Perspectives on halogen bonding and other σ-hole interactions: Lex parsimoniae (Occam’s Razor)

σ-Hole Bonding: A Physical Interpretation

“…This has been demonstrated for covalentlybonded atoms of Group VI [35], Group V [36], and Group IV [37]. This is again caused by the anisotropic charge distributions of the atoms [21,25,38,39], which result in σ-holes on the extensions of single (and sometimes multiple) bonds to these atoms. Accordingly Group VI, V, and IV atoms can have two, three, and four σ-holes, respectively (or more if the atoms are hypervalent [37,40]).…”

“…Its electronic density distribution is, on average, spherically symmetrical [18] and the electrostatic potential V(r) created by its nucleus and electrons is positive for all r < 1 [19]; the positive contribution of the nucleus, which is usually treated as a point charge, outweighs the negative contribution of the dispersed electrons. When the atom participates in forming a covalent bond, however, its electronic density undergoes a redistribution which causes it to become anisotropic [20][21][22][23][24][25], less on the outer side of the atom (i.e., along the extension of the bond) than on the lateral sides. This can be seen, for instance, for the chlorine in Cl-OH in Fig.…”

Halogen Bonding with Phosphine: Evidence for Mulliken Inner Complexes and the Importance of Relaxation Energy

“…has proved somewhat controversial. It is possible to argue that electrostatic forces and their polarisation encompass all other contributions, [22][23][24] yet these other concepts do provide a level of insight into why some interactions are stronger than others and, indeed, are mentioned in the IU-PAC features of a halogen bond. Shaik and co-workers meanwhile argue that polarisation arrises from electron excitations within the sub-units of a complex and as charge transfer is associated with excitations between the sub-units the two must be physically distinct.…”

Using beryllium bonds to change halogen bonds from traditional to chlorine-shared to ion-pair bonds

“…Hence, when FCl interacts simultaneously with the beryllium derivatives and the nitrogen bases, synergistic cooperativity between the beryllium bond and the halogen bond should be expected, as has been observed for other weak interactions. 47,48 The structures, total energies, and molecular graphs of the binary complexes are reported in Table S2 of the ESI, † and their binding energies in Table 2. In complexes of FCl with the beryllium derivatives, binding energies range from À8.5 to À16.0 kJ mol À1 , and increase in the order…”