Isolobal main-group and transition-metal fragments

1 Isolobal main-group and transition-metal fragments 

In the first three chapters, instances were noted where the number, symmetry characteristics and occupation numbers of the frontier orbitals of a transition-metal fragment were similar to those of a main-group fragment. Such fragments are said to be isolobal to emphasize similar bonding capabilities. Since its enunciation by Hoffmann and Mingos, the concept has been used effectively for the analysis of both organometallic and cluster problems. Let s explore the idea in a more systematic 

Take the EH2 fragment, for example. It contains 3,2,1 electrons in the two frontier orbitals for E = N, C, B, respectively, with energies that increase in the order N C B. We assume this partitioning of the valence electrons and orbitals remains valid in the construction of molecules or clusters from these fragments. 

Exercise 4.1. Fragment ethane into two CH3 radicals. Construct a neutral M(CO)m fragment isolobal with CH3. Give the structures and compositions of all compounds analogous to ethane that can be formed with these two isolobal main-group and transition-metal fragments. 

The effectiveness of a fragment analysis depends on the extent to which the perturbation of the fragment electronic structures is restricted to the frontier orbital 

---