Nuclear Resonance Study of Structure and Molecular Motions in Some Cobalt Complex Salts

Abstract

Line widths and second moments of the proton and F¹⁹ resonances in Co(NH₃)₆X₃ (X=Cl, Br, I, NO₃, BF₄, PF₆) were measured from 90° to 400°K. In the first four, unusual and very broad (∼200°K) line‐width transitions were observed. These are interpreted by postulating crystal structures containing nonequivalent sites for Co(NH₃)₆⁺⁺⁺ ions in relative abundances 2:1, 1:1, 1:1, and 2:1, respectively. This interpretation was verified in one case by x‐ray diffraction. Co(NH₃)₆(BF₄)₃ showed a normal line‐width transition which is interpreted both by the usual correlation theory and by a very detailed analysis to give ΔE≅12 kcal/mole and τ₀≅10^—14 sec. At 90°K rapid reorientation of —NH₃ groups and of BF₄^— and PF₆^— ions occurs in all of these salts. At 330°K and above, rapid tumbling of Co(NH₃)₆⁺⁺⁺ ions occurs.