The electronic structures of several butterfly clusters containing carbon, nitrogen, and oxygen atoms have been studied by using Fenske-Hall molecular orbital calculations. Comparison of the calculated electronic structures of [Fe4X(CO)12]n-(X = C, N, O) shows that the major effect of substituting the smaller N and O atoms is a weakening of the X-wingtip Fe bond. These results suggest that the O-Fe interactions may not be strong enough to maintain the same butterfly cluster geometry as that observed for [Fe4C(CO)12]2- and [Fe4N(CO)12]-. The coordinates for the homonuclear ruthenium nitride cluster were obtained from the single-crystal X-ray crystallographic analysis of [PPN][Ru4N(CO)12] [P1 space group, a = 10.815 (2) A, b = 14.185 (2) A, c = 16.433 (9) A, a = 91.98 (3)°, β= 94.68 (3)°, ϒ= 97.79 (2)°, Z = 2], which established that it was isomorphous with the tetrairon nitrido cluster. The major difference between the electronic structure of [Fe4N(CO)12]- and that of [ru4n(co)i2]-is the stronger metal-metal bonding in the Ru4N cluster. Calculations for the two isomers of [FeRu3N(CO)12]-show that substitution of Fe into the Ru4framework of [Ru4N(CO)12]-results in relatively small perturbations of the electronic structure of the cluster.