A molecular mechanics (MM) force field is reported for modeling square pyramidal complexes of the Tc(V) oxo group, which, as the Tc-99m radioisotope, are important in diagnostic nuclear medicine. A novel aspect of the parametrization is the inclusion of significant barriers to rotation about the Tc-L bonds. These barriers to rotation are interpreted in terms of overlap between filled p pi orbitals on the donor atoms and oxygen and d orbitals on the Tc.