LUMO manifold

Two more states of the 7r-symmetry ( ) and u)) on the metal ion remain unchanged as in the free metal ion and both are empty. The frontier orbitals here are the a ) (HOMO) and those in the 7r-manifold ( ) and u) - LUMO). 

The lowest energy absorption for each of six dA monomers containing a diaminoalkyne ligand has been assigned as a transition into the dir LUMO of the complexes. In the coordinate system employed by Lippard and co-workers (Fig. 27) the b2 symmetry of the dyz LUMO restricts excitations to originating orbitals of a1( a2, and b2 symmetry. The relatively low extinction coefficients favor assignment as a d-d transition, and dxi yi (a ) to dyz (b2) is a rational assignment within the d-rr manifold. Molybdenum-to-bypyridine MLCT bands near 530 and 600 nm with extinction coefficients of the order of 3000 M l cm-1 obscure d-d transitions which may be present in [Mo(RNHC=CNHR)(CNR)3(bpy)]2+ (118). 

Fig. 14 A schematic of the photoinduced charge separation and charge recombination processes in 18(w). A simple orbital diagram is provided which captures the essentials of the ET processes. HD, donor HOMO LD, donor LUMO HA, acceptor HOMO LA, acceptor LUMO. Note that all depicted processes are assumed to take place on the singlet multiplicity manifold.

The Bradsher reaction is formally a [4 + 2] Diels-Alder reaction. However, as a consequence of the aza cationic nature of the diene, this reaction proceeds by the inverse electron demand manifold. The classical Diels-Alder reaction employs the partnering of an electron-rich diene and an electron-deficient dienophile to provide the proper interaction of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) as prescribed by frontier molecular orbital theory (FMO) to generate the observed adducts. Thus FMO theory interprets this reaction proceeding via the HOMO of the diene with the LUMO of the dienophile. In the case of the inverse electron demand reaction, the electronics of the reaction are inverted. Therefore, in the Bradsher reaction, the electron-deficient aza cation diene s LUMO interacts with the HOMO of an electron rich dienophile. This mechanistic pathway provided a rationalization for the lack of reactivity of the electron-deficient tetracyanoethylene (TCNE), while electron-rich styrenes afforded the predicted product from reaction of 1 to generate 2. ... 

---