The Structure of Cluster Compounds of Transition Metals and the Limits of Applicability of the Electron Counting Rules for Polyhedral Molecules

The current theoretical ideas concerning the electronic structure of clusters and electron counting schemes for them derivable from molecular orbital analysis, whereby one can determine magic numbers of valence and skeletal electrons corresponding to energetically favourable characteristic geometrical configurations of cluster polyhedra, are examined. Among the multiplicity of polynuclear compounds, the class of superaromatic clusters obeying Wade's rule (the 2n + 2 rule) has been isolated. The structures of transition metal clusters M_n with n = 3–38, including small, trigonal-bipyramidal, and octahedral clusters, polyhedra of medium dimensions, and large clusters, are considered and the applicability of the existing schemes to the determination of magic numbers for each of these groups of compounds is discussed.
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