Antibonding pairs

Starting from hybrid orbitals, it is possible to construct bond orbitals by using normalized combinations. For every couple of hybrids pointing to each other along a bond direction, a bond-antibond pair can be obtained2 ... 

Three p AO s give three molecular orbitals, as indicated in Fig. 8-4. Since there are an odd number of p AO s in this linear system, the middle-energy molecular orbital is nonbonding (w"). Note that the node of this MO" is at a C, indicated by a 0. An MO" can be recognized if the number of bonding pairs equals the number of antibonding pairs (see Problem 8.26) or if there is no overlap. 

The number of CSFs is now reduced to 24. We can constrain the wave function further by requiring that there is two electrons in each of the bonding-antibonding pairs of n orbitals ... 

The reader should now complete the bonding model for CH4 by constructing a bonding-antibonding pair for each of the other three interacting pairs of atomic orbitals. 

As an example, let us look at a bonding model for the H20 molecule. The Lewis structure (2) leads to the choice of sp3 hybridization on the oxygen. We then use two of the hybrids to make a bonding and antibonding pairs by combi-... 

It is a straightforward task to carry the procedure one step further. In order to find the butadiene orbitals, it is easiest to allow two ethylene units to come together end to end. Figure 10.20 illustrates the results. The important interactions are between orbitals of the same energy we simply treat the ethylene orbitals as the basis, and combine the bonding pair in the two possible ways and the antibonding pair in the two possible ways. 

When we examine the results of the in- and out-of-phase combination of py and pz orbitals, we get the bonding and antibonding pairs that we would expect, but the resulting molecular orbitals have a different symmetry rather than being rotationally symmetric about the line of centers, these orbitals extend in both perpendicular directions from this line of centers.Orbitals having this more complicated symmetry are called 7i (pi) orbitals. There are two of them, py and pz differing only in orientation, but otherwise completely equivalent. 

It is 2. The Is and 2s orbitals form bonding and antibonding pairs, each with two electrons, and so do not contribute to the bond order. There are six electrons, i.e. three pairs, in the bonding orbitals formed by the 2p orbitals, and one electron, or half a pair, in the antibonding orbitals 2tc, giving a bond order of 3 - = 2 . 

Molecule Number of electrons Electronic configuration Bonding pairs Antibonding pairs Bonds... 

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