Structural, spectroscopic and angular-overlap studies of the nature of metal-ligand bonding for tripod ligands

The metal(II) complexes [M{(pz)₂(py)CH}₂][NO₃]₂(M = Fe, Co, Ni, Cu or Zn), where (pz)₂(py)CH is the unsymmetrical tripodal nitrogen-donor ligand bis(pyrazol-1-yl)(pyridin-2-yl)methane, have been prepared and examined by single-crystal X-ray diffraction and single-crystal electronic spectroscopy. The structural studies, and the application of the angular overlap model to the spectroscopic results, provide new information on the bonding characteristics of tripod ligands and a comparison of pyrazole and pyridine groups. The zinc complex crystallizes in both the monoclinic (P2₁/c) and triclinic (P) systems, the monoclinic complex being isomorphous with those of cobalt and copper. The cations are centrosymmetric, but exhibit disorder in the orientation of the pyrazole and pyridine rings, except for the copper complex which exhibits a Jahn–Teller distortion involving two Cu–N (pz) bonds which are ca. 0.038 Å longer than the other Cu–N distances. Except for the copper complex, the ligands form ‘bite’ angles N–M–N of 83.8(2)–86.9(2)°, resulting in a slight trigonal distortion from octahedral geometry. The tripod ligand produces a relatively strong ligand field, consistent with the rather short metal–nitrogen bond lengths in the complexes. The pyridine group is a slightly stronger σ donor than the pyrazole groups, with both amines acting as weak π donors.