) were found to be [1.5, 1.4, 2.5] MHz with e(2)qQ/h = 1.4 MHz and eta = 0.9. 111 ENDOR also revealed three sets of HA tensors corresponding to the nearly equivalent H-2/H-4 protons in addition to the H-5 and H-1 protons of the Im ring. The spin Hamiltonian parameters for the geometry optimized structures of [Cu(acac)(2)Im(n=2)], including cis-mixed plane, trans-axial, and trans-equatorial, were calculated. The best agreement between theory and experiment indicated the preferred coordination is trans-equatorial [Cu(acac)(2)Im(n=2)]. A number of other Im derivatives were also investigated. 4(5)-methyl-imidazole forms a [Cu(acac)(2)(Im-3)(n=2)] trans -equatorial adduct, whereas the bulkier 2-methyl-imidazole (Im-2) and benzimidazole (Im-4) form the [Cu(acac)(2)(Im-2,4)(n=1)] monoadduct only. Our data therefore show that subtle changes in the substrate structure lead to controllable changes in coordination behavior, which could in turn lead to rational design of complexes for use in catalysis, imaging, and medicine.