Copper complexes of pyrazine (1,4-C4H4N2), pyrimidine (1,3-C4H4N2), and pyridazine (1,2-C4H4N2) are produced in laser-vaporization supersonic molecular beams and studied by pulsed-field ionization zero electron kinetic energy (ZEKE) spectroscopy and second-order Moller-Plesset perturbation theory. Both sigma and pi complexes are considered by these ab initio calculations; only sigma structures are identified in these experiments. Adiabatic ionization energies and metal-ligand vibrational frequencies of the sigma complexes are measured from the ZEKE spectra. Metal-ligand bond dissociation energies of these complexes are obtained from a thermochemical cycle. The ionization energies follow the trend of Cu pyridazine (43,054 cm(-1)) < Cu pyrimidine (45,332 cm(-1)) < Cu pyrazine (46,038 cm(-1)); the bond energies are in the order of Cu pyridazine (56.2 kJ mol(-1)) > Cu pyrazine (48.5 kJ mol(-1)) approximately Cu pyrimidine (46.4 kJ mol(-1)). The stronger binding of pyridazine is due to its larger electric dipole moment and possibly bidentate binding.