Double bond molecular orbitals

The narrower gap between the HOMO and the LUMO in 1,3-butadiene results from the conjugation of the double bonds. Molecular orbital calculations indicate that a much larger gap should occur in isolated alkadienes. This is borne out experimentally. 

The double bond shown in different locations in the two resonance stmctures represents a pair of electrons in a delocalized n bonding molecular orbital that spans all three of the oxygen atoms, as shown in Figure I0-37a. One lone pair also appears in different positions in the two resonance stmctures, again signaling a delocalized orbital. This lone pair is spread over both outer atoms but not across the inner atom, as shown in Figure I0-37Z>. This is a nonbonding molecular orbital,. The lone pair shown in different positions in the resonance stmctures occupies the delocalized orbital. 

In marked contrast to these are the fully unsaturated aromatic compounds, epitomized by benzene, in which the carbocyclic rings formally consist of a conjugated set of alternating single and double bonds. Such systems have a specially stabilized cyclic ir-electron system in which all of the bonding molecular orbitals are completely filled and the antibonding orbitals are all empty. The concept of... 

Structure of the double bond in ethylene. The first pair of electrons forms a a bond. The second pair forms a it bond. The it bond has its electron density centered in two lobes, above and below the bonding molecular orbital constitute one bond. 

Two more electrons must go into the carbon-carbon bonding region to form the double bond in ethylene. Each carbon atom still has an unhybridized p orbital, and these overlap to form a pi-bonding molecular orbital. The two electrons in this orbital form... 

We now compose a wave function for each tz electron in a conjugated system which describes its behaviour in the whole system, thus beforehand without restriction of the electron to particular bonds (molecular orbital, M.O.). It is now customary to compose these M.O. according first to Lennard-Jones as a linear combination of atomic orbitals (A.O.) provided with coefficients (L.C.A.O. approximation). These atomic orbitals in a molecule with double bonds are the p2 functions of the carbon atoms. The square of a particular coefficient indicates the contribution of the electron in question to the charge around this particular atom. 

The kind of bond with respect to type B may be interpreted analogous to the ethylene complexes as a dative x-alternating effect (Fig. 24). As opposed to type A complexes, the properties of type B complexes are determined in a characteristic way by the back donation into the unoccupied n molecular orbital (LUMO) of the double bond (Fig. 24). This strengthens the metal-ligand bond, as proved by a shorter interatomic distance, and weakens the double bond (occupying anti bonding molecular orbitals), as is evident by a widened PC distance. The phosphaalkene P atom has, so to speak, more of a phosphane... 

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