Density functional theory (DFT) studies on Fe(CS)(CO)(4) using the B3LYP and BP86 methods show the axially and equatorially substituted trigonal bipyramidal structures to be essentially degenerate, in accord with the experimental observation of an equilibrium of these two isomers in Fe(CS)(CO)(4) synthesized from Na2Fe(CO)(4) and S=CCl2, Furthermore, the apically substituted square pyramidal structure of Fe(CS)(CO)(4) lies similar to 5 kcal/mol above the trigonal bipyramidal structures, implying a highly fluxional system. The lowest energy structures for the unsaturated Fe(CS)(CO)(n) (n = 3, 2) can be derived from the trigonal bipyramidal or square pyramidal structures of Fe(CS)(CO)(4) by removal of one or two carbonyl groups, respectively. For the binuclear Fe-2(CS)(2)(CO)(n) (n = 7, 6, 5, 4) derivatives there is a clear energetic preference for bridging CS groups over bridging CO groups in most cases. Thus, the global minimum for Fe-2(CS)(2)(CO)(7) is a triply bridged structure analogous to Fe-2(CO)(9) but with two bridging CS groups and one bridging CO group. The lowest energy structures for the unsaturated Fe-2(CS)(2)(CO)(n) (n = 6, 5, 4) also contain two bridging CS groups, including at least one four-electron donor eta(2)-mu-CS group bonded to the iron atom not only through the carbon atom but also through the sulfur atom as indicated by relatively short Fe-S distances of similar to 2.6 angstrom. The Fe=Fe distances of similar to 2.4 angstrom in the highly unsaturated Fe-2(CS)(2)(CO)(n) (n 5, 4) derivatives with one or two four-electron donor bridging CS groups, respectively, suggest a formal bond order no higher than two, which is sufficient to give both iron atoms the favored 18-electron configuration.