Five-coordination and Paramagnetic Anisotropy

Abstract

CONSIDERABLE effort has been devoted recently to the establishment of simple experimental criteria by which the stereochemistry of five-coordinate transition metal species can be recognized. They have been primarily concerned with examination of the electronic spectra and, when paramagnetic, the bulk (powder) susceptibilities. These techniques have long been used successfully to differentiate between octahedral and tetrahedral coordination in complexes. It must be emphasized, however, that this success is dependent on only these two alternative stereochemistries having been previously established. A substantial number of five-coordinate complexes of nickel(II) and cobalt(II) have been prepared and some of these have sufficiently distinctive electronic reflectance spectra to reveal their geometry. The point dipole and point charge calculations of Ciampolini¹, of Ciampolini and Bettini (personal communication) and of Wood² have provided bases for their assignment and also for the low temperature polarized single crystal spectra (unpublished work of Kohl, Gerloch and Lewis) of the molecules, M (arsine oxide)₄X₂ (M = Co, Ni; X = ClO₄−, NO₃−). By contrast the reflectance spectrum of β-Co(paphy)Cl₂ may be equally well assigned³ on the basis of octahedral or tetrahedral stereochemistry although the X-ray data⁴ established this as five-coordinate.