Binodal Orbital Aromaticity

The concept of binodal orbital aromaticity in reduced early transition-metal polyoxometalates may be related to their classification as mixed valence compounds. Robin and Day divide mixed valence compounds into the following three classes ... 

The concept of binodal orbital aromaticity in reduced early transition metal polyoxometalates may be related to their... 

ESR studies on the one-electron reduced polyoxometalates M60 9" and XMi204o" suggest class II mixed valence species." " Although such species are delocalized at accessible temperatures, they behave as localized systems at sufficiently low temperatures this behavior is similar to that of semiconductors. This behavior is in accord with the much smaller overlap (i.e., lower P in equation 26) of the metal dxy orbitals associated with binodal orbital aromaticity as compared with the boron sp hybrid anodal internal orbitals in the deltahedral boranes or the carbon uninodal p... 

The Hiickel approach (Section 10.3.1) can be used to treat the type I polyoxometalates as aromatic systems in which the d . orbitals on each of the transition metal vertices overlap to form molecular orbitals. Since these orbitals (Figure 17a) have two nodes, the resulting aromaticity can be called binodal aromaticity. Such binodal aromaticity is much weaker than anodal or uninodal orbital aromaticity since the metal atom vertices furnishing the orbitals participating in the delocalization are much farther apart being separated by M-O-M bridges rather than direct M-M bonds. 

Figure 14 Types of venex orbitals participating in the delocalization of aromatic systems of various types as classified by their nodalities. (a) The anodal sp hybrid unique internal orbitals of a B-H vertex in the deltahedral boranes. (b) The uninodal p orbital of a C-H vertex in planar polygonal aromatic hydrocarbons such as benzene, (c) The binodal d y orbital of a (p -0)5M0 vertex in polyoxometalates...

The topology of the overlap of the d y orbitals in Type I polyoxometalate structures can be described by a graph G whose vertices and edges correspond to the vertices and edges, respectively, of the macropolyhedron (octahedron or cubocta-hedron). Equation (26) (Section 3.2) can then be applied to these binodal aromatic systems to relate the spectrum of G... 

---