Valence electron counts, iron clusters

In both [Fe4X4(N0)4](X = S or Se) and [Fe4S2(NO)4(NCMe3)2] the total valence electron count is 60. This is the number characteristic of tetrahedral tetranuclear metal clusters, such as [Ir4(CO)12], in the Wade and Mingos skeletal-electron counting schemes (76, 77) and, furthermore, each iron atom in these clusters obeys the 18-electron rule, provided that it forms single Fe-Fe bonds to each of the other iron atoms in the tetrahedron. 

For a metal with eight valence electrons, electron precise carbonyl structures see Electron Precise Compound) are M(C0)5 and [M(CO)4]3, which are known for all three metals of the iron group, albeit with quite different properties for the different metals. The next member, [M(CO)3]6, is not formed since a four-connected vertex requires more metal-metal bonding orbitals than can be arranged on the surface of a sphere. Bonding orbitals directed towards the inside of the cluster, however, form MOs that will accommodate more electrons than would be predicted by an 18 valence electron count for each metal see Eighteen Electron Compounds), that is, 86 instead of 84 for a six-vertex closo cluster see Closo Cluster). This situation is realized in the iron group by the anions [M6(CO)i8], M = Fe, Ru, Os (isolectronic to M6(CO)i6, M = Co, Rh, Ir). 

For biological systems such as ferredoxins, problems arise when counting electrons by the valence electron method. This system assumes six Fe-Fe bonds within the tetrahedral iron-sulfur clusters, but Fe-Fe bond distances within biological iron-sulfur clusters, as found by X-ray crystallography, often... 

Exercise 4.5. Count the skeletal electron pairs (or cluster valence electrons) in the organometallic complex shown below at the left. Is there a relationship to cyclobutadiene iron tricarbonyl shown next to it ... 

The number of cluster valence electrons (CVE) expected for a linear M3 array is 50 (110), however the formulation from the X-ray and magnetic susceptibility data suggests a 48 CVE count for the [Fe3(CO)u]2- moiety. It is possible that the Fe3(CO)u unit is electron deficient, but another explanation is that it is a 50-electron system with two hydride ligands. No evidence for an M—H bond is present in the H-NMR spectrum. Similarly, the disposition of the carbonyl ligands on the terminal iron atoms does not suggest the presence of hydrogen ligands. 

Clusters with triangular metal arrays are common and typically clusters in this class contain 48 cluster valence electrons. Examples include Fe3(CO)i2, (Fig. 2) and [Fe3(CO)ii] . A good illustration of the influence of electron count, on the chemistry of a cluster compound, is the reduction of the neutral molecule Fe3(CO)i2, which has a triangular array of iron atoms. When this cluster is reduced by sodium, two electrons are transferred to Fe3(CO)i2, and to accommodate these electrons, a CO ligand (which is a... 

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