In this work, the CCS-mass trends of ion mobility-derived collision cross section (IM-derived CCS) of negatively and singly charged iron metal center (Fe(II) and Fe(III)) coordinated with 3 or 4 ligands being halides or carboxylate generated by electrospray operating in negative ionization mode have been reported. The CCS-mass trends were fitted using the equation CCS = A ×mass ^pow (where A is an apparent density parameter and pow is an apparent rotationally averaged shape parameters). The value of the pow parameter is generally between 0.5 to 1, where 2/3 corresponds to the pow value describing a spherical shape as sensed by ion mobility. Iron-halide complexes led to pow parameters well below the typical limit of 0.5, which could only be explained by refining the fitting equation using a linear combination of these A and pow parameters. The latter find their physical meaning in terms of inhomogeneous mass distribution within the rotationally averaged volume of the iron-ligands complex ions. By acquiring the CCS-mass trend of iron-halide and iron-carboxylate complexes, it was possible to predict the IM-derived CCS and the CCS-mass trends of any combination of iron-halides/carboxylates complexes. The results show no differences in trend between planar trigonal and tetrahedral geometries according to valence shell electron pair repulsion (VSEPR) theory (Gillespie-Nyholm models)