Equatorial site

Even though the methyl group occupies an equatorial site, the conformation shown is not the most stable one for methylcyclohexane. Explain. 

The structures of several adducts can be rationalized on the basis [128] that in a 5-coordinate low-spin d8 tbp system, the acceptor ligands prefer to occupy an equatorial site (IrCl(CO)2(PPh3)2) whereas a 7r-donor prefers an axial site. In a square pyramidal situation, it is weakly bonded acceptors that prefer the apical position, e.g. (IrCl(S02)(C0)(PPh3)2. 

Three- and pentacoordinate organic phosphorus compounds can be oxidized through a free radical Arbuzov reaction, i.e., formation and p-scission of a phosphoranyl radical (Scheme 24). The P-scission is regioselective homolysis occurs on a ligand located in an equatorial site. Both a- and P-scissions are strongly dependent on the strength (bond dissociation energy) of the cleaved... 

With a steric number of 5, chlorine has trigonal bipyramidal electron group geomehy. This means the inner atom requires five directional orbitals, which are provided hymsp d hybrid set. Fluorine uses its valence 2 p orbitals to form bonds by overlapping with the hybrid orbitals on the chlorine atom. Remember that the trigonal bipyramid has nonequivalent axial and equatorial sites. As we describe in Chapter 9, lone pairs always occupy equatorial positions. See the orbital overlap view on the next page. 

Phosphorous (V) fluorides have some interesting structural and dynamic features that influence their 19F NMR spectra. All such compounds appear to have a trigonal bipyramid structure, which features two axial and three equatorial sites. Hexafluorophosphate, PF6 , is of course octahedral with all sites equivalent (Scheme 7.6). 

There are some unique structural aspects of some of the sulfur fluorides that will need to be discussed in order to understand the 19F NMR spectra. The geometry of tetracoordinate group VI compounds is predicted on the basis of Gillespie s electron-pair repulsion theory to be trigonal bipyramid, with an electron pair occupying one of the equatorial sites.2 Thus, the SF3 substituent as well as the molecule SF4 have structures as depicted in Scheme 7.12, with nonequivalent (axial and equatorial) fluorines, and thus their 19F NMR spectra consist of two 19F signals, with the fluorines being coupled if the system is scrupulously dry. 

Figure 4.22 The less electronegative ligands always preferentially occupy the less crowded equatorial sites of a trigonal bipyramid, leaving the more electronegative ligands in the more crowded axial sites.

Figure 9.13 The geometry of some phosphorus chlorofluorides and some phosphorus methyl fluorides in which the larger and less electronegative ligands occupy the equatorial sites.

D2EF1PA is thus a poor extractant for nickel as this shows a preference for a pseudo-octahedral structure in which two axial ligands are either fully protonated extractant molecules, fully protonated extractant dimers, or water molecules depending on extractant concentration, and four equatorial sites are occupied by deprotonated isolated D2EHPA molecules. 

Kinetic data were interpreted in terms of a mechanism in which initial heterolytic cleavage of H2 involved the basic 0 ligand coordination of olefin at the generated free equatorial site, followed by insertion to give alkyl and then protonation of the Pd-alkyl bond by the phenolic OH group, completes the catalytic cycle ... 

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