Electronic and geometrical structure of the pyrazole ligand co-ordinated to metal centres

Abstract

The electronic and geometrical structure of pyrazole (Hpz) and pyrazolate (pz^–) ligands co-ordinated to metal centres such as Na⁺, Cu⁺, CuF and Cu(NH₃)⁺ or perturbed by a positive point charge has been investigated by means of ab initio, all-electron Hartree–Fock calculations. Extensive geometry optimization of the internal coordinates of Hpz and pz^– shows that the structure of the ligand is little affected by the presence of chemically different co-ordination centres, thus confirming the pronounced rigidity of the pyrazole ring. The co-ordination process appears to influence only the valence angles external to the ring; their values correlate well with the characteristics of the molecular orbitals localized on bonds involving the nitrogen donor atom. An analysis of the localized molecular orbitals has led to conclusions in agreement with the valence shell electron pair repulsion approach and Bent's rule.