N* Antibonding molecular orbitals

For a 7t system to function as an electron acceptor, it must have unfilled orbitals available to accept electrons. In the case of olefins or dienes those are n antibonding molecular orbitals. Thus interaction of the HOMO of one n system with file LUMO of a second n system produces a donor-acceptor pair (HOMO donating to LUMO) enabling electrons to be transferred from one n system to another with resulting bond formation. 

Therefore, in the vast majority of deltahedral clusters which belong to these point groups, the total representation splits into two distinct sub-shells and there are n bonding and n antibonding molecular orbitals. Since such boranes also have a single radial bonding molecular orbital belonging to the totally symmetric representation, they are characterised by a total of n -t- 1 skeletal electron pairs or 4 n -I- 2 valence electrons in total. 

Figure 1.24 How two isolated carbon p orbitals combine to form two n (pi) molecular orbitals. The bonding MO is of lower energy. The higher energy antibonding MO contains an additional node. (Both orbitals have a node in the plane containing the C and H atoms.)...

The other two n molecular orbitals are antibonding molecular orbitals, i) In the ground state these orbitals are unoccupied. 

As we have already seen, two molecular orbitals form when two atomic orbitals overlap - a bonding molecular orbital and an antibonding molecular orbital. End-on overlap of atomic orbitals along the axis of the bond results In cr and cr molecular orbitals forming. Slde-on overlap of atomic orbitals at an angle perpendicular to the axis of the bond results In the formation of n and molecular orbitals. 

Defined as N = (n - n )/2, where n and n are the numbers of electrons in the bonding and antibonding molecular orbitals, respectively, of the corresponding dioxygen species. e R = alkyl. 

The third step hypothesized [eq. (9)] is electron demotion from the singly occupied antibonding molecular orbital of XIII to the low energy, singly occupied oxygen p orbital which originally lost the electron in the n-ir excitation process. It is important and of interest to note that this demotion process is an electronic transition thus XIII and XIV are... 

Conversely, electrons that occupy any of the antibonding molecular orbitals are predominantly metal electrons. Any electrons in the t2g orbitals will be purely metal electrons when there are no n molecular orbitals. Thus, the t2g and antibonding e orbitals are predominantly of metal 3d orbital character. 

The n orbital results from combining the two 2p orbitals of the separate carbon atoms. Remember that when we combine two atomic orbitals we get two molecular orbitals. These result from combining the p orbitals either in-phase or out-of-phase. The in-phase combination accounts for the bonding molecular orbital (ji), whilst the out-of-phase combination accounts for the antibonding molecular orbital (jc ). As we progress to compounds with more than one alkene, so the number of Jt orbitals will increase but will remain the same as the number of Tt orbitals. 

In a molecular system which clearly possess a ct LUMO the photoexcitation process may involve promotion of an electron from a n HOMO to the ct LUMO. This process may or may not produce bond cleavage. Direct observation of a 7t — ct electronic transition is often difficult due to the localized nature of the ct molecular orbitals resulting in a low probability for the transition [91]. More likely ait->it electronic transition takes place initially and ET, i.e., a - ct, is required to eventually populate the lower energy ct antibonding molecular orbital. Onium salts are examples of chemical structures that possess a ct LUMO and are expected to behave in this manner (see in Sect. 3.3). 

The effect of the n acceptor interaction is the opposite of the a interactions an electron pair on the metal is now stabilized when a molecular orbital is formed, whereas the energy of the (empty) ligand orbital is destabilized as it becomes a higher energy (antibonding) molecular orbital. 

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