A rotational potential has been quantified by correlating the rotation
al barrier of the NH, groups in the series Ni(NH3)(6) I2-xBrx (0 < x <
2), with the measured changes in interatomic distances in the crystal
. It transpires that the interaction with the anion is slight and that
the effective exponent for intermolecular NH3-NH3 interaction is clos
e to six.