Orbitals, antibonding bond formation with

The positively charged allyl cation would be expected to be the electron acceptor in any initial interaction with ethylene. Therefore, to consider this reaction in terms of frontier orbital theory, the question we need to answer is, do the ethylene HOMO and allyl cation LUMO interact favorably as the reactants approach one another The orbitals that are involved are shown in Fig. 1.27. If we analyze a symmetrical approach, which would be necessary for the simultaneous formation of the two new bonds, we see that the symmetries of the two orbitals do not match. Any bonding interaction developing at one end would be canceled by an antibonding interaction at the other end. The conclusion that is drawn from this analysis is that this particular reaction process is not favorable. We would need to consider other modes of approach to analyze the problem more thoroughly, but this analysis indicates that simultaneous (concerted) bond formation between ethylene and an allyl cation to form a cyclopentyl cation is not possible. 

The attractive nature of XB causes D- -X distances shorter than the sum of van der Waals radii of involved atoms the stronger the interaction, the shorter the D- -X interaction lengths. Consistent with the rationalization of XB as an electron donation from D to the antibonding X-Y orbital [59], XB formation results in an elongation of the X-Y covalent bond. The D- -X interaction length is usually a more sensitive probe for XB strength than the X - Y covalent bond elongation (Table 3). 

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. 

The same group of coordination polymerisations in which alkene undergoes re complex formation with the metal atom includes the copolymerisation of ethylene, a-olefins, cycloolefins and styrene with carbon monoxide in the presence of transition metal-based catalysts [54-58], In this case, however, the carbon monoxide comonomer is complexed with the transition metal via the carbon atom. Coordination bond formation involves the overlapping of the carbon monoxide weakly antibonding and localised mostly at the carbon atom a orbital (electron pair at the carbon atom) with the unoccupied hybridised metal orbitals and the overlapping of the filled metal dz orbitals with the carbon monoxide re -antibonding orbital (re-donor re bond) [59], The carbon monoxide coordination with the transition metal is shown in Figure 2.2. 

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