Electron-counting procedures in clusters of the iron triad

Until recently all known high-nuclearity carbonyl clusters of the iron triad had metal core structures based on an octahedron with formulations corresponding to seven skeletal electron pairs (S = 7), in agreement with the predictions of polyhedral skeletal electron counting procedures. The isolation of the monoanion [HOs₈(CO)₂₂]^?, which also has S = 7, first broke this pattern, as it has a fused tetrahedral core geometry. The extension of skeletal electron counting procedures to include multiple “capping” of incomplete polyhedra permits a simple rationalisation of this structure as a tetra-capped arachno-octahedral. This approach has allowed the development of a matrix correlation for the main structural types observed in this triad. Structural relationships between the first members of a new class of high-nuclearity clusters based on the fusion of two are also discussed.