Density functional theory calculations were carried out on the structurally characterized [(Cl-4-cat)Mo(py)Fe3S3 (CO)(4)((PPr3)-Pr-n)(3)], A, and (Cl-4-cat)Mo(py)Fe3S3(CO)(6)(PEt3)(2), B, and also on A(2-) and B(2+)Clusters. The Fe-Fe distances in these molecules depend on the total number of valence electrons (60 e(-) in A and B2+ and 62 e(-) in A(2-) and B) and undergo great structural changes upon addition or removal of electrons. The changes are consistent with known electron-counting rules in organometallic chemistry. The weak nature of the Fe-Fe bonding interactions in these clusters is apparent in the very similar energies of states with widely different Fe-F:e distances.