On the instability of iodides of heavy main group atoms in their higher oxidation state

Abstract

The inert pair effect—the tendency of the s orbital of heavy atoms to stay unreactive, is a consequence of the relativistic contraction of the s orbitals. While the manifestations of this on the reactivity depend on the nature of the substituents, this aspect is often overlooked. Divalent Pb prefers inorganic substituents, whereas tetravalent Pb prefers organic substituents. Among the inorganic substituents, again there are specific preferences—tetravalent Pb prefers F and Cl more than Br and I. It is as though the relativistic contraction of the s orbital of Pb is more significant with Br and I substituents than with Cl, F, and alkyl substituents. Herein, we address this problem using the molecular orbital approach and support it with quasi-relativistic density functional computations. We explain why typical hypervalent systems, like 12-X-6, and 10-X-5 (X is a heavy atom, the number preceding X is the number of valence electrons surrounding X, and the number after X is the coordination number) with less electronegative substituents carrying a lone pair (such as iodine), and Lewis octet molecules like PbI₄ are unstable, but their dianions (14-X-6, 12-X-5, PbI₄²⁻) are not. For heavy atoms, the relativistic contraction of the s orbital renders the antibonding combination of s with ligand orbitals (σ₁*) very low-lying, making it a good acceptor of electrons. Thus, compounds where σ₁* is empty are kinetically unstable when an electron donor with appropriate energy (such as the lone pair on iodine or bromine) is present in the vicinity. Donor–acceptor interaction between σ₁* and the lone pair on I or Br (F and Cl lone pairs are energetically far away from σ₁*) is responsible for the instability of such compounds. The kinetic stability of tetraalkyl lead compounds is due to the absence of lone pairs on the alkyl substituents. This work illustrates the key factor responsible for the instability of heavy element iodides by taking into consideration the covalent nature of the bonds, while the existing explanations assume a purely ionic bonding, which is an oversimplification.