Bonding site preference

We have already used the HSAB principle as it applies to linkage isomers in metal complexes. This application to bonding site preference can also be used to show the behavior of other systems. For example, the reactions of organic compounds also obey the principles when reacting with nucleophiles such as SCN- or N02 ... 

The values of the fractionation factors in structures [15]-[21] are not strictly comparable since they are defined relative to the fractionation in different solvent standards. However, in aqueous solution, fractionation factors for alcohols and carboxylic acids relative to water are similar and close to unity (Schowen, 1972 Albery, 1975 More O Ferrall, 1975), and it seems clear that the species [15]-[21] involving intermolecular hydrogen bonds with solvent have values of cp consistently below unity. These observations mean that fractionation of deuterium into the solvent rather than the hydrogen-bonded site is preferred, and this is compatible with a broader potential well for the hydrogen-bonded proton than for the protons of the solvents water, alcohol and acetic acid. 

Wang and Rikvold [60] have applied ab initio total-energy density-functional methods in combination with supercell models to calculate the c(2 x 2) structure of bromide adsorbed on Au(lOO) and Ag(lOO) surfaces. The preferred bonding sites have been determined. The calculations have shown that bromide favorably binds the bridge site on the Au(lOO) surfaces. These results explain experimental observations that adsorption of bromide on the Au(lOO) and Ag( 100) surfaces proceeds via different bonding configurations. 

Co(II) in chloroform (40), In this latter case, the AF difference is about 5 kcal/mole, so the Ni-Co AF difference is smaller. Compare this with the 20 kcal/mole site-preference energy for octahedral Ni(II) over Co(II), according to crystal-field theory [for example, (56)], The fundamental assumption of crystal-field theory, of course, is that the radial factors in cation-coordination-sphere relations are constant, which is tantamount to saying that bonding does not change. As we have seen in deriving Eq. (24), binding factors are very important (8) and, as the above numerical relations confirm, play by far the dominant role. 

Atom jumping in a crystal can occur by several basic mechanisms. The dominant mechanism depends on a number of factors, including the crystal structure, the nature of the bonding in the host crystal, relative differences of size and electrical charge between the host and the diffusing species, and the type of crystal site preferred by the diffusing species (e.g., anion or cation, substitutional or interstitial). 

Naphthalene endoperoxides have been synthesized with methyl groups located at the 1,4-, 1,8-, 1,2,4-, 1,4,5-, 1,2,3,4-positions, and 1,2,3,4,5,6,7,8-positions in good yield after recrystallization of 58%, 88%, 93%, 54%, 83%, and 69%, respectively [48, 49]. The C—O bond formation preferably takes place at aryl sites bonded by the methyl groups. The stabilities of the endoperoxides were greater with increased numbers of methyl groups on naphthalene [at 25 °C in CDC13, compare... 

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