Ji-antibonding orbitals

They are delocalized inside a molecule. The highest Jt-bonding orbital that is occupied by electrons is called the highest occupied molecular orbital (HOMO). The lowest Ji-antibonding orbital, which is unoccupied, is called the lowest unoccupied molecular orbital (LUMO) [14]. The HOMO and LUMO are, in some sense, analogous to the valance band and conduction band of an inorganic semiconductor. 

A bonding orbital always is lower than the antibonding orbital if they are derived from the same atomic orbitals. 

Overlap of the filled Jt bonding MO of C2H4 with a Pt hybrid orbital of o symmetry, leading to donation from ligand to Pt. Overlap of a filled Pt 5d orbital with the empty ji antibonding MO of C2H4, which amounts to n donation from Pt to C2H4 ... 

The leading term of the inter-bond contributions (terms involving the exchange integrals (ij ji ) and (if i j ) associated to the bonding and antibonding orbitals between the carbon and the two gem protons) is equal to —27.075 Hz... 

The reactivity of the active center is attributed primarily to the presence of d-orbitals in the transition metal. The initial state of the active center shows that the =CH2 group will be capable of considerable distortion from its equilibrium position, because of the availability of adjacent d-orbitals. Complexing (b) takes place when the 7t-bonding orbitals of the olefin overlap with the vacant d 2 y2 orbital of the TP+ while at the same time the 7t -antibonding orbitals can overlap the d, orbitals of the Ti +. Formation of the transition state is aided by the ability of the =CH2 group to migrate by partial overlap with the dy, and ji -orbitals (see Figures 7.1 and 7.2). 

Figure 7.3 A representation of bonding in metal alkene complexes (a) the o bond in which a n MO on the alkene overlaps with a metal orbital (6) the rbond in which the Ji antibonding MO of the alkene overlaps with a metal d orbital.

The use of TEP is also not without some drawbacks. The weakening of the CO bond via back-donation into the r co antibonding orbital is dependent on the ability of the metal center to undergo d —+ it co backbonding while this is typically correlated to the electron density of the metal center, it also depends somewhat on the Ji-acceptor nature of the NHC ligand. Nolan, Jacobsen, and Cavallo... 

Fig. 11. Antibonding interaction between a Pr f z(ji —y ) orbital and O p orbitals. The Pr atom, in the center, is surrounded octahedrally by eight oxygen ions. Light and dark shadings indicates different signs of the lobes.

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