The crystal structures of cobalt and nickel acetate tetrahydrate have been determined at room-temperature and liquid-helium temperature by neutron powder diffraction of the fully deuterated salts. Molecular mechanics and ab initio methods based on these structural results have then been used to calculate the rotational potentials experienced by the methyl groups. We have also used inelastic neutron scattering to measure the rotational potential via the rotational tunneling spectrum of the methyl groups, and this has enabled us to compare different methods for the calculation of partial charges in these ionic compounds. Good agreement between the observables and calculations has been obtained for both compounds when nb initio methods are used to recalculate partial charges at every step of the methyl rotation.