Structural, spectroscopic and angular-overlap studies of tripodal pyridine ligands with nickel(II) and zinc(II)

Zinc(II) and nickel(II) complexes [M{(X(C₅H₄N)₃}₂]²⁺], where X(C₅H₄N)₃ are symmetrical tripodal nitrogendonor ligands with X = CH, N or P, 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, confirm previous results on the bonding characteristics of pyridine and provide a unique way of establishing the effect of the bridgehead atom, X. The cations are all centrosymmetric with the ligand 'bite' angles N–M–N 85.2(1)–88.5(2)°, resulting in a slight trigonal distortion from octahedral geometry. The ligand fields in the three nickel compounds are very similar and the large ligand-field splitting is consistent with the rather short metal-nitrogen bond lengths. The pyridine groups act as moderately strong σ-donor and weak π-donor ligands, with no evidence of conjugation of the π system across the bridgehead atom. The crystal structure of [Ni{CH(C₅H₄N)₃}₂][NO₃]₂ shows the complex cation to have crystallographic 3^⁻ symmetry such that there is one independent Ni–N interaction of 2.069(2)Å. The corresponding zinc complex, isolated as its dibromide nonahydrate salt, has crystallographic 2/m symmetry with two Zn–N contacts of 2.123(5)Å being shorter than the others, i.e. 2.141(3)Å. In the phosphine analogue, [Zn{P(C₅H₄N)₃}₂]²⁺, isolated as its diperchlorate monohydrate salt, the Zn²⁺ cation is situated on a site of symmetry 1^⁻ with one Zn–N distance [2.145(4)Å] being shorter than the other two, 2.162(4) and 2.173(4)Å. Small trigonal distortions from the ideal octahedral geometry are due to the restricted bite distances of the tripodal ligands