U-bonding

FIGURE 5 1 (a) The planar framework of u bonds in ethylene showing bond distances and angles (b) and (c) The p orbitals of two sp hybridized carbons overlap to produce a tt bond (d) The electrostatic potential map shows a region of high negative potential due to the tt elec trons above and below the plane of the atoms... 

Recall from Section 115 that effects that are transmitted by the polarization of u bonds are called inductive effects... 

The C—C single bond in vmylacetylene is a a bond generated by overlap of an sp hybridized orbital on one carbon with an sp hybndized orbital on the other Vmylacetylene has three u bonds and three tt bonds... 

TT bond (Section 2 20) In alkenes a bond formed by overlap of p orbitals in a side by side manner A tt bond is weaker than a u bond The carbon-carbon double bond in alkenes con sists of two sp hybridized carbons joined by a a bond and a TT bond... 

The reaction begins with an attack on the electrophile, HBr, by the electrons of the nucleophilic tt bond. Two electrons from the 7t bond form a new u bond between the entering hydrogen and an alkene carbon, as shown by the curved arrow at the top of Figure 6.7. The carbocation intermediate that results is itself an electrophile, which can accept an electron pair from nucleophilic Br ion to form a C Brbond and yield a neutral addition product. 

The presence of a carbon-carbon double bond strongly influences the shape of a molecule because it prevents one part of a molecule from rotating relative to another part. The double bond of ethene, for example, holds the entire molecule flat. Figure 3.19 shows that the two 2p-orbitals overlap best if the two CH2 groups lie in the same plane. In order for the molecule to rotate about the double bond, the u-bond would need to break and reform. 

Answer Linear bond angles all 180° each C atom is sp hybridized and forms one u-bond and one -ir-bond to each adjacent C or O atom.]... 

Radical abstraction reactions that involve molecules with u-bonds in the vicinity of the reaction center are characterized by higher Ee0 values than the corresponding reactions... 

The added primary amine may facilitate the cleavage of the Ti-N bond of the key intermediate 107 through the coordination to the titanium center followed by u-bond metathesis. Such an intramolecular exchange process is expected to be facile. The amine exchange product is 106, which can then be rapidly converted to 105 and the corresponding silylated amine to complete the catalytic cycle. 

Recently, calculations have suggested that diuranium compounds should be stable with a multiple U-U bond and short bond distances.100 We have studied two chlorides, U2Clg and U2Clg, both with U(III) as the oxidation state of uranium (see Figure 5), and three different carboxylates (see Figure 6), U2(OCHO)4, U2(OCHO)6, and U2(OCHO)4Cl2. All species have been found to be bound with a multiply bonded U2 unit. 

Creutz-Taube ion [bis(pentaammine-ruthenium)pyrazine]D (30) provides an example of this. There is good reason to suppose (in spite of many earlier arguments to the contrary) that this is a fully delocalized mixed-valence system (27). In symmetry, the one-electron levels separated by energy gap 2J are calculated to have b u (bonding) and b (antibonding) symmetry,... 

A new class of conjugated hydrocarbons is that of the fullerenes [11], which represent an allotropic modification of graphite. Their electrochemistry has been studied in great detail during the last decade [126]. The basic entity within this series is the Ceo molecule (23). Because of its high electron affinity, it can be reduced up to its hexaanion (Fig. 4) [14,127]. Solid-state measurements indicate that the radical anion of Ceo reversibly dimerizes. NMR measurements confirm a u-bond formation between two radical anion moieties [128,129]. 

When a proton addition complex is formed, the particular even-numbered hydrocarbon is converted to an odd hydrocarbon ion. The TT-electron number remains even, but is reduced by two as a result of the formation of the u-bond. Thus we are dealing with a diamagnetic positive hydrocarbon ion. The theoretical treatment of this 7r-electron system has to reflect the following ... 

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