Diamagnetic nickel (I1) complexes were allowed to react with anhydrous pyridine. In several instances it was possible to isolate well-defined crystalline compounds of hexacoordinated nickel (11) containing two molecules of pyridine. These solid compounds were paramagnetic and removal of pyridine at reduced pressures yielded the original diamagnetic materials. It is therefore concluded that the paramagnetism of certain pyridine ...