Side-on overlap

Both carbon atoms in ethene undergo sp2 hybridization. The C-H bonds involve overlap of sp1 carbon orbitals with Is orbitals of the H atoms. The carbon-carbon double bond involves the overlap of sp2 orbitals from each carbon to give the o bond and the side-on overlap of a p orbital from each carbon atom to give the n bond. 

Sigma (o) bonds are formed by the end-on overlap of atomic orbifals and n bonds resulf from side-on overlap. 

The side-on overlap of two p orbitals forms an MO that is no longer symmetrical about the inter-nuclear axis. If we rotate about this axis, the phase of the orbital changes. The orbital is described as having n symmetry—a n orbital is formed and the electrons in such an orbital make up a K bond. Since there are two mutually perpendicular pairs of p orbitals that can combine in this fashion, there are a pair of degenerate mutually perpendicular n bonding MOs and a pair of degenerate mutually perpendicular n antibonding MOs. 

Figure 8.4 Symmetry properties of MOs formed by the side-on overlap of two p-orbitals.

MOs of ethene The carbon atoms in ethene are sp hybridized. The double bond between two carbon atoms comprises a a- and a ir-bond. The carbon-carbon a-bond is formed by the head-on overlap of two sp -orbitals. The overlapping results in two MOs a bonding (ct) and an antibonding (ct ). The TT-bond is formed by the side-on overlap of p-orbitals. The bonding orbital is formed by the overlap of in-phase p-orbitals, and the antibonding orbital arises from the interference between two p-orbitals of opposite phases and has a node (or a region of minimum electron density between the nuclei). These orbitals are designated as TV and 77, respectively (Fig. 8.5). 

The two C atoms interact by head-on (end-to-end) overlap of sp hybrids pointing toward each other to form a sigma, (a) bond and by side-on overlap of the unhybridized 2p orbitals to form a pi (tt) bond. 

A pi bond is a bond resulting from side-on overlap of atomic orbitals. The regions of electron sharing are on opposite sides of an imaginary line connecting the bonded atoms and parallel to this line. 

The unhybridized atomic 2py and 2p orbitals are perpendicular to each other and to the line through the centers of the two sp hybrid orbitals (Figure 8-8). The side-on overlap of the 2py orbitals on the two C atoms forms one pi bond the side-on overlap of the 2p orbitals forms another pi bond. 

Prepare sketches of the overlaps of the following atomic orbitals (a) s with r (b) r with p along the bond axis (c) p with p along the bond axis (head-on overlap) (d) p with p perpendicular to the bond axis (side-on overlap). 

If we had chosen the 2 axis as the axis of head-on overlap of the Ip orbitals in Figure 9-3, side-on overlap of the 2p,-2p,. and 2py-2py orbitals would form the ir-type molecular orbitals. 

Pi (77) orbital A molecular orbital resulting from side-on overlap of atomic orbitals. 

Here the spz hybrid orbitals lie in the xy plane. The possibility of side-on overlap of the 2pz orbitals, unused in hybridisation, is also shown. 

Now, each carbon atom has unused partly filled 2py and 2pz orbitals, and these will experience side-on overlap to give two tt bonds. On molecular rBital fheory, the electrons in these orbitals will enter the tiry2p and ttz2p... 

However, the oxygen atoms have unused partly filled 2py and 2pz orbitals between them, and the carbon atom has unused partly filled 2py and 2pz orbitals. We may therefore visualise side-on overlap between these orbitals to give 77 bonds. There is therefore a a bond and a 7r bond between the carbon atom and each oxygen atom. 

Each atom presents its other two 2p orbitals for side-on overlap. This is what the antibonding MO formed by out-of-phase combination of two side-on p orbitals looks like ... 

Because the 2p orbitals have a node at the atomic nucleus, the electron distribution of the VB wavefunction formed by their side-on overlap does not have cylindrical symmetry, but instead changes sign when rotated by 180° about the bond axis. Bonds that have this property are called pi (tt) bonds. Thus, the double bond in the oxygen molecule consists of a sigma bond and a pi bond, which are not equivalent. The existence of two different types of bonds in double bond formation is something that is not predicted by simple Lewis theory. A triple bond such as that in N2 is described within the VB approach as a sigma bond formed from the head-on overlap of 2p orbitals and two pi bonds formed by the overlap of the 2py and 2p orbitals on one N atom with their counterparts on the other atom (Figure 3.7). 

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