Transition metal rings valence molecular orbitals

FIGURE16.8 The valence molecular orbitals of four-member transition metal ring-like molecules. The atomic components of the each of the four molecular orbitals of each of the six sets, (a) the Oj set, and (h) the set 8 arising from the orbitals, are shown at the bottom. At the top row, (c) the arising from the orbitals, (d) the o, arising from the d y orbitals, (e) the Jt, arising from the d orbitals, and (f) the Jt, arising from the d y orbitals, are displayed. 

In particular, we have reviewed here how aromaticity can account for the electronic structure of ring-like molecules made of both main group metals and metalloids and transition metals. Of the many ways and indexes to characterize aromaticity, a loosely defined concept in itself, we have demonstrated that the very first of them, namely the analysis of the valence molecular orbitals complanented with the Aufbau principle and the Hund s rule for their occupation, and the Hiickel electron counting rules, yields a very appealing, albeit approximate, picture to assess the aromaticity of any particular ring-like molecule. 

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