'Ionic Radii,' Spin-Orbit Coupling and the Geometrical Stability of Inorganic Complexes

The van Santen and van Wieringen theory of "ionic radii" is briefly reviewed and extended to include spin-orbit influences. It is noted that, although for the most part spin-orbit forces have little effect upon stereochemical predictions made for the first transition series, noteworthy exceptions to this rule occur for octahedral complexes of Co++ and tctrahedral complexes of Ni++ and C u . Indeed, it is found that spin-orbit coercions render the ground states of these molecules Jahn-Teller resistant. Diagrams are displayed and tables compiled to illustrate the variation of ionic radii with atomic number for the second and third transition series. Paths for future theoretical research are indicated and an exhortation for closer theoretical-experimental alliance is promulged. Now, although the ligand field theory is in its 30th year, it has only been within the last decade that its chemical fruits have been earnestly harvested. One of the earliest pickings of its ripened orchards was accomplished by van Santen and van Wieringen. 1 These researchers noticed that the enigmatic irregularities of the existent transition metal "ionicradii''tabulations could be neatly rationalized on the basis of the BetheKramers-Van Vleck2, ' crystalline field formalism. I now wish to extend their argument to include metals which possess a nonnegligible spinorbit interaction and to point out certain modifications (due to Jahn") of the Jahn-Teller stability rules 6 which such interactions engender. But before we may embark upon this tour of the crystallographic implications of the existence of spin-orbit forces in inorganic complexes, it is necessary to review briefly the basic precepts of ligand (or crystalline) field theory.