Pi-bonding

Pi bonding and antibonding molecular orbitals. The sideways overlap of two p orbitals leads to a 77 bonding MO and a 77 antibonding MO. A pi bond is not as strong as most sigma bonds. 

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 x bond. Together, the two lobes of the it bonding molecular orbital constitute one bond. 

The X and z axes (and their corresponding orbitals) must be taken as a single set of 12, because each axis can be converted into every other axis by one of the symmetry operations (C4 or one of the a). The reducible representation for these 12 orbitals is in the top row of Table 10-8. The reducible representation is 

Of these four representations, Tig and T2u have no match among the metal orbitals, Tig matches the d y, dy orbitals, and matches the Px, Py, Pz orbitals of the metal. The p orbitals of the metal are already used in a bonding and will not overlap well with the ligand tt orbitals because of the larger bond distances in coordination complexes therefore, they are unlikely to be used also for tt bonding. There are then three orbitals on the metal (dxy, available for tt bonds distributed over the six 

Pi bonding in coordination complexes is possible when the ligand has p or tt molecular orbitals available. Because the effects are smaller for occupied orbitals, we will first treat the more important case of ligands with empty tt orbitals, or TT-acceptor ligands. 

FIGURE 10-10 Overlap of d, TT, and p Orbitals with Metal d Orbitals. Overlap is good with ligand d and TT orbitals, but poorer with ligandp orbitals. 

L) TT bonding is also called tt back-bonding, with electrons from d 

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Geometrical Isomerism. Rotation about a carbon-carbon double bond is restricted because of interaction between the p orbitals which make up the pi bond. Isomerism due to such restricted rotation about a bond is known as geometric isomerism. Parallel overlap of the p orbitals of each carbon atom of the double bond forms the molecular orbital of the pi bond. The relatively large barrier to rotation about the pi bond is estimated to be nearly 63 kcal mol (263 kJ mol-i). 

The carbon—carbon double bond is the distinguishing feature of the butylenes and as such, controls their chemistry. This bond is formed by sp orbitals (a sigma bond and a weaker pi bond). The two carbon atoms plus the four atoms ia the alpha positions therefore He ia a plane. The pi bond which ties over the plane of the atoms acts as a source of electrons ia addition reactions at the double bond. The carbon—carbon bond, acting as a substitute, affects the reactivity of the carbon atoms at the alpha positions through the formation of the aHyUc resonance stmcture. This stmcture can stabilize both positive and... 

Along the bond axis itself, the electron density is zero. The electron pair of a pi (tt) bond occupies a pi bonding orbital. There is one tt bond in the C2H4 molecule, two in QH The geometries of the bonding orbitals in ethylene and acetylene are shown in Figure 7.13. 

Reality Check There are two pi bonds in a triple bond, one in a double bond, and none in a single bond. 

Bonding orbitals in ethylene (CH2=CH2) and acetylene (CH=CH). The sigma bond backbones are shown in blue. The pi bonds (one in ethylene and two in acetylene) are shown in red. Note that a pi bonding orbital consists of two lobes. 

Give the number of sigma and pi bonds in the molecule in Question 57. 

A compound of chlorine and fluorine, CIF, reacts at about 75°C with uranium to produce uranium hexafluoride and chlorine fluoride, C1F. A certain amount of uranium produced 5.63 g of uranium hexafluoride and 457 mL of chlorine fluoride at 75°C and 3.00 atm. What is x Describe foe geometry, polarity, and bond angles of foe compound and foe hybridization of chlorine. How many sigma and pi bonds are there ... 

You may recall that we discussed the bonding in ethene in Chapter 7. The double bond in ethene and other alkenes consists of a sigma bond and a pi bond. The ethene molecule is planar. There is no rotation about the double bond, since that would require breaking the pi bond. The bond angle in ethene is 120°, corresponding to sp2 hybridization about each carbon atom. The geometries of ethene and the next member of the alkene series, QHg, are shown in Figure 22.6. 

The most important alkyne by far is the first member of the series, commonly called acetylene. Recall from Chapter 7 that the C2H2 molecule is linear, with 180° bond angles. The triple bond consists of a sigma bond and two pi bonds each carbon atom is sp-hybridized. The geometries of acetylene and the next member of the series, C3H4, are shown in Figure 22.7. 

In a complex such as the sulfate ion the sulfuiMjxygen bond assumes multiple-bond character through resonance involving one sigma bond and two pi bonds. An empirical equation has been formulated connecting interatomic distances and bond number for resonance of this sort. On application of this equation it is found that in many complexes the amounts of multiple-bond character are such as to cause all atoms to conform rather closely to the principle of electroneutrality. 

The rationale behind this choice of bond integrals is that the radical stabilizing alpha effect of such radicals are explained not by the usual "resonance form" arguments, but by invoking frontier orbital interactions between the singly occupied molecular orbital of the localized carbon radical and the highest occupied molecular orbital (the non-bonding electrons atomic orbital) of the heteroatom (6). For free radicals the result of the SOMO-HOMO interaction Ts a net "one-half" pi bond (a pi bond plus a one-half... 

First we need to locate the part of the molecule where resonance is an issue. Remember that we can push electrons only from lone pairs or bonds. We don t need to worry about all bonds, because we can t push an arrow from a single bond (that would violate the first commandment). So we only care about double or triple bonds. Double and triple bonds are called pi bonds. So we need to look for lone pairs and pi bonds. Usually, only a small region of the molecule will possess either of these features. 

Can we convert any lone pairs into pi bonds without violating the two commandments ... 

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