A systematic study of the iron–M(3d) molecules (M = Mn, Fe, Co, Ni, Cu) was carried out using matrix isolation techniques in conjunction with Mössbauer spectroscopy. From the Mössbauer parameter of the isolated species it was possible to determine their electronic ground state. The bonding between the various transition metal atoms and iron varies appreciably. The strongest bond is associated with the molecules having an odd number of electrons. The isomer shift of FeNi is very close to that obtained for iron monomers in rare gas solids (weakest bond molecule). In contrast, FeCo has a more positive isomer shift, closer to that expected for 3d74s configuration at the iron atom.