Chemical Bonding in Hypervalent Molecules:  Is the Octet Rule Relevant?

Abstract

The bonding in a large number of hypervalent molecules of P, As, S, Se, Te, Cl, and Br with the ligands F, Cl, O, CH(3), and CH(2) has been studied using the topological analysis of the electron localization function ELF. This function partitions the electron density of a molecule into core and valence basins and further classifies valence basins according to the number of core basins with which they have a contact. The number and geometry of these basins is generally in accord with the VSEPR model. The population of each basin can be obtained by integration, and so, the total population of the valence shell of an atom can be obtained as the sum of the populations of all the valence basins which share a boundary with its core basin. It was found that the population of the V(A, X) disynaptic basin corresponding to the bond, where A is the central atom and X the ligand, varies with the electronegativity of the ligand from approximately 2.0 for a weakly electronegative ligand such as CH(3) to less than 1.0 for a ligand such as F. We find that the total population of the valence shell of a hypervalent atom may vary from close to 10 for a period 15 element and close to 12 for a group 16 element to considerably less than 8 for an electronegative ligand such as F. For example, the phosphorus atom in PF(5) has a population of 5.37 electrons in its valence shell, whereas the arsenic atom in AsMe5 has a population of 9.68 electrons in its valence shell. By definition, hypervalent atoms do not obey the Lewis octet rule. They may or may not obey a modified octet rule that has taken the place of the Lewis octet rule in many recent discussions and according to which an atom in a molecule always has fewer than 8 electrons in its valence shell. We show that the bonds in hypervalent molecules are very similar to those in corresponding nonhypervalent (Lewis octet) molecules. They are all polar bonds ranging from weakly to strongly polar depending on the electronegativity of the ligands. The term hypervalent therefore has little significance except to indicate that an atom in a molecule is forming more than four electron pair bonds.

Meaningful Structural Descriptors from Charge Density

“…In the valence shell of one (central) atom in hypovalent molecules there are less than eight electrons, while in hypervalent molecules there are more than eight electrons. The bonding situation in hypervalent [80,81] molecules is especially a topic of constant debate. In the framework of the valence bond (VB) theory, the bonding in hypervalent molecules is described in terms of d n sp 3 hybrid orbitals pursuant to the sp n hybridization in molecules obeying the octet rule.…”

Comments on the nature of the bonding in oxygenated dinuclear copper enzyme models

“…The topological analysis of the ELF function [24] has been extensively used for bonding analysis [25][26][27][28][29][30][31][32][33]. Indeed, ELF is interpretable in term of an excess of kinetic energy due to the Pauli repulsion [34].…”

Characterisation of the reaction mechanism between ammonia and formaldehyde from the topological analysis of ELF and catastrophe theory perspective

“…Hydrogen transfer is an exception since the proton bearing some electron density is described by the asynaptic attractor V(H) [29]. To aid better understanding of the methodology used here, the reader is encouraged to familiarise themselves with the papers published by Silvi's group [30][31][32][33][34][35]. Particularly those describing the BET theory [5] and BET applications for reaction mechanisms such as isomerisation in XNO (X=H, Cl) [36], hydrogen transfer in H 5 O 2 + [33], in malonaldehyde [37], in the HF-(OH)…”