The crystal and molecular structure of diiodotetrakis(pyridine)nickel(II) [Nipy₄I₂]

Abstract

The crystal structure of diiodotetrakis(pyridine)nickel (II), [Nipy₄I₂] has been determined accurately by three-dimensional, single-crystal X-ray analysis. The orthorhombic cell of space group Pbcn has a = 9.678(4) Å, b = 16.077(6) Å, and c = 14.004(4) Å, with four molecules per unit cell (density = 1.88 g/cm³ experimental, and 1.92 g/cm³ calculated). The structural refinement was terminated at R = 0.042. The molecule was found to have the geometric trans configuration, and this supports electronic spectroscopic conclusions. The nickel atom is surrounded by the four nitrogen atoms of pyridine molecules so that the group (NiN₄) defines a plane. The four nitrogen atoms deviate but slightly from defininig a square. The INiI moiety is nearly linear so that any one NiI) vector deviates only slightly (∼2) from being perpendicular to the (NiN₄) plane. The average NiN distance in [Nipy₄I₂] is the largest (2.127(7) Å) such bond length yet observed for high-spin Ni^II-pyridine complexes. Regarding the pyridine ring, the average CC bond distances of coordinated pyridine are found to be shorter than those of the free pyridine molecule. It is also interesting that the two planes of each pair of trans pyridine molecules are found to be staggered (∼90°) whereas both eclipsed and staggered configurations were previously reported for other, but somewhat related molecules. Each of the four pyridine molecules has a pitch angle of ca.45° with respect to the near-C₄ axis along NiI. The pyridine molecules also define a four-blade propeller, since this “C₄” axis brings one pyridine molecule nearly into coincidence with the others. The structural relationships of [Nipy₄I₂] are also compared with those of other metal-pyridine complexes.