Carbon pi bonds

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. 

The forms of the localized metal-carbon pi bond and antibond NBOs are again highly recognizable and transferable from species to species. This transferability is... 

Figure 4.15 Metal-carbon pi-bond (7tMc) and antibond (7TMc ) NBOs of saturated group 6-10 transition-metal alkylidenes H M=CH2.

Because the two metal-carbon pi bonds now extend into both dimensions perpendicular to the axis of the metal-carbon bond, the residual metal-hydride bonds are all constrained to lie essentially orthogonal to the M—C axis (i.e., in the nodal hollows of the pi-bonding dxz and d, orbitals). The optimized structures, as shown in Fig. 4.16, all reveal this common structural tendency, with near-perpendicular (91-96°) H—M—C bond angles in all cases. 

However, the chemical properties of an alkene are dramatically affected by the presence of the double bond. Recall that a carbon-carbon pi bond is considerably weaker than a carbon-carbon or carbon-hydrogen sigma bond. It is possible to selectively cause a reaction to occur at a pi bond under conditions that do not affect the sigma bonds. The pi bond is the weak spot of an alkene, and it is there that most chemical reactions occur. This is why unsaturated fats spoil more readily than saturated fats. Their pi bonds provide a place for reaction with oxygen to occur, which leads to spoilage. 

Figure 1.12 The bonding and antibonding orbitals of a carbon-carbon pi bond are made from the combination of two 2p orbitals by subtraction and addition. Atoms adjacent to the pi bond lie in a plane, as shown on the right. The R groups in this example are cis to each other.

Figure 1.17 The resonance forms and pi overlap of an unconjugated system (top) and a conjugated system (bottom) of a carbon-carbon pi bond and a carbon-oxygen pi bond.

When two orbitals of differing energy interact, the molecular orbital contains a greater percentage of the atomic orbital closest to it in energy. This is easily demonstrated (Fig. 2.10) by examining the two extremes. As we have seen for a carbon-carbon pi bond, when the two orbitals are of equal energy each contributes equally to the... 

Figure 4.34 Top view of the simplified energy surface for addition to a carbon-carbon pi bond.

The three routes in Figure 4.40 are similar to those for a carbon-carbon pi bond with the exception that the carbonyl lone pair is protonated instead of the carbonyl pi bond. This is due to the nonbonding electron pair on oxygen being energetically more... 

Variations An additional six-membered transition state example is the cyclic decarboxylation. The overlap limitation for the new carbon-carbon pi bond requires the breaking C-C bond to align coplanar with the carbon p orbital of the carbonyl. 

This reaction is a minor variant of the Ac1e2 in which the electrophile, usually a proton, attacks the lone pair rather than the less available heteroatom-carbon pi bond. The lone pair is protonated, path p.t., to give a highly polarized multiple bond that can be viewed as a stabilized carbocation. The nucleophilic attack on this carbocation could also be viewed as path An, trapping of a cation by a nucleophile, instead of path AdN-... 

The stabilization of a radical by conjugation with a carbon-carbon pi bond can be understood if we consider the interaction of a singly occupied p orbital with a carbon-carbon double bond to form the allylic system. Although the singly occupied orbital is not changed in energy, the two electrons in the Jt orbital are stabilized by the interaction (Fig. 11.1). 

In 1958 the American biochemist George Wald and his co-workers discovered that visible light isomerizes 11-cm retinal to ail-trans retinal by breaking a carbon-carbon pi bond. With the pi bond broken, the remaining carbon-carbon sigma bond is free to rotate and does so. Within 200 femtoseconds after it has absorbed a photon, the 11-cm retinal is transformed into all-fran retinal. 

Includes bibliographical references and index. Contents category 1. OrganometaUics. v. 1. Compounds with transition metal-carbon [pi -bonds and compounds of groups 10-8 (Ni, Pd, Pt, Co, Rh, Ir, Fe, Ru, Os) / volume editor, M. Lautens... 

We cannot make a similar description for acetylene. Since the 2p orbitals are all involved in the C-C bonds, there are no orbitals left to form the C-H bonds. A better description is obtained by constructing two 2sp hybrids on each carbon. The orbital regions of the 2sp hybrids on the same atom point in opposite directions along the z axis. We form a carbon-carbon sigma bonding orbital from the two 2sp hybrid orbitals that overlap between the carbon atoms. We call this the Icr bonding orbital. There is also an antibonding orbital, but we will not need to occupy it. The carbon-carbon pi bonds are constructed from the unhybridized... 

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