Pi bonds formation

Overlap like the E2, the C-H bond and the C-Z bond involved in the elimination must be roughly coplanar for strong, untwisted pi bond formation (Fig. 7.10). 

FIGURE 10.16 Another view of pi bond formation in the C2H4 molecule. Note that all six atoms are in the same plane. It is the overlap of the 2p orbitals that causes the molecule to assume a planar structure. 

Although both carbon and silicon are in Group 4A, very few Si=Si bonds are known. Account for the instability of silicon-to-silicon double bonds in general. Hint Compare the atomic radii of C and Si in Figure 8.5. What effect would the larger size have on pi bond formation )... 

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... 

However, the important new feature of metal alkylidenes (4.51) is metal-carbon pi-bonding. As discussed in Section 2.8, pi bonds between transition metals and main-group elements are of d -p type, much stronger than corresponding p —pn bonds between heavier main-group elements. Compared with simple metal hydrides and alkyls, metal-carbon pi-bonding in metal alkylidenes affects the selection of metal d orbitals available for hybridization and skeletal bond formation, somewhat altering molecular shapes. 

In contrast to the severe difficulty of cracking a sigma bond, insertion of a transition metal into a pi bond can proceed in facile fashion. This can be illustrated by the attack of Ti on the pi bond of ethylene, which leads to metallacycle formation in the reaction... 

Formation of a sigma bond (o) from two s orbitals and formation of a pi bond (n) from two adjacent p orbitals. 

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