7r-bonding

Total syntheses of tazettine and 6a-epi-pretazettine have been carried out by application of the Pd-catalyzed cydization of 169, in which a single pentacyclic product 170 was obtained, establishing a preference for an eclipsed orientation of the Pd—C (7-bond and alkene 7r-bond in the key intramolecular insertion step[l40]. 

Finally, the CN and CC 7r-bond orders can be determined by use of a relation between bond orders and bond length (122) and the experimentally measured bond length (159). 

In Table 1-9 we have collected only the 7r-bond orders calculated by allvalence-electrons methods and compared their values with those deduced from experimental bond lengths. Both data are indicative of an aromatic molecule with a large dienic character. The 2-3 and 4-5 bonds especially present a large double-bond character, whereas both C-S bonds are relatively simple. 

The resonance integral of the 7r-bond between the heteroatom and carbon is another possible parameter in the treatment of heteroatomic molecules. However, for nitrogen compounds more detailed calculations have suggested that this resonance integral is similar to that for a C—C bond and moreover the relative values of the reactivity Indices at different positions are not very sensitive to change in this parameter. 

Although essentially free rotation is possible around single bonds (Section 3.6), the same is not true of double bonds. For rotation to occur around a double bond, the -rrbond must break and re-form (Figure 6.2). Thus, the barrier to double-bond rotation must be at least as great as the strength of the 7r bond itself, an estimated 350 kj/mol (84 kcal/mol). Recall that the barrier to bond rotation in ethane is only 12 kj/mol. 

The alkene adsorbs to the catalyst surface, using its 7r bond to complex to the metal atoms. 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

Pi (77 ) bond (Section 1.8) The covalent bond formed by sideways overlap of atomic orbitals. For example, carbon-carbon double bonds contain a 7r bond formed by sideways overlap of two / orbitals. 

Resonance effect (Section 16.4) The donation or withdrawal of electrons through orbital overlap with neighboring 7r bonds. For example, an oxygen or nitrogen substituent donates electrons to an aromatic ring by overlap of the O or N orbital with the aromatic ringp orbitals. 

There are, therefore, three unused d orbitals per ruthenium one of which is used to form a 7r-bond with an unused oxygen p orbital (it has already used the 2s and 2p orbitals in the (7-bonds to the three rutheniums). 

Some bis(dinitrogen) complexes exist, generally as m-isomers (presumably this minimizes competition for the metal t2g electron density in 7r-bonding). Unlike ruthenium, osmium(III) dinitrogen complexes do exist, showing osmium(III) to be a better 7r-donor not surprisingly, they are more labile than the osmium(II) species. 

A compound frans-OsCl2(PMe2Ph)4 has been isolated from the solution and is believed to contain one very loosely bound phosphine, possibly attached through a metal-ring 7r-bond or Os—H—C agostic interaction. 

All the OsNX4 complexes are distorted square pyramids (with N-Os-X angles of 103.7 to 104.5°) [188]. The stability of an osmium(VI) to iodine bond is unusual and is presumably owing to the extensive Os=N 7r-bonding reducing the positive charge on the metal and stabilizing it to reduction. 

Like rhodium(I), the iridium(I) complexes are stabilized by 7r-bonding ligands such as PR3 and CO, with 4- and 5-coordinate geometries. 

Although Zeise s salt is a complex of a 7r-bonding ligand, this compound must be included in an account of the chemistry of these metals, if only as... 

Back-bonding, with formation of a 7r-bond, from a filled metal d orbital to an anti-bonding it -ethene orbital. 

In many cases, it has been found that 7r-bonding ligands favour S-bonding. In a complex with both N- and S-bonded thiocyanate (Figure 3.74) the N-bonded group is trans to P while the sulphur-bonded thiocyanate is trans to the harder nitrogen ( anti-symbiosis ). 

Figure 3.78 Postulated 7r-bonding in cis- and rnms-phosphine complexes. (Reproduced with permission from S.A. Cotton and F.A. Hart, The Heavy Transition Elements, Macmillan...

Theoretical explanation of the trans-effect (and /rans-influence) has centred on two theories, one based on cr-bonding the other on 7r-bonding. The tr-bonding argument considers two frans-ligands sharing a metal p orbital (Figure 3.83). 

A 7r-bonding explanation notes that several ligands high in the trans-effect series are good -acceptors and thus siphon off 7r-density, making the region trans to it electron deficient and thus attractive to ligands that are electron rich. 

Since n bonding is believed to be more important in low oxidation states, as d orbitals contract with increasing oxidation state leading to poorer dw-pw overlap, this would not be expected on the basis of a 7r-bonding mechanism. Similarly, one can compare /(Pt-P) for pairs of isomers in the +2 and +4 states in a planar platinum(II) complex, the platinum 6s orbital is shared by four ligands whereas in an octahedral platinum(IV) complex it is shared by six ligands. Therefore, the 6s character is expected to be only 2/3 as much in the platinum(IV) complexes, correlating well with the 7(Pt-P) values, which can be taken to be a measure of the a-character in the bond. 

Support for this view is found in the I95Pt-15N coupling constants for dodecylamine complexes of platinum(II) and platinum(IV), where 7r-bonding cannot of course occur, which exhibit similar trends (Table 3.24) [156]. 

As already mentioned, a purely 7r-bonding mechanism cannot account for the position of hydride in trans-effect and trans-influence series. Overall, therefore, a major role (though not necessarily the only one) for <7-bonding is implied. 

The carhon-carbon double bond in alkenes is more reactive than carbon-carbon single bonds and gives alkenes their characteristic properties. As we saw in Section 3.4, a double bond consists of a a-bond and a 7r-bond. Each carbon atom in a double bond is sp2 hybridized and uses the three hybrid orbitals to form three cr-bonds. The unhvbridized p-orbitals on each carbon atom overlap each other and form a Tr-bond. As we saw in Section 3.7, the carbon-carbon 7r-bond is relatively weak because the overlap responsible for the formation of the 7r-bond is less extensive than that responsible for the formation of the a-bond and the enhanced electron density does not lie directly between the two nuclei. A consequence of this weakness is the reaction most characteristic of alkenes, the replacement of the 77-bond by two new a-bonds, which is discussed in Section 18.6. 

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