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

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.