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

Zinc(I) and nickel(II) complexes [M{X(C5H4N)(3)}(2)](2+)], where X(C5H4N)(3) are symmetrical tripodal nitrogen-donor 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)degrees, 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 sigma-donor and weak pi-donor ligands, with no evidence of conjugation of the pi system across the bridgehead atom. The crystal structure of [Ni{CH(C5H4N)(3)}(2)][NO3](2) shows the complex cation to have crystallographic (3) over bar symmetry such that there is one independent Ni-N interaction of 2.069(2) Angstrom. The corresponding zinc complex, isolated as its dibromide nonahydrate salt, has crystallographic 2/m symmetry with two Zn-N contacts of 2.123(5) Angstrom being shorter than the others, i.e. 2.141(3) Angstrom. 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) over bar with one Zn-N distance [2.145(4) Angstrom] being shorter than the other two, 2.162(4) and 2.173(4) Angstrom. Small trigonal distortions from the ideal octahedral geometry are due to the restricted bite distances of the tripodal ligands.