Ir bonding

A rational classification of reactions based on mechanistic considerations is essential for the better understanding of such a broad research field as that of the organic chemistry of Pd. Therefore, as was done in my previous book, the organic reactions of Pd are classified into stoichiometric and catalytic reactions. It is essential to form a Pd—C cr-bond for a synthetic reaction. The Pd— C (T-bond is formed in two ways depending on the substrates. ir-Bond formation from "unoxidized forms [1] of alkenes and arenes (simple alkenes and arenes) leads to stoichiometric reactions, and that from oxidized forms of alkenes and arenes (typically halides) leads to catalytic reactions. We first consider how these two reactions differ. 

The discussion of the ir-bond orders is interesting because it gives a picture of the distribution of Tr-electrons along the cr-frame of the ring... 

F. 1-26. (a) ir-Bond order of the C-S bonds in the ground state, (fc) ir-Bond order of the C-S bonds in the first excited state, (c) Free-valence number of the intermediate diradicaf. (Most probable bicyclic intermediate resulting from the ring closure of the diradicai. 

The activation energy for ro tation about a typical carbon-carbon double bond IS very high—on the order of 250 kj/mol (about 60 kcal/ mol) This quantity may be taken as a measure of the ir bond contribution to the to tal C=C bond strength of 605 kJ/mol (144 5 kcal/mol) in ethylene and compares closely with the value esti mated by manipulation of thermochemical data on page 191... 

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

Fenocene has an even more interesting stmcture. A central iron is ir-bonded to two cyclopentadienyl ligands in what is aptly described as a sandwich. It, too, obeys the 18-electron rule. Each cyclopentadienyl ligand contributes five electrons for a total of ten and iron, with an electron configuration of [Ar]45 34i contributes eight. Alternatively, fenocene can be viewed as being derived from Fe " (six valence electrons) and two aromatic cyclopentadienide rings (six electrons each). 

Section 14.15 Coordination polymerization of ethylene and propene has the biggest economic impact of any organic chemical process. Ziegler-Natta polymerization is canied out using catalysts derived from transition metals such as titanium and zirconium. ir-Bonded and a-bonded organometallic compounds aie intennediates in coordination polymerization. 

When two sp2-hybridized carbons approach each other, they form a cr bond by sp2-sp2 head-on overlap. At the same time, the unhybridized p orbitals approach with the correct geometry for sideways overlap, leading to the formation of what is called a pi (ir) bond. The combination of an >p2-sp2 a bond and a 2p-2p 77 bond results iii the sharing of four electrons and the formation of a carbon-carbon double bond (Figure 1.14). Note that the electrons in then-bond occupy the region centered between nuclei, while the electrons in the 77 bond occupy regions on either side of a line drawn between nuclei. 

Figure 1.14 The structure of ethylene. Orbital overlap of two sp hybridized carbons forms a carbon-carbon double bond. One part of the double bond results from a (head-on) overlap of sp2 orbitals (green), and the other part results from (sideways) overlap of unhybridized p orbitals (red/blue). The ir bond has regions of electron density on either side of a line drawn between nuclei.

Allene (see Problem 1.46) is related structurally to carbon dioxide, C02. Draw a picture showing the orbitals involved in the cr and -ir bonds of C02, and iden-... 

Figure 8.2 The structure of a secondary vinylic carbocation. The cationic carbon atom is sp-hybridized and has a vacant p orbital perpendicular to the plane of the tt bond orbitals. Only one R group is attached to the positively charged carbon rather than two, as in a secondary alkyl carbocation. The electrostatic potential map shows that the most positive (blue) regions coincide with lobes of the vacant p orbital and are perpendicular to the most negative (red) regions associated with the ir bond.

Because a [1,5] sigmatropic rearrangement involves three electron pairs (two ir bonds and one cr bond), the orbital-symmetry rules in Table 30.3 predict a suprafacial reaction. In fact, the 1,5] suprafacial shift of a hydrogen atom across... 

For the cyclotrimerization of alkynes, several mechanisms have been proposed. The most plausible ones are a concerted fusion of three ir-bonded alkyne molecules, and stepwise processes involving a cyclobutadiene complex or a five-membered metallocyclic intermediate (98). In the case of the cyclotrimerization of a-alkynes it is possible to discriminate between a reaction pathway via a cyclobutadiene complex and the other reaction pathways, by analysis of the products. If cyclotrimerization proceeds via a cyclobutadiene complex and if steric factors do not affect the reaction,... 

With the increase in electronegativity of the element M the degree of covalence of the bonds M —O and M—0 should increase, as a result of which an increase in electron density on the ion M can be expected. As in the formation of the ir-bond with olefin the ir-backbonding mechanism plays a large role, that should result in an increase in the ir-complex stability. 

FIGURE 3.11 A iT-bond is formed when electrons in two 2p-orbitals pair and overlap side by side. The middle diagram shows the extent of the overlap, and the bottom diagram shows the corresponding boundary surface. Even though the bond has a complicated shape, with two lobes, it is occupied by one pair of electrons and counts as one bond. In this text, ir-bonds are usually colored yellow. 

In valence-bond theory, we assume that bonds form when unpaired electrons in valence-shell atomic orbitals pair the atomic orbitals overlap end to end to form cr-bonds or side by side to form ir-bonds. 

FIGURE 3.21 Unhybridized carbon 2p-orbitals can form a ir-bond with either of their immediate neighbors. Two arrangements are possible, each one corresponding to a different Kekule structure. One Kekule structure and the corresponding ir-bonds are shown here. 

Form the bonds. A -ir-bond forms by the overlap of the p-orbital on... 

Self-Test 3.9A Describe the structure of the carbon suboxide molecule, C302, in terms of hybrid orbitals, bond angles, and tr- and -ir-bonds. The atoms lie in the order OCCCO. 

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.]... 

Multiple bonds are formed when an atom forms a tr-bond by using an sp or sp2 hybrid orbital and one or more ir-bonds by using unhybridized p-orbitals. The side-by-side overlap that forms a ir-bond makes a molecule resistant to twisting, results in bonds weaker than tr-bonds, and prevents atoms with large radii from forming multiple bonds. 

Cyclopropane, C.H, is a hydrocarbon composed of a three-membered ring of carbon atoms, (a) Determine the hybridization of the carbon atoms, (b) Predict the CCC and HCH bond angles at each carbon atom on the basis of your answer to part (a), (c) What must the real CCC bond angles in cyclopropane be (d) What is the defining characteristic of a cr-bond compared with a ir-bond, for example (e) How do the C—C cr-bonds in cyclopropane extend the definition of conventional o-bonds (f) Draw a picture depicting the molecular orbitals to illustrate your answer. 

The following molecules are bases that are part of the nucleic acids involved in the genetic code. Identify (a) the hybridization of each C and N atom, (b) the number of a- and ir-bonds, and (c) the number of lone pairs of electrons in the molecule. 

Which d-orbitals on the metal ion are used to form ir-bonds between octahedral metal ions and ligands ... 

Molecules of the oxides of phosphorus P4Oe and P4O10 also contain bridging oxygen atoms. The predicted amounts of ir-bond character, 0.73 X 0.32 = 0.23 and 0.77 X 0.32 = 0.25, respectively, lead with use of the single-bond distance... 

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