Binding energies and electronic structures of Cu(OH2)n and Cu(NH3)n (n=1–4): anomaly of the two ligand Cu complexes

Ab initio calculations up to MP4(SDTQ) level and density functional theory have been used to estimate binding energies and electronic structures of Cu⁺(L)_n (L=OH₂, NH₃, n=1–4) complexes using TZP basis set types. The computed binding energies agree well with experimental values. General trends in structures and energetics are recorded for both Cu⁺(OH₂)_n and Cu⁺(NH₃)_n systems. The first two ligands are more strongly bound to Cu⁺ than the third and fourth molecules. The 4s–3dσ hybridization and electrostatic interactions are the main factors behind the higher binding energies for the first two ligands. Analysis of HOMO mixed orbitals in the copper ion as well as in complexes indicates shrinking of the orbital lobes directed to the ligand with shrinking more effective in the two ligand system. The lower binding energies for the third and fourth ligands were attributed to the attenuation of sdσ hybridization and decreasing of Cu–L attraction at long separation which is necessary to relieve Cu–L and L–L exchange repulsions. NBO analysis and charge-model calculations support the presence of sdσ hybridization and electron transfer to the copper ion in case of the first two ligands.