Rr bonding orbital

The electronic structures of PTH, PbTH, PtTH and poly(3-alkylthiophene)s have been the subject of a number of XPS and UPS studies [117,123,124], A systematic evolution of the rr-bonding orbital was observed by going successively from thiophene to bithiophene and terthiophene. It finally led to the formation of an intrinsic rc-bonding band for undoped... 

Figure 5.6 Schematic energy diagram of two p orbitals on adjacent atoms forming a rr bonding orbital and a tt antibonding orbital.

Some simpler ligands can also serve as ir-donors. We saw earlier that acetylenes can act as CT-donors and ir-acceptors in the same way as olefins. However, the rr-bonding orbitals that are perpendicular to those oriented toward the metal can also serve as ir-donors, as shown in Figure 1.29. Acetylene ligands in complexes that possess less than 18 electrons without such TT-donation are sometimes considered to be "four-electron donors." This is the origin of the listing of alkynes as 2-electron or 4-electrori ligands in Table 1.1. 

Recall (Chapter 5) that a rr bond has a nodal surfrice that includes (he bond axis and that a rr bonding orbital will have lobes of opposite sign on each side of this nodal surface. From (he standpoint of orbital symmetry, an octahedral complex could have up to twelve such bonds—two between the metal and each of the six ligands, although this number is never realized in an actual complex. Metal aixl ligand orbitals par-ticipaUng in rr bonds w9l lie perpendicular to the internudear axes. Consider four potential metal-ligand ir interactions (I) (2) (3) and (4) [Pg.420]

Neighboring group participation (a term introduced by Winstein) with the vacant p-orbital of a carbenium ion center contributes to its stabilization via delocalization, which can involve atoms with unshared electron pairs (w-donors), 7r-electron systems (direct conjugate or allylic stabilization), bent rr-bonds (as in cyclopropylcarbinyl cations), and C-H and C-C [Pg.150]

As portrayed m Figure 2 20 the two carbons of acetylene are connected to each other by a 2sp-2sp cr bond and each is attached to a hydrogen substituent by a 2sp-ls CT bond The unhybndized 2p orbitals on one carbon overlap with their counterparts on the other to form two rr bonds The carbon-carbon triple bond m acetylene is viewed as a multiple bond of the ct + rr + rr type... 

Section 2 20 Carbon is sp hybridized in ethylene and the double bond has a ct com ponent and a rr component The sp hybridization state is derived by mix mg the 2s and two of the three 2p orbitals Three equivalent sp orbitals result and their axes are coplanar Overlap of an sp orbital of one car bon with an sp orbital of another produces a ct bond between them Each carbon still has one unhybridized p orbital available for bonding and side by side overlap of the p orbitals of adjacent carbons gives a rr bond between them... 

Although both stereoisomers yield 4 tert butylcyclohexene as the only alkene they do so at quite different rates The cis isomer reacts over 500 times faster than the trans The difference in reaction rate results from different degrees of rr bond develop ment in the E2 transition state Since rr overlap of p orbitals requires their axes to be parallel rr bond formation is best achieved when the four atoms of the H—C—C—X unit he in the same plane at the transition state The two conformations that permit this are termed syn coplanar and anti coplanar... 

Bonding m alkenes is described according to an sp orbital hybridization model The double bond unites two sp hybridized carbon atoms and is made of a ct component and a rr component The ct bond arises by over lap of an sp hybrid orbital on each carbon The rr bond is weaker than the CT bond and results from a side by side overlap of p orbitals... 

We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

An orbital overlap description of electron delocalization mil dimethylallyl cation H2C=CH—C(CH3)2 is given m Figure 10 2 Figure 10 2a shows the rr bond and the vacant p orbital as independent units Figure 10 2b shows how the units can overlap to give an extended rr orbital that encompasses all three carbons This permits the two rr electrons to be delocalized over three carbons and disperses the positive charge... 

You might recall from Section 1.9 that a carbon-carbon triple bond results from the interaction of two sp-hybridized carbon atoms. The two sp hybrid orbitals of carbon lie at an angle of 180° to each other along an axis perpendicular to the axes of the two unhybridized 2py and 2pz orbitals. When two sp-hybridized carbons approach each other, one sp-sp a bond and two p-p -rr bonds are... 

Figure 3-3. A schematic picture of a (irin.v-polyaceiylenc chain, showing the 2p electronic orbitals, which are perpendicular to the plane formed hy the rr-bonds (solid lines).

A carbon-carbon double bond is stronger than one carbon-carbon single bond but weaker than the sum of two single bonds (Section 2.15). A carbon-carbon triple bond is weaker than the sum of three carbon-carbon single bonds. Recall that a single C—C bond is a o-bond, but the additional bonds in a multiple bond are TT-bonds. One reason for the difference in strength is that the side-by-side overlap of p-orbitals that results in a rr-bond is not as great as the end-to-end overlap that results in a o-bond. 

Coordination to an isocyanide can either raise or lower the value of cn depending on the relative contributions of a- and rr-bonding. These contributions are dependent on metal orbital energies, which are influenced by oxidation state and by other ligands present. 

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