Ethene molecular orbital description

The actual E of a particular system is often not as informative as is the comparison of the delocalized system with a reference system having localized orbitals. Figure 4.11 shows the reference system for allyl a double bond separated by an imaginary barrier from a p orbital that may have 0 (cation), 1 (radical), or 2 (anion) electrons. The molecular orbital description of that system, then, is simply a sum of the HMOs of the double bond and of the p orbital. Again, it does not matter whether we are talking about the cation, radical, or anion in Figure 4.11. The HMOs of the reference system are simply those of ethene (E = a -H /3, = a — /8) superimposed on the one HMO for an isolated p orbital (E = a). 

Molecular orbital descriptions of the ethene analogues of the heavier Group 14 elements... 

MOLECULAR ORBITAL DESCRIPTIONS OF THE ETHENE ANALOGUES HOMO... 

The description of the bonding of ethene to transition metals is known as the Dewar-Chatt-Duncanson model. This builds on the previous descriptions for CO and CH2 with the exception that we must now consider the frontier orbitals of the ligand to be molecular orbitals delocalized over the two (or more) donor atoms. The basic features are presented for ethene however, the bonding scheme applies in principle to the side-on coordination of any multiple bond to a transition metal. 

Let s review how the tt molecular orbitals of ethene are constructed. An MO description of ethene is shown in Figure 7.8. The twop orbitals can be either in-phase or out-of-phase. (The different phases are indicated by different colors.) Notice that the number of orbitals is conserved—the number of molecular orbitals equals the number of atomic orbitals that produced them. Thus, the two atomic p orbitals of ethene overlap to produce two molecular orbitals. Side-to-side overlap of in-phase p orbitals (lobes of the same color) produces a bonding molecular orbital designated i/ i (the Greek letter psi). The bonding molecular orbital is lower in energy than the p atomic orbitals, and it encompasses both carbons. In other words, each electron in the bonding molecular orbital spreads over both carbon atoms. 

Description of the electronic stmcture of molecules by using this molecular orbital model provides important information about the nature of molecules. One of the most important pieces of information is the electron density distribution. While in a -bonds the electron density is largest between the atoms i.e. along the line of the chemical bond in % bonds the electron density is concentrated not between the atoms but above and below the plane which contains the bond line . This is represented in the next figure, hi an ethane molecule, which possess only a -bonds, the electron cloud is situated between the C-atoms. However, in the ethene molecule, with a x-bond, the electron density is highest above and below the plane in which all the atoms are located. Electron clouds in ethene are not blocked by the atoms and are situated on the open side of the molecule where other particles can attack it. Consequently an ethene molecule is much more chemically reactive then ethane. 

A much older description of ethene is known as the bent bond formulation. The double bond is described as the result of overlap of two sp hybrid orbitals on each of the two carbon atoms, as shown in Figure 1.29. This model also predicts that ethene should be a planar molecule, but it predicts H-C-H bond angles of 109.5°— which is even further from the observed 117° than was the prediction based on sp hybridization. Furthermore, except for cyclopropane, we feel imcomfortable about drawing molecular pictures with bonds that... 

Pauling s prediction that the use of the molecular structure are in better agreement with the bent bond description than with the a,n description. Figure 1.35(a) shows calculated contour lines of a carbon orbital in a plane that is perpendicular to the molecular plane of ethene, and Figure 1.35(b) shows contour lines for an orbital in a plane containing the carbon atoms of cyclopropane. Clearly, much of the orbital lies outside the intemuclear bond line in each case. It is generally agreed that the bonds in cyclopropane are bent the picture from this theoretical calculation reinforces Ae view that they are bent in ethene also. 

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