O , electronic structure

Sinanoglu, O., Electronic Structure of Atoms and Molecules, Harvard Lecture Notes, Spring, 1962. 

Since the topological charge density parameters are excellent descriptors of the electronic structure, it may be concluded that HOMA and NICS alike are associated with this characteristic of 7t and a electron systems, at least as far as benzenoid hydrocarbons are concerned. This result supports a view that there is an interplay between the ji and o electron structure. The charge density analysis shows that both structures contribute to the overall effect observed as aromaticity via indices as HOMA or NICS. 

There are a few examples of -i- d cases giving a (o) electronic structure and many d -i- d compounds giving a (a) (jt)" (6) (6 ) (3t ) electronic structure.Each of the two metal fragments can be considered as a radical and thus as an X ligand for the neighboring metal. The binuclear complex is then a dimer, if the two halves are the same as in the examples below. Since each metal of a d + d system gives its available electron to the M-M bond, the two metals do not have any non-bonding electrons available in the bimetallic complex (NNBE = 0). Each metal of a d + d system becomes d (NNBE = 6) in the binuclear complex. 

---