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(C5H 4N)(3) are symmetrical tripodal nitrogen-donor ligands with X = CH, N or P, have been prepared and examined by single-crystal X-ray diffract ion and single-crystal electronic spectroscopy. The structural studies , and the application of the angular overlap model to the spectroscopi c results, confirm previous results on the bonding characteristics of pyridine and provide a unique way of establishing the effect of the br idgehead 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 trig onal distortion from octahedral geometry. The ligand fields in the thr ee nickel compounds are very similar and the large ligand-field splitt ing is consistent with the rather short metal-nitrogen bond lengths. T he pyridine groups act as moderately strong sigma-donor and weak pi-do nor ligands, with no evidence of conjugation of the pi system across t he bridgehead atom. The crystal structure of [Ni{CH(C5H4N)(3)}(2)][NO3 ](2) shows the complex cation to have crystallographic (3) over bar sy mmetry such that there is one independent Ni-N interaction of 2.069(2) Angstrom. The corresponding zinc complex, isolated as its dibromide n onahydrate salt, has crystallographic 2/m symmetry with two Zn-N conta cts 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+), isolate d as its diperchlorate monohydrate salt, the Zn2+ cation is situated o n a site of symmetry (1) over bar with one Zn-N distance [2.145(4) Ang strom] being shorter than the other two, 2.162(4) and 2.173(4) Angstro m. Small trigonal distortions from the ideal octahedral geometry are d ue to the restricted bite distances of the tripodal ligands.